Sublethal effects of four insecticides on the reproduction and wing formation of brown planthopper, Nilaparvata lugens

Hormetic dose response induced by sublethal-dose sulfoxaflor leads to reproductive stimulation of Aphis gossypii

Aphis gossypii Glover is one of the most agriculturally important phloem-feeding economic pests, causing tremendous loss in crop yield annually. The hormesis is an important cause of A. gossypii resistance formation, population resurgence, and re-outbreak. However, whether the hormesises induced by different insecticides interact mutually remain largely unclear. In the study, four-generation A. gossypii experiment found that the 24-h sublethal-dose (LC20) sulfoxaflor treatment on G0 significantly increased the net reproductive rate (R0) and fecundity of G1 and G2 generation A. gossypii, but it did not significantly affect the fecundity of G3 and G4 individuals. Transcriptomic analyses revealed that the insecticide-induced significant up-regulation of pathways ribosome, energy metabolism, and the DNA replication and reparation might be responsible for the enhancement of fecundity in G1 and G2 A. gossypii. Notably, G0 exposure to LC20 sulfoxaflor followed by G1 exposure to LC30 deltamethrin resulted in a stronger reproductive stimulation than sulfoxaflor or deltamethrin exposure alone. Our findings provide valuable reference for optimizing sulfoxaflor application in integrated pest management strategies.

Sublethal effects of imidacloprid on the fitness of two species of wheat aphids, Schizaphis graminum (R.) and Rhopalosiphum padi (L.)

Imidacloprid is a neonicotinoid insecticide that efficiently controls piercing-sucking mouthparts pests. However, the impact of low lethal concentration of imidacloprid on key demographic parameters of wheat aphids, Schizaphis graminum (R.) and Rhopalosiphum padi (L.) has been scarcely studied. In this study, we used the age stage, two-sex life table approach to investigate the sublethal effects of imidacloprid on the biological traits of S. graminum and R. padi. Bioassays showed that imidacloprid possesses high toxicity to adult S. graminum and R. padi, with LC50 of 3.59 and 13.78 mg L-1 following 24 h exposure. A low lethal concentration of imidacloprid (LC25) significantly decreased adult longevity and total longevity of progeny generation aphids (F1) of S. graminum. Nevertheless, imidacloprid (LC25) had no significant effects on the fecundity and longevity of directly exposed parental parental S. graminum and R. padi (F0). Our results showed that the low lethal concentration of imidacloprid affected the demographic parameters that ultimately impact on the population of S. graminum. This study provides detailed information about the overall effects of imidacloprid on S. graminum and R. padi that might help to manage these two key pests.

Hormesis effects of sulfoxaflor on Aphis gossypii feeding, growth, reproduction behaviour and the related mechanisms

Sulfoxaflor, an important alternative insecticide in integrated pest management (IPM) strategies, can effectively control sap-feeding insect pests such as Aphis gossypii. Although the side effects of sulfoxaflor have recently attracted widespread attention, its toxicological characteristics and mechanisms are still largely undefined. Therefore, the biological characteristics, life table and feeding behaviour of A. gossypii were studied to evaluate the hormesis effect of sulfoxaflor. Then, the potential mechanisms of induced fecundity associated with the vitellogenin (Ag. Vg) and vitellogenin receptor (Ag. VgR) genes were investigated. Although the LC₁₀ and LC₃₀ concentrations of sulfoxaflor significantly reduced the fecundity and net reproduction rate (R₀) of the directly exposed sulfoxaflor-resistant and susceptible aphids, hormesis effects on fecundity and R₀ were observed in the F₁ generation of Sus A. gossypii when the parental generation was exposed to the LC₁₀ of sulfoxaflor. Moreover, the hormesis effects of sulfoxaflor on phloem feeding were observed in both A. gossypii strains. Additionally, enhanced expression levels and protein content of Ag. Vg and Ag. VgR were observed in progeny generations when F₀ was subjected to the trans- and multigenerational sublethal sulfoxaflor exposure. Therefore, sulfoxaflor-induced resurgence might occur in A. gossypii after exposure to sublethal concentrations. Our study could contribute to a comprehensive risk assessment and provide convincing reference to optimize sulfoxaflor in IPM strategies.

Evaluation of Transgenerational Effects of Sublethal Imidacloprid and Diversity of Symbiotic Bacteria on Acyrthosiphon gossypii

Symbiotic bacteria and hormesis in aphids are the driving forces for pesticide resistance. However, the mechanism remains unclear. In this study, the effects of imidacloprid on the population growth parameters and symbiotic bacterial communities of three successive generations of Acyrthosiphon gossypii were investigated. The bioassay results showed that imidacloprid had high toxicity to A. gossypii with an LC₅₀ of 1.46 mg·L^-1. The fecundity and longevity of the G0 generation of A. gossypii decreased when exposed to the LC₁₅ of imidacloprid. The net reproductive rate (R₀), intrinsic rate of increase (r_m), finite rate of increase (λ), and total reproductive rate (GRR) of G1 and G2 offspring were significantly increased, but those of the control and G3 offspring were not. In addition, sequencing data showed that the symbiotic bacteria of A. gossypii mainly belonged to Proteobacteria, with a relative abundance of 98.68%. The dominant genera of the symbiotic bacterial community were Buchnera and Arsenophonus. After treatment with the LC₁₅ of imidacloprid, the diversity and species number of bacterial communities of A. gossypii decreased for G1-G3 and the abundance of Candidatus-Hamiltonella decreased, but Buchnera increased. These results provide insight into the resistance mechanism of insecticides and the stress adaptation between symbiotic bacteria and aphids.

Sublethal Effects of Thiamethoxam on Biological Traits and Detoxification Enzyme Activities in the Small Brown Planthopper, Laodelphax striatellus (Fallén)

The small brown planthopper (Laodelphax striatellus (Fallén), Hemiptera: Delphacidae), is an important agricultural pest of rice, and neonicotinoid insecticides are commonly used for controlling L. striatellus. However, the sublethal effects of thiamethoxam on L. striatellus remain relatively unknown. In this study, an age-stage life table procedure was used to evaluate the sublethal effects of thiamethoxam on the biological parameters of L. striatellus. Additionally, activities of carboxylesterase, glutathione S-transferase, and cytochrome P450 monooxygenase in the third instar nymphs were analyzed. The results indicated that the survival time of F0 adults and the fecundity of female adults decreased significantly after the third instar nymphs were treated with sublethal concentrations of thiamethoxam (LC15 0.428 mg/liter and LC30 0.820 mg/liter). The developmental duration, adult preoviposition period, total preoviposition period, and mean generation time of the F1 generation increased significantly, whereas the fecundity of the female adults, intrinsic rate of increase (ri), and finite rate of increase (λ) decreased significantly. The oviposition period was significantly shorter for the insects treated with LC30 than for the control insects. Neither sublethal concentrations had significant effects on the adult longevity, net reproduction rate (R0), or gross reproduction rate (GRR) of the F1 generation. The activities of carboxylesterase, glutathione-S-transferase, and cytochrome P450 monooxygenase increased significantly after the thiamethoxam treatments. These results indicate that sublethal concentrations of thiamethoxam can inhibit L. striatellus population growth and enhance detoxification enzyme activities.

Reproductive Outbreaks of Sogatella furcifera Mediated by Overexpression of the Nuclear Receptor USP under Pressure from Triflumezopyrim

Long-term pesticide-driven selection pressure is one of the main causes of insect outbreaks. In this study, we found that low doses of triflumezopyrim could increase the fecundity of white-backed planthoppers (Sogatella furcifera). By continuously screening 20 generations with a low dose of triflumezopyrim, a triflumezopyrim-resistant strain (Tri-strain, resistance ratio = 20.9-fold) was obtained. The average oviposition quantity and longevity of the Tri-strain (208.77 eggs and 21.31 days, respectively) were significantly higher than those of the susceptible strain (Sus-strain) (164.62 eggs and 17.85 days, respectively). To better understand the mechanism underlying the effects on reproduction, we detected the expression levels of several reproduction-related transcription factors in both the Tri- and Sus-strains. Ultraspiracle (USP) was significantly overexpressed in the Tri-strain. Knockdown of USP by RNAi severely inhibited the moulting process of S. furcifera and disrupted the development of female adult ovaries. Among the potential downstream target genes of USP, Kr-h1 (0.19-fold), Cht8 (0.56-fold) and GPCR A22 (0.31-fold) showed downregulated expression after USP-RNAi. In contrast, the expression of EcR (2.55-fold), which forms heterodimers with USP, was significantly upregulated. Furthermore, RNAi was performed on Kr-h1 in the Tri-strain, and the results show that larval moulting and the development of female adult ovaries were inhibited, consistent with the USP-RNAi results in S. furcifera. These results suggest that the transcription factors USP and Kr-h1 play important roles in the reproductive development of S. furcifera, and overexpression of USP and Kr-h1 in the Tri-resistant strain may result in reproductive outbreaks of pests.

The development, reproduction and P450 enzyme of the white-backed planthopper, Sogatella furcifera (Hemiptera: Delphacidae) under the sublethal concentrations of clothianidin

Clothianidin is a second-generation neonicotinoid insecticide that can effectively prevent piercing-sucking pests, such as white-backed planthopper (WBPH), Sogatella furcifera (Horváth). In this study, the sublethal effects of clothianidin on the biological traits of S. furcifera were evaluated via the age-stage, two-sex life table procedure. Our results exhibited that the female adult longevity, fecundity and hatchability of F₀ generation were significantly decreased after LC₁₀ and (or) LC₃₀ of clothianidin exposure compared to the control. Transgenerational effects showed that the pre-adult period, female adult longevity, total longevity, oviposition days (Od), fecundity and mean generation time (T) of F₁ generation were significantly decreased in the LC₁₀ and LC₃₀ groups compared to the control. Moreover, the development times of the third- and fifth-instar nymphs, total preoviposition period (TPOP) and doubling time (DT) were significantly shorter in the LC₁₀ group than in the control and LC₃₀ groups. Furthermore, the intrinsic rate of increase (r_i) and finite rate of increase (λ) values of the LC₁₀ group were significantly higher than those of the control group. However, there was no significant difference in the male adult longevity, adult preoviposition period (APOP) and net reproductive rate (R₀) between the treated groups and the control. Enzyme activity and gene expression results showed that the P450 enzyme activity and mRNA levels of many P450 genes were significantly increased by clothianidin treatment. In addition, the knockdown of CYP4CE3 and CYP6FJ3, which showed the highest inducing levels, by RNA interference (RNAi) dramatically increased the toxicity of clothianidin against S. furcifera. These results indicated that sublethal concentrations of clothianidin showed a stimulatory effect on the development, but it could adversely affect the survival and reproduction of S. furcifera. Additionally, CYP4CE3 and CYP6FJ3 might play an important role in the detoxification and evolution of clothianidin resistance in S. furcifera.

Sublethal and transgenerational effects of exposures to the thiamethoxam on the seven-spotted lady beetle, Coccinella septempunctata L. (Coleoptera: Coccinellidae)

The seven-spot ladybird beetle, Coccinella septempunctata L. (Coleoptera: Coccinellidae) has been used as the major natural enemy insect against many kinds of aphids in environment. Thiamethoxam is a second-generation neonicotinoid insecticide and commonly used to control various aphids, but it also has adverse effects on natural enemies. Therefore, the sublethal effects of thiamethoxam on the offspring of C. septempunctata were studied. For the adults of C. septempunctata, the LD₃₀ of F₀, F₅, F₁₀ and F₁₅ generations were 0.039, 0.136, 0.243, 0.417 μg adult^-1, respectively. The LD₅₀ of F₀, F₅, F₁₀ and F₁₅ generations were 0.072, 0.275, 0.435, 0.819 μg adult^-1, respectively. The resistance ratio of the F₁₅ generation was 11.4-fold higher than that of the F₀ generation. The preadult duration ( F₀ = 17.4 d, F₅ = 20.0 d, F₁₀ = 19.1 d, F₁₅ = 18.0 d) and adult preoviposition period ( F₀ = 7.39 d, F₅ = 8.07 d, F₁₀ = 9.32 d, F₁₅ = 8.03 d) of C. septempunctata were prolonged, and the fecundity ( F₀ = 1690.10, F₅ =1686.93, F₁₀ = 1119.40, F₁₅ = 752.87), oviposition period ( F₀ = 42.84 d, F₅ =40.33 d, F₁₀ = 40.72 d, F₁₅ = 30.84 d) and total longevity ( F₀ = 86.7 d, F₅ =81.3 d, F₁₀ = 82.0 d, F₁₅ = 74.0 d) were reduced by long-term exposure to sublethal dosage (LD₃₀) of thiamethoxam. Population parameters (r, λ, R₀, and T) were significantly reduced in F₅, F₁₀ and F₁₅ generation. In addition, the predation function of F₁₅ generation was reduced by sublethal dosage exposure of thiamethoxam. These results showed the negative impacts of sublethal thiamethoxam on the offspring of predators.

Flupyradifurone induces transgenerational hormesis effects in the cowpea aphid, Aphis craccivora

With low-dose stimulation and high-dose inhibition, insecticide-induced hormesis, a biphasic phenomenon, can contribute to pest resurgence. The cowpea aphid, Aphis craccivora (Koch) (Homoptera: Aphididae), is a vital insect that infests legume crops. Its hormesis of flupyradifurone has not been previously established. Age-stage two-sex life analysis is used to investigate the sublethal and transgenerational effects of flupyradifurone on two successive generations of A. craccivora. A leaf-dip bioassay method revealed high toxicity of flupyradifurone against A. craccivora, with lethal concentration 50% value (LC₅₀) of 1.82 mg L^-1 after 48 h exposure. Treatment of parent generation (F₀) with LC₁₀ and LC₂₅ of flupyradifurone significantly increased the longevity and fecundity of the directly exposed adults. The results of transgenerational effects showed that the treatment of (F₀) with LC₂₅ induced significant hormetic effects in progeny generation (F₁). Furthermore, flupyradifurone at LC₂₅ significantly enhanced the biological traits, such as intrinsic rate of increase (r), finite rate of increase (λ), and net reproductive rate (R₀) compared with the control. Similarly, both LC₁₀ and LC₂₅ induced a significant increase in the mean generation time T (d). Conversely, both treatments caused a significant decrease in the doubling time (DT). Data in the present study demonstrate that the exposure of (F₀) to flupyradifurone at LC₁₀ and LC₂₅ enhanced longevity and fecundity in the directly exposed adults of A. craccivora, and induced transgenerational hormesis across the subsequent (F₁) generation. These results should be taken into consideration when using flupyradifurone for controlling cowpea aphid.

Sublethal Exposure to Deltamethrin Stimulates Reproduction and Alters Symbiotic Bacteria in Aphis gossypii

In aphids, hormesis and symbiotic bacteria are the drivers for the development of pesticide resistance. However, the related mechanism remains unclear. Here, we evaluated the sublethal and transgenerational effects of the extensively used pyrethroid pesticide deltamethrin (DMT) on the population dynamics in Aphis gossypii and tested its influence on symbiotic bacterial communities. The leaf-dip bioassay revealed that DMT was highly toxic to A. gossypii, and at a low lethal concentration of DMT, the intrinsic (r) and finite rates of increase (λ) of the initially exposed aphids (G0) significantly decreased. Intriguingly, the r, λ, and net reproductive rate (R₀) of G1 and G2 significantly increased, but the r and λ decreased in G3. The adult and total preoviposition period increased in G3 but decreased in G4. Additionally, the diversity of the bacterial community decreased, while the abundance values of Buchnera, Pseudomonadaceae, and Burkholderiaceae increased after 24 h of exposure to LC₃₀ DMT in G0 aphids, and the latter two decreased in G1 but increased in G2. In summary, sublethal DMT has intergenerational hormesis effect on cotton aphids in G1-G2 and remarkably altered their symbiotic bacterial community and abundance. These results broaden our understanding of the relationship of hormesis and symbiotic bacteria in aphids under insecticide exposure.

Lethal and Sublethal Effects of Proteus, Matrine, and Pyridalyl on Frankliniella occidentalis (Thysanoptera: Thripidae)

The western flower thrips (WFT), Frankliniella occidentalis (Pergande) is one of the most harmful pests of crops in greenhouses and fields. Considering the need for studies that introduce new insecticides for control of the WFT, the leaf dip method was carried out to study the acute toxicity of Proteus, matrine, and pyridalyl to adult thrips, and life tables were constructed to assess the impacts of sublethal concentrations (LC25) of these insecticides on the development and reproduction of the F1 generation. Bioassays showed that the toxicity of matrine (LC50: 45.9 µl ml-1) and Proteus (LC50: 54.5 µl ml-1) was higher than pyridalyl (LC50: 176.5 µl ml-1). At LC25 concentration, both Proteus and matrine prolonged the development period and reduced the survival rate of eggs, larval stages, and pupae in the F1 generation. Also, the adults' longevity, oviposition duration, and the cumulative number of eggs laid per female (fecundity) were decreased significantly. Sublethal concentrations of Proteus and matrine inhibited the population growth rate relative to the control based on the predicted number of offspring. The lowest net reproductive rate (R0), intrinsic rate of increase (r), and finite rate of increase (λ) were estimated for Proteus (7.02 offspring/individual, 0. 0838 d-1, and 1. 08 d-1, respectively). In contrast, the WFT F1 generation that resulted from parent adults treated with pyridalyl was neither affected in their developmental time, nor fecundity, or the intrinsic rate of increase. According to our findings, all tested insecticides, especially Proteus, showed good potential for use in integrated pest management strategies against F. occidentalis.

Evaluation of sublethal and transgenerational effects of sulfoxaflor on Aphis gossypii via life table parameters and 16S rRNA sequencing

BACKGROUND

Aphis gossypii, a polyphagous and recurrent pest induced by pesticides, causes tremendous loss crop yields each year. Previous studies on the mechanism of pesticide-induced sublethal effects mainly focus on the gene level. The symbiotic bacteria are also important participants of this mechanism, but their roles in hormesis are still unclear.

RESULTS

In this study, life table parameters and 16S rRNA sequencing were applied to evaluate the sublethal and transgenerational effects of sulfoxaflor on adult A. gossypii after 24-h LC₂₀ (6.96 mg L^-1 ) concentration exposure. The results indicated that the LC₂₀ of sulfoxaflor significantly reduced the finite rate of increase (λ) and net reproductive rate (R₀ ) of parent generation (G0), and significantly increased mean generation time (T) of G1 and G2, but not of G3 and G4. Both reproductive period and fecundity of G1 and G2 were significantly higher than those of the control. Furthermore, our sequencing data revealed that more than 95% bacterial communities were dominated by the phylum Proteobacteria, in which the maximum proportion genus was the primary symbiont Buchnera and the facultative symbiont Arsenophonus. Compared to those of the control, the abundance and composition of symbiotic bacteria of A. gossypii for three successive generations (G0-G2) were changed after G0 A. gossypii was exposed to sulfoxaflor: the diversity of the bacterial community was decreased, but the abundance of Buchnera was increased (G0), while the abundance of Arsenophonus was decreased. Contrary to G0, G1 and G2 cotton aphid exhibited an increased relative abundance of Arsenophonus in the sublethal treatment group.

CONCLUSION

Taken together, our results provide an insight into the interactions among pesticide resistance, aphids, and symbionts, which will eventually help to better manage the resurgence of A. gossypii. © 2021 Society of Chemical Industry.

Synergism and unintended effects of the association between imidacloprid and sodium chloride (NaCl) on the management of Euschistus heros

BACKGROUND

The use of insecticidal solutions containing sodium chloride (NaCl) has been proposed as a more environmentally friendly alternative to managing stink bug infestations of Neotropical soybean fields. The potential sublethal and undesirable effects of this practice have, however, been overlooked, especially with novel insecticides. Here, we have evaluated experimentally whether the addition of NaCl (0.5% w/v) to imidacloprid-containing solutions could alter insecticide toxicity and modify the reproductive responses of the Neotropical brown stink bug Euschistus heros.

RESULTS

Adding NaCl to imidacloprid solutions significantly increased imidacloprid toxicity against E. heros. The exposure to E. heros to sublethal concentrations of imidacloprid affected the insect's mating abilities in a concentration-dependent manner. The addition of NaCl to solutions containing imidacloprid at concentrations as low as 0.126 μg a.i. cm^-2 (i.e. the equivalent to 3% of field rate recommendation) also impacted the sexual behavior of E. heros, reducing mating duration. NaCl-exposed stink bugs, however, exhibited higher fecundity and fertility rates than those insects that were unexposed to NaCl or those that were exposed to sublethal levels of imidacloprid only.

CONCLUSIONS

The addition of low amounts of NaCl resulted in a higher toxicity of imidacloprid. This practice, however, can also lead to undesirable effects as increasing reproductive output of E. heros that can potentially compromise the management of these insect pests.

Evaluation of the sublethal effect of tetrachlorantraniliprole on Spodoptera exigua and its potential toxicity to two non-target organisms

Tetrachlorantraniliprole (TCAP) is a novel anthranilic diamide insecticide that specifically targets the ryanodine receptors of lepidopteran insect species with excellent insecticidal activity. Previous studies have reported the sublethal effects of multiple diamides on several lepidopteran species, whereas the sublethal and non-target effects of TCAP remain largely unknown. We assessed the sublethal effects of TCAP on Spodoptera exigua. We also investigated the effects of TCAP on non-target Harmonia axyridis and Eisenia fetida, S. exigua was more sensitive to TCAP than to chlorantraniliprole, as the LC₅₀ (10.371 μg L^-1 at 72 h) of TCAP was relatively lower. Compared with those of the control, sublethal concentrations of TCAP (LC₁₀ and LC₃₀) not only prolonged the duration of the larval and pupal stages as well as the mean generation time but also reduced certain population parameters. On the other hand, TCAP exposure, even at the highest concentration, did not induce toxic effects in H. axyridis ladybugs (1^st instar larvae and adults) or E. fetida earthworms. Taken together, our results suggest that TCAP can be used as a novel and promising component of the integrated pest management (IPM) program against S. exigua due to its robust target effects and negligible non-target risks.

Mobility and Dispersal of Two Cosmopolitan Stored-Product Insects Are Adversely Affected by Long-Lasting Insecticide Netting in a Life Stage-Dependent Manner

Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and Trogoderma variabile Ballion (Coleoptera: Dermestidae) are two stored-product insects that cause extensive damage to a variety of postharvest commodities. Long-lasting insecticide-incorporated netting (LLIN), commonly used to control vector-borne diseases in tropical regions, has only been recently studied in an agricultural setting. While prior research showed that LLIN was successful against stored-product beetles, little is known about differential susceptibility among stored-product insect life stages. The aim of this study was to evaluate LLIN efficacy against immature T. castaneum and T. variabile compared with adults. Movement and dispersal ability were evaluated after exposure to LLIN or an untreated, control netting. For the movement assay, video-tracking software recorded the postexposure effects of LLIN on distance traveled and velocity of the insects in 2-h trials. For the dispersal assay, insects were exposed to the netting then released into one end of a PVC pipe and allowed 48 h to disperse to a novel food patch located at the opposite end of the pipe. Our study found that movement and dispersal ability of T. variabile and T. castaneum are significantly reduced, often by multiple-fold, after LLIN exposure, with the larval stage of each species more tolerant to the insecticide netting than adults. These results indicate that LLIN is a promising tool for use in intercepting immigrating insects of different life stages in food facilities to protect stored products.

Sublethal and transgenerational effects of sulfoxaflor on the demography and feeding behaviour of the mirid bug Apolygus lucorum

Sulfoxaflor, the first commercially available sulfoximine insecticide, has been used for the control of sap-feeding insect pests such as plant bugs and aphids on a variety of crops. However, its sublethal effects on the mirid bug Apolygus lucorum, one of the key insect pests of Bt cotton and fruit trees in China, have not been fully examined. Here, we evaluated the demography and feeding behaviour of A. lucorum exposed to sulfoxaflor. The leaf-dipping bioassay showed that the LC₁₀ and LC₃₀ of sulfoxaflor against 3^rd-instar nymphs of this insect were 1.23 and 8.37 mg L^-1, respectively. The LC₁₀ significantly extended the nymphal duration and decreased the oviposition period by 5.29 days and female fecundity by 56.99% in the parent generation (F0). The longer duration of egg, 5^th-instar nymphs, preadult, and male adult longevity were observed in the F1 generation (F1) at LC₁₀. At the LC₃₀, the duration of egg and 1^st-instar nymph, female adult longevity, and oviposition period of the F1 were significantly shorter, while the nymphal duration in the F0 and duration of 5^th-instar nymphs, preadult survival rate, and male adult longevity in the F1 significantly increased. The net reproductive rate (R₀), intrinsic rate of increase (r), and finite rate of increase (λ) in the F1 were not significantly affected by these two concentrations, whereas the mean generation time (T) was lower at the LC₃₀. Additionally, the probe counts and cells mixture feeding time were markedly lengthened by the LC₁₀ and LC₃₀, respectively, when A. lucorum nymphs exposed to sulfoxaflor fed on Bt cotton plants without insecticides. These results clearly indicate that sulfoxaflor causes sublethal effects on A. lucorum and the transgenerational effects depend on the tested concentrations.

Thiamethoxam induces transgenerational hormesis effects and alteration of genes expression in Aphis gossypii

Insecticide induced-hormesis, a bi-phasic phenomenon characterized by low dose stimulation and high dose inhibition following exposure to insecticide, is crucial to insect pest resurgence. In this study, the effects of low or sublethal concentrations of thiamethoxam on biological traits and genes expression were investigated for Aphis gossypii Glover following 72 h exposures. Leaf-Dip bioassay results showed that thiamethoxam was very toxic against adult A. gossypii with an LC₅₀ of 1.175 mg L^-1. The low lethal (LC₁₅) and sublethal (LC₅) concentrations of thiamethoxam significantly reduced longevity and fecundity of the directly exposed aphids. However, stimulatory effects on pre-adult stage, longevity, and fertility were observed in the progeny generation (F₁) of A. gossypii, when parental aphids (F₀) were exposed to LC₁₅ of thiamethoxam. Subsequently, biological traits such as intrinsic rate of increase (r), finite rate of increase (λ), and net reproductive rate (R₀) increased significantly to F₁ individuals due to LC₁₅ treatment. No significant responses were observed for LC₅ of thiamethoxam. The LC₁₅ of thiamethoxam significantly increased the expression level of vitellogenin and ecdysone receptors genes in progeny generation, while no effects were observed for treatment with LC₅. Additionally, the expression levels of P450 genes including CYP6CY14, CYP6CZ1, CYP6DC1, CYP6CY9, and CYP6DD1 were up-regulated in the exposed aphids. Taken together, our results show the hormetic effects of thiamethoxam on F₁ individuals, which might be due to the intermittent changes in expression of genes involved in fertility, growth and insecticide detoxification in A. gossypii.

Neonicotinoid insecticides hinder the pupation and metamorphosis into adults in a crabronid wasp

Neonicotinoid insecticides are associated with a decline in the diversity and distribution of bees and wasps (Hymenoptera: Aculeata). The effects of neonicotinoids on the metamorphosis of aculeates have never been addressed in detail; however, recent evidence suggests that neonicotinoids induce wing abnormalities. We hypothesized that the metamorphosis success of bees and wasps differs in response to contact exposure to field-realistic concentrations of neonicotinoid insecticides or in response to combined exposure to neonicotinoid insecticides and benzimidazole fungicides. We treated prepupae of the model crabronid wasp Pemphredon fabricii with field-realistic concentrations of four neonicotinoids, acetamiprid, imidacloprid, thiacloprid and thiamethoxam, and/or with the benzimidazole fungicide thiabendazole. Treatment with acetamiprid or imidacloprid decreased the pupation rates to only 39% and 32%, respectively. Treatment with thiacloprid or thiamethoxam did not affect the pupation rate when applied alone, but the subsequent treatment of thiacloprid- or thiamethoxam-treated prepupae with thiabendazole led to significant decreases in pupation rates. A high concentration of acetamiprid, which severely affected the pupation rates, had moderate effects on metamorphosis into adults, resulting in 53% metamorphosis success (as opposed to 95% metamorphosis success in the water-treated group). However, imidacloprid or thiamethoxam treatment resulted in only 5%-10% metamorphosis success into adults. Overall survival decreased in response to treatment with any of the neonicotinoids or benzimidazoles or their combinations, with extremely low survival (<2%) following combined treatment with imidacloprid and thiabendazole or thiamethoxam and thiabendazole. In conclusion, neonicotinoids alter insect metamorphosis success, which can be further potentiated by their combination with other agrochemicals, such as benzimidazoles.

Evaluation of the pesticide Oberon on a model organism Drosophila melanogaster via topical toxicity test on biochemical and reproductive parameters

Spiromesifen (Oberon® 240 SC), a pesticide widely used to control pests like mites and whiteflies, was investigated using Drosophila melanogaster Meigen, 1830 (Diptera, Drosophilidae) as a model organism. The compound was applied topically at two concentrations (LC₁₀: 21.45 and LC₂₅: 39.53 μg active ingredient/pupa), on newly molted pupae and assessed on morphometric measurements of ovaries and the progeny of surviving adults. Results showed that spiromesifen inhibited the growth and development of ovaries, reducing at the highest dose (LC₂₅) the number of oocytes, the volume of basal oocytes and ovarian weight. Biochemical analysis revealed that the tested compound reduced the ovarian levels of carbohydrates and glycogen during the sexual maturation. Moreover, fecundity, fertility and number of descendants from parents that survived to the treatment of pupae were significantly reduced. The sex ratio determined indicated a significant decrease in treated series and males seemed more sensitive to spiromesifen than females. Lastly, the compound was found to affect the sexual behavior.

Sublethal and transgenerational effects of dinotefuran on biological parameters and behavioural traits of the mirid bug Apolygus lucorum

The mirid bug, Apolygus lucorum, has become a major pest of many crops and fruit trees since the widespread adoption of Bt cotton in northern China. Neonicotinoid insecticides, such as dinotefuran, applied to control this pest may show sublethal effects, but evidence for such effects is lacking. Here, we investigated the sublethal and transgenerational effects of dinotefuran on biological parameters and feeding behavioural traits of A. lucorum using the age-stage, two-sex life table and electrical penetration graphs (EPGs), respectively. The LC₁₀ and LC₃₀ of dinotefuran against 3^rd-instar nymphs of A. lucorum were 14.72 and 62.95 mg L⁻¹, respectively. These two concentrations significantly extended the development duration from 3^rd-instar nymph to adult in parent generation (F0). LC₃₀ also increased the oviposition period and male adult longevity and reduced nymphal survival rate in the F0. For offspring generation (F1), the egg duration, preadult duration, and total preoviposition period were significantly lower at LC₁₀ than in the control, and the egg duration, duration of 4^th-instar nymphs, preadult duration, oviposition period, and fecundity were also decreased at LC₃₀. However, the four demographic parameters of F1 generation, namely, net reproductive rate (R₀), intrinsic rate of increase (r), finite rate of increase (λ), and mean generation time (T), were not affected by dinotefuran. The significant differences in the number of probes and duration of each of four feeding waveforms failed to be detected when A. lucorum nymphs treated by dinotefuran feed on Bt cotton plants without insecticide exposure. Overall, the dinotefuran concentrations tested here have sublethal, but no transgenerational impacts on A. lucorum.

Assessment of the effects of lethal and sublethal exposure to dinotefuran on the wheat aphid Rhopalosiphum padi (Linnaeus)

The wheat aphid Rhopalosiphum padi (Linnaeus) (Hemiptera: Aphididae) is a devastating pest of wheat crops worldwide. Dinotefuran, a novel neonicotinoid insecticide, has been used to prevent piercing-sucking agricultural insects, such as R. padi. This research showed that the dinotefuran not only caused direct mortality but also affected the physiology of R. padi via sublethal effects. In this study, residual film bioassay results indicated that there were no significant differences in the toxicity of dinotefuran between field in 2017 and laboratory strains. However, the longevity, fecundity and female preoviposition of the F₀ generation were significantly decreased by exposure to different sublethal doses (L₁₀, L₂₀ and L₃₀) of dinotefuran. In contrast, the fecundity and female preoviposition of the F₁ generation were significantly increased by the sublethal treatment L₂₀, although this dose reduced net reproductive rate, intrinsic rate of increase and finite rate of increase. These findings are the first laboratory evidence of hormesis attributable to low dinotefuran doses. Developmental duration of nymphs was significantly increased by the sublethal doses L₂₀ and L₃₀ but not L₁₀. Sublethal exposure to dinotefuran can increase the transgenerational population growth of R. padi and affected demographic parameters of the target insect. This study provides useful data for developing management strategies for R. padi involving the use of dinotefuran.

Lethal and Sublethal Effects of Neonicotinoid Insecticides on the Adults of Phenacoccus solenopsis (Hemiptera: Pseudococcidae) on Tomato Plants

Acetamiprid and imidacloprid are two important neonicotinoid insecticides that are widely utilized under field conditions for the management of sucking insect pests, including the solenopsis mealybug Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae). Although some information is available regarding their lethal effects, nothing is currently known about the sublethal effects of these insecticides. We, therefore, performed a series of experiments to test the lethal and sublethal effects of these chemicals on oviposition duration and fecundity. We also assessed sublethal effects on feeding behavior using the electrical penetration graph (EPG) technique. The results of this study reveal that acetamiprid toxicity is higher than imidacloprid and that both insecticides have negative effects on the oviposition, fecundity, and feeding behavior of P. solenopsis when applied at sublethal dosages. These chemicals also significantly reduce oviposition duration and fecundity and significantly prolong nonprobing duration, increase penetration problems, and reduce phloem and xylem feeding activities when compared with adults exposed to just water. No significant differences were detected in all waveform durations and events when adults previously exposed to foliage treated with each of these two insecticides were compared. The results of this study, therefore, suggest that both insecticides are capable of protecting crops from mealybug damage by not only killing these pests directly but also reducing their fecundity and inhibiting feeding behaviors when applied at sublethal dosages.

Exposure to Insecticides Reduces Populations of Rhynchophorus palmarum in Oil Palm Plantations with Bud Rot Disease

The South American palm weevil (SAPW), Rhynchophorus palmarum Linnaeus (Coleoptera: Curculionidae) is the main pest of Elaeis guineensis and damages palm trees with bud rot disease in the Americas. The effects of six neurotoxic insecticides (abamectin, carbaryl, deltamethrin, fipronil, imidacloprid and spinosad) were evaluated against SAPW for toxicity, survival, reproduction, and mortality. Abamectin (LC₅₀ = 0.33 mg mL⁻¹), Carbaryl (LC₅₀ = 0.24 mg mL⁻¹), deltamethrin (LC₅₀ = 0.17 mg mL⁻¹), and fipronil (LC₅₀ = 0.42 mg mL⁻¹) were the most toxic to SAPW. Adult survival was 95% without exposure to insecticides, decreasing to 78–65% in insects treated with the LC₂₅ and 49–35% in insects exposed to LC₅₀. Sublethal doses of carbaryl, fipronil and imidacloprid showed significant effect on the reproduction of this insect. Mortality of SAPW populations caused by insecticides had similar effects in the laboratory and field conditions. The results suggest that carbaryl, deltamethrin, fipronil, and imidacloprid caused significantly higher mortality as compared to the control in SAPW and may be used to control its populations in oil palm trees where bud rot appears as the key disease for SAPW attraction and infestation.

Transgenerational hormetic effects of sublethal dose of flupyradifurone on the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae)

Both inhibitory and stimulatory (known as hormesis) effects of the sublethal flupyradifurone, a butenolide insecticide, on Myzus persicae Sulzer (Hemiptera: Aphididae) were investigated for incorporating it into integrated pest management (IPM). A leaf-dip bioassay showed that flupyradifurone was very toxic against adult M. persicae with a 48 h LC50 of 8.491 mg/L. Using the age-stage two-sex life table approach, we assessed the effects of LC25 of flupyradifurone on adult M. persicae and its progeny (F1 and F2). On the one hand, aphids exposed to flupyradifurone had significantly negative effects on the life history traits acrossing the generations, such as reduced the adult longevity and fecundity of F0, shortened the duration of third instar and fourth instar nymphs, preadult period and the pre-reproductive period of F1, and decreased the reproductive days and adult longevity of F2. On the other hand, stimulatory effects on the duration of pre-adult, adult reproductive days, and reproduction of F1 were observed in the flupyradifurone-treated aphids. Consistently with the stimulation on individual traits, a higher net reproductive rate (R0) of F1 and a shorter mean generation time (T) of F2 were observed in the flupyradifurone-treated aphids, although the other population parameters including the intrinsic rate of increase (r), finite rate of increase (λ) and T of F1 and R0, r and λ of F2 were not significantly affected. These results revealed that adult M. persicae exposed to sublethal concentration of flupyradifurone can induce hormetic effects on F1, and also cause negative effects on F2. Our results would be useful for assessing the overall effects of flupyradifurone on M. persicae and the hormetic effects should take into consideration when use flupyradifurone for control M. persicae.

Silencing of triazophos-induced Hexokinase-1-like reduces fecundity in Nilaparvata lugens (Stål) (Hemiptera: Delphacidae)

Hexokinase is a rate-limiting enzyme that plays pivotal roles in glucose homeostasis and energy metabolism via glucose (Glc) phosphorylation and Glc signaling mediation. Previous investigations have revealed the modulatory role of Hexokinase (Hex) genes involved in proper glucose regulation during insect diapause and embryo development, whereas whether it functions in insect fecundity remains largely unknown. We aimed to explore the relationship between Triazophos (TZP)-induced Hex-1 and fecundity of female Nilaparvata lugens. In this study, Hex-1 expression were characterized at different developmental stages and in various tissues of N. lugens, with the highest expression registered in brain tissues and 5th instar nymph. The present findings indicated that TZP + dsHex-1 silencing significantly reduced protein synthesis, including the fat body and ovarian protein content of female adults. Meanwhile, the glycometabolism with respect to the soluble sugar, trehalose and glucose content in female adults were strikingly influenced as a result of Hex-1 knockdown. The relative transcript level of Hex-1, vitellogenin (NlVg) and vitellogenin receptor (NlVgR) considerably decreased in TZP + dsHex-1 treated females compared to TZP and TZP + dsGFP-treated groups. More importantly, TZP + dsHex-1 silencing led to reduced number of eggs laid and vitellogenin (Vg) accumulation as well as retarded ovary development compared with TZP-treated and TZP + dsGFP-treated groups. Taken together, it is proposed that Hex-1 implicates in N. lugens fecundity by exerting profound effects on glycometabolism, protein sythesis and NlVg expression.

Sublethal Effects of Imidacloprid on the Population Development of Western Flower Thrips Frankliniella occidentalis (Thysanoptera: Thripidae)

The Western flower thrips (WFT, Frankliniella occidentalis) is a global polyphagous pest that is often dependent on chemical control. Imidacloprid has been a commonly used chemical insecticide for effective control of WFT. Low concentrations of insecticides can have sublethal effects on individual insects. However, no more information is known about the effects of exposure at low concentrations of imidacloprid on WFT. Here, we evaluated the effects of imidacloprid at sublethal concentrations on WFT population growth parameters. We first exposed the parental generation to LC₁₀ (56.8 mg/L) and LC₂₅ (79.2 mg/L) concentrations of imidacloprid. We then quantified various parameters related to the development, survival, and fecundity of the F₁ generation also exposed to these same concentrations. The development time of the treatment groups exposed to imidacloprid was significantly shorter than the control group, and the net reproductive rate (R₀) was significantly higher for treatment groups than for the control group. For both variables, there was no significant difference between LC₁₀ and LC₂₅ exposure. The generational survival rate was significantly higher for the control group, followed by the LC₁₀ treatment group and then the LC₂₅ treatment group. However, the opposite was true for fecundity and intrinsic rate of increase (r_m) of WFT. In summary, exposure to low concentrations of imidacloprid was positive for the population development of WFT, which may contribute to the development of insecticide resistance and cause resurgence in WFT populations.

Evaluation of Insecticides induced hormesis on the demographic parameters of Myzus persicae and expression changes of metabolic resistance detoxification genes

Insecticide induced-hormesis is a bi-phasic phenomenon generally characterized by low-dose induction and high-dose inhibition. It has been linked to insect pest outbreaks and insecticide resistance, which have importance in the integrated pest management (IPM). In this paper, hormesis effects of four insecticides on demographic parameters and expression of genes associated with metabolic resistance were evaluated in a field collected population of the green peach aphid, Myzus persicae Sulzer. The bioassay results showed that imidacloprid was more toxic than acetamiprid, deltamethrin and lambda-cyhalothrin. After exposure to sublethal doses of acetamiprid and imidacloprid for four generations, significant prolonged nymphal duration and increased fecundity were observed. Subsequently, mean generation time (T) and gross reproductive rate (GRR) was significantly increased. Moreover, expression of CYP6CY3 gene associated with resistance to neonicotinoids was increased significantly compared to the control. For pyrethriods, across generation exposure to sublethal doses of lambda cyhalothrin and deltamethrin prolonged the immature development duration. However, the expression of E4 gene in M. persicase was decreased by deltamethrin exposure but increased by lambda cyhalothrin. Based on results, demographic fitness parameters were effected by hormetic dose and accompanied with detoxifying genes alteration, hence, which would be evaluated in developing optimized insect pest management strategies.

Development and evaluation of pymetrozine controlled-release formulation to control paddy planthopper

Continuous outbreaks of rice planthoppers in rice-growing regions in China indicates the importance of redesigning several planthopper management programs. Chemical control remains the main strategy for planthopper control in China and other subtropical and temperate regions. Most common chemical insecticides are emulsifiable concentrates, suspension concentrates, soluble concentrates, and wettable powders. These insecticides are applied by dusting or spraying using simple equipment. The active ingredient, with short effectiveness time, is degraded rapidly in natural paddy ecosystems. Thus, repeated pesticide applications are required to control rice planthoppers. Altering the short-term effect formulation of pesticides to a long-acting formulation may be an alternative solution. A pymetrozine controlled-release granule (CRG; 1%) was developed by loading the pesticide on bentonite and coating the solid pesticide with resin. Analysis of pymetrozine release indicated that the 1% pymetrozine CRG release was more than 80% for 60 days. In the field trial screening, the 1% pymetrozine CRG showed a controlled effect of 61.96-78.87% at 48 days after CGR application. Application of 1% pymetrozine CRG at the recommended dosage and 1.5 times the recommended dosage resulted in terminal residues on brown rice below the maximum residue limit (0.1 mg kg-1) of China and Japan. Moreover, the pesticide granules showed low toxicity against all tested beneficial organisms in the environment. Pymetrozine CRG (1%) showed good controlled release and efficacy for controlling paddy planthoppers. The compound exhibited a low terminal residue and low toxicity against all tested beneficial organisms. Pymetrozine CRG (1%) showed great potential for field applications to control paddy planthoppers, because it overcame the rapid loss of biological function during treatment.

Comparative transcriptome analysis of Sogatella furcifera (Horváth) exposed to different insecticides

White-backed planthopper, Sogatella furcifera (Horváth) (Hemiptera: Delphacidae), one of the main agricultural insect pests in China, is resistant to a wide variety of insecticides. We used transcriptome analysis to compare the expression patterns of resistance- and stress-response genes in S. furcifera subjected to imidacloprid, deltamethrin, and triazophos stress, to determine the molecular mechanisms of resistance to these insecticides. A comparative analysis of gene expression under imidacloprid, deltamethrin, and triazophos stress revealed 1,123, 841, and 316 upregulated unigenes, respectively, compared to the control. These upregulated genes included seven P450s (two CYP2 clade, three CYP3 clade, and two CYP4 clade), one GST, one ABC transporter (ABCF), and seven Hsps (one 90 and six Hsp70s) under imidacloprid stress; one P450 (CYP3 clade), two ABC transporters (one ABCF and one ABCD), and one Hsp (Hsp90) under deltamethrin stress; one P450 (CYP3 clade) and one ABC transporter (ABCF) under triazophos stress. In addition, 80 genes were commonly upregulated in response to the three insecticide treatments, including laminin, larval cuticle protein, and fasciclin, which are associated with epidermal formation. These results provide a valuable resource for the molecular characterisation of insecticide action in S. furcifera, especially the molecular characteristics of insecticide cross resistance.

Sublethal effects of chlorfenapyr on the life table parameters, nutritional physiology and enzymatic properties of Bradysia odoriphaga (Diptera: Sciaridae)

Bradysia odoriphaga (Diptera: Sciaridae) is the major pest affecting Chinese chive production. Chlorfenapyr is a halogenated pyrrole-based pro-insecticide that is currently used to control insects and mites on a variety of crops. In the present study, fourth-instar larvae of B. odoriphaga were exposed to chlorfenapyr at LC₁, LC₂₀ and LC₅₀ concentrations. The developmental duration of the treated larvae was not significantly different, but fecundity was significantly increased in the LC₁ and LC₂₀ treatment groups compared with the control group. The population parameters of the LC₁ treatment group were increased significantly, whereas those of the LC₅₀ treatment group were reduced significantly compared with the control. The food consumption by larvae and pupal weight were significantly increased under the LC₁ treatment and decreased under the LC₅₀ treatment compared with the control. Moreover, chlorfenapyr decreased the lipid, carbohydrate and trehalose contents significantly, whereas the total protein content was increased compared with the control. Additionally, the activities of protease, lipase and trehalase were significantly decreased. Chlorfenapyr treatment for 24 h also induced the activities of glutathione S-transferase (GST), carboxylesterase (CarE) and O-demethylation. The results of this study suggest that low lethal concentrations of chlorfenapyr can affect oviposition, population development, the activities of digestion and detoxification enzymes, and nutrient accumulation in B. odoriphaga. This study provides valuable information for the assessment and rational application of chlorfenapyr for effective control of this pest.

PHF7, a novel male gene influences female fecundity and population growth in Nilaparvata lugens Stål (Hemiptera: Delphacidae)

PHF7 exhibits male-specific expression in early germ cells, germline stem cells and spermatogonia in insects, and its expression promotes spermatogenesis in germ cells when they are present in a male soma. However, the influence of male-specific PHF7 on female reproductive biology via mating remains unclear. Thus, we investigated the potential impacts of male PHF7, existed in seminal fluid of Nilaparvata lugens (NlPHF7), on fecundity and population growth via mating. Our results revealed that suppressing male NlPHF7 expression by RNAi led to decreases in body weight, soluble accessory gland protein content, arginine content, and reproductive organ development in males, resulting in significant reduction of oviposition periods and fecundity in females, and significant decrease in body weight, fat body and ovarian protein content, yeast-like symbionts abundance, ovarian development and vitellogenin gene expression in their female mating partners. Similarly, suppression of NlPHF7 expression in males mated with the control female reduced population growth and egg hatching rate, but did not influence gender ratio. We infer that NlPHF7 play a role important in stimulating female fecundity via mating. This study provides valuable information by identifying a potentially effective target gene for managing BPH population through RNAi.

Expression and RNA Interference of Ribosomal Protein L5 Gene in Nilaparvata lugens (Hemiptera: Delphacidae)

The ribosomal proteins play important roles in the growth and development of organisms. This study aimed to explore the function of NlRPL5 (GenBank KX379234), a ribosomal protein L5 gene, in the brown planthopper Nilaparvata lugens. The open reading frame of NlRPL5 was cloned from N. lugens based on a previous transcriptome analysis. The results revealed that the open reading frame of NlRPL5 is of 900 bp, encoding 299 amino acid residues. The reverse transcription quantitative PCR results suggested that the expression of NlRPL5 gene was stronger in gravid females, but was relatively low in nymphs, males, and newly emerged females. The expression level of NlRPL5 in the ovary was about twofolds of that in the head, thorax, or fat body. RNAi of dsNlRPL5 resulted in a significant reduction of mRNA levels, ∼50% decrease in comparison with the dsGFP control at day 6. Treatment of dsNlRPL5 significantly restricted the ovarian development, and decreased the number of eggs laid on the rice (Oryza sativa) plants. This study provided a new clue for further study on the function and regulation mechanism of NlRPL5 in N. lugens.

Sublethal Concentration of Beta-Cypermethrin Influences Fecundity and Mating Behavior of Carposina sasakii (Lepidoptera: Carposinidae) Adults

The purpose of this study was to evaluate the sublethal effects of the beta-cypermethrin on calling behavior and fecundity of a major fruit-boring pest of apple, Carposina sasakii Matsumura. The mating rate, fertility (total number of eggs laid per female), and adult longevity of adults were remarkably decreased as compared with that in control when the adults were exposed to 10% lethal concentrations (LC10) of beta-cypermethrin (LC10♀ × LC10♂), and the age-specific survival rate (lx) was also negatively affected by sublethal beta-cypermethrin especially for the LC10♀× LC10♂ mating combination. However, the age-specific fecundity (mx) was stimulated particularly in the combination of CK♀× LC10♂. Furthermore, the oviposition period was prolonged and the number of eggs was significantly increased for combinations of CK♀× LC10♂ and LC10♀× CK♂. In the mating experiments, males in control or LC10-beta-cypermethrin treatments preferred to mate with females in control. It might be because of lower calling rate of female survivors treated with sublethal beta-cypermethrin. Our data indicate that treatment of beta-cypermethrin had a sublethal effect on the development and production of C. sasakii, and their mating behavior changes in surviving adults that may contribute to assortative mating.

JNK signaling mediates wing form polymorphism in brown planthoppers (Nilaparvata lugens)

Wing polyphenism is considered to be an adaptive trade-off between migration (long winged forms) and reproduction (short winged forms), determined by various environmental conditions. The c-Jun NH2-terminal kinase (JNK) is crucial for the regulation of the activity of a number of transcription factors, and is activated under stress and environmental fluctuations where it functions in maintaining cell viability and proliferation. We used RNA interference and a pharmacological inhibitor of JNK to test the role of JNK signaling in regulating the wing dimorphism of the brown planthopper, Nilaparvata lugens. Silencing NlJNK increased the proportion of short winged female adults, reminiscent of the effect of silencing inhibitory components of the insulin-signaling pathway, such as NlAkt. However, silencing of the JNK-activated transcription factors NlJun and NlFos did not change the wing form ratio significantly, indicating that NlJNK may not act through NlJun and NlFos in mediating this process. In summary, JNK signaling may play a role in determining wing polymorphism in N. lugens females.

Chlorpyrifos-induced hormesis in insecticide-resistant and -susceptible Plutella xylostella under normal and high temperatures

Hormesis induced by insecticides at the dosage lower than what ostensibly directly causes death on insects was studied. This paper reports the effects of the in vivo application of varied concentrations of chlorpyrifos (CPF) on Plutella xylostella (DBM). The insecticide concentrations applied included 0.000025-2.5 mg l-1, which are far lower than LC1 (7.2 mg l-1), for the CPF-susceptable (Si) DBM, and 250 mg l-1 which is far below LC1 (1286 mg l-1), for the CPF-resistant (Rc) DBM, as well as LC10- and LC50-doses for both strains. Significant hormesis was found with the 'hermetic-CPFs', i.e., 0.0025 mg l-1 for Si DBM and 2.5 mg l-1 for Rc DBM, at the normal or high temperature either in a 24 h or under a long-term treatment. These doses of CPF significantly stimulated the development and increased the fecundity of Si and Rc DBM at 25°C with approximately 23.5-29.8% activity increase on acetylcholinesterase (AChE) and 30.5-91.3% increase on glutathione S-transferases (GSTs) at 25 or 38°C in 4-24 h. The enzymatic activities were significantly reduced by LC50-CPF at 25°C in vivo, but the inhibition was relieved significantly, if the insects were first subjected to a hormetic-CPF pretreatment. It was remarkable that the average rates of enzymatic activity increase were 67.5-76.6% for AChE and 366-546% for GSTs. Consequently, it was concluded that the hormesis on Si and Rc DBM could be induced by CPF doses far below LC1 at normal or high temperature in short- or long-term treatment. These findings might help to improve the current insect control practices in the field.

Impact of imidacloprid on new queens of imported fire ants, Solenopsis invicta (Hymenoptera: Formicidae)

Neonicotinoid insecticides are commonly used in managing pest insects, including the imported fire ant, Solenopsis invicta Buren. There is increasing evidence that neonicotinoid insecticides at sublethal concentrations have profound effects on social insects. However, the sublethal effect of neonicotinoids on S. invicta has never been investigated. In this study, the newly mated queens were fed with water containing 0.01 or 0.25 μg/ml imidacloprid. Imidacloprid at both concentrations did not cause any increase in queen mortality during the founding stage; however, it significantly reduced queens’ brood tending ability. In the 0.25 μg/ml imidacloprid treatment, the time to larval emergence was significantly delayed and no pupae or adult workers were produced. This study provides clear evidence that imidacloprid at sublethal concentrations has a significant detrimental impact on S. invicta queens and the development of incipient colonies.

Evaluation of Sublethal Effects of Sulfoxaflor on the Green Peach Aphid (Hemiptera: Aphididae) Using Life Table Parameters

The green peach aphid, Myzus persicae (Sulzer), is an important insect pest of many crops around the world. Pesticide-induced hormesis may be an alternative mechanism for pest resurgence. In this study, life table parameters were applied to the estimation of sulfoxaflor-induced hormesis of adult M. persicae following 2-d LC25 concentration exposure. Leaf-dip bioassays showed that the sulfoxaflor possessed high toxicity against M. persicae, with an LC50 of 0.059 mg/liter. The results indicated that the exposure of the parent generation of M. persicae to sublethal sulfoxaflor induced increase in reproduction and prolongation of immature development duration in the first progeny generation. Both R0 and GRR of aphids for treatment group were significantly higher than for the control in F1 generation, and the mean generation time was significantly postponed in treated group. These results suggest a hormesis induced by lower concentration of sulfoxaflor in M. persicae. It would be useful for assessing the overall effects of sulfoxaflor on M. persicae.

Status of insecticide resistance and selection for imidacloprid resistance in the ladybird beetle Propylaea japonica (Thunberg)

Field populations or strains of Propylaea japonica collected from four places in southern China (Guangzhou, Nanning, Guilin, and Yuxi) were tested for susceptibility to four insecticides (abamectin, imidacloprid, beta-cypermethrin, and chlorpyrifos) by the Petri-dish Potter tower method and compared with an insecticide-susceptible strain. Concentrations that proved lethal for 50% of the tested individuals (LC50) were estimated by probit analysis, and resistance factors (RF) were calculated at the LC50 level, which ranged from 1.6 to 10.1, depending on the insecticide. In addition, the Guangzhou strain formed the original population for imidacloprid resistance selection. After selection for 20 generations, the resistance had increased 39.3-fold. Fitness analysis in terms of such traits as fecundity, days to maturity, and survival showed that although both resistant and susceptible populations developed at comparable rates, the resistant strain was less fecund (it laid fewer eggs and a smaller proportion of those eggs hatched and resulted in adults), attaining a fitness score of only 0.56 relative to the susceptible strain. These observations suggest that it is possible to detect strains of P. japonica highly resistant to insecticides under laboratory conditions, and that resistance to imidacloprid carries considerable fitness costs to P. japonica. The study served to expand our understanding of the impact of imidacloprid resistance on biological parameters of P. japonica in more detail and to facilitate the deployment of natural enemies resistant to insecticides in integrated pest management.

Demonstration of an adaptive response to preconditioning Frankliniella occidentalis (Pergande) to sublethal doses of spinosad: a hormetic-dose response

Sublethal doses of some insecticides have been reported to either stimulate or reduce the survival and fecundity of insects. Many sublethal-effect studies have been conducted after exposure of only one generation to sublethal insecticides, and there is little information about the sublethal effects on insects after long-term exposure to sublethal insecticides. In this study, changes in biological characteristics were investigated in spinosad-susceptible (Spin-S) and sublethal-spinosad-treated (Spin-Sub) strains of Frankliniella occidentalis (Pergande) after exposure to their corresponding sublethal concentrations of spinosad. The results showed that for the Spin-S strain, the LC10 concentration of spinosad slightly affected the biotic fitness both in parents and offspring of F. occidentalis. The LC25 concentration of spinosad prolonged the development time, reduced the fecundity, and significantly reduced the intrinsic rate of increase, the net reproductive rate and the finite rate of increase in the Spin-S strain. However, the negative effects were not as pronounced in the offspring (F1 generation) as in the parent generation. For the Spin-Sub strain, the LC10 and LC25 concentrations of spinosad had little negative effect on the development and fecundity, and no significant difference was found between the effects of the LC10 and LC25 treatments on the Spin-Sub strain. The Spin-Sub strain exhibited a shorter developmental time, and larger intrinsic rates of increase and net reproductive rates, compared with the corresponding treatments of the Spin-S strain. These findings combined with our previous studies suggest that the biotic fitness increased in the Spin-Sub strain and the strain became more adaptable to sublethal doses of spinosad, compared with the Spin-S strain. Physiological and biochemical adaptation may contribute to these changes after long treatment times at sublethal doses.

Assessment of Sublethal and Transgenerational Effects of Pirimicarb on the Wheat Aphids Rhopalosiphum padi and Sitobion avenae

The wheat aphids, Rhopalosiphum padi (Linnaeus) and Sitobion avenae (Fabricius), are key pests on wheat crops worldwide. Management practices rely primarily on insecticides. The pirimicarb (carbamate) is used extensively as an effective insecticide to control these two aphids. In addition to the mortality caused by pirimicarb, various sublethal effects may occur in aphids when exposed to low lethal or sublethal doses. Understanding the general effect of pirimicarb on aphids could help increasing rational use of this insecticide. Under laboratory conditions, we assessed the sublethal effects of a low lethal concentration of pirimicarb (LC25) on biological traits and acetylcholinesterase (AChE) activity of R. padi and S. avenae. Both direct and transgenerational effects, i.e. on parent and the F1 generations were assessed, respectively. We found that R. padi and S. avenae responded differentially to the LC25 of pirimicarb. The parent generation of R. padi showed a 39% decrease in fecundity and multiple transgenerational effects were observed in the F1 generation; overall juvenile development, reproductive period, adult longevity and lifespan were longer than those of the control group. By contrast, LC25 of pirimicarb showed almost no effects on S. avenae biological traits in both the parent and F1 generations; only the pre-reproductive duration was reduced in F1 generations. Demographic parameter estimates (e.g. rm) showed similar trend, i.e. significant negative effect on R. padi population growth and no effect on S. avenae. However, AChE activity decreased in both R. padi and S. avenae treated by the LC25 of pirimicarb. We demonstrated sublethal and transgenerational effects of pirimicarb in the two wheat aphid species; it hinted at the importance of considering sublethal effects (including hormesis) of pirimicarb for optimizing Integrated Pest Management (IPM) of wheat aphids.

Sublethal and hormesis effects of imidacloprid on the soybean aphid Aphis glycines

The soybean aphid, Aphis glycines Matsumura, is a major pest in soybean crop. Current management of this pest relies mainly on insecticides applications, and the neonicotinoid imidacloprid has been proposed as an effective insecticide to control A. glycines in soybean field. Imidacloprid at lethal concentrations not only exerts acute toxicity to A. glycines, but also cause various biological changes when aphids are chronically exposed to lower concentrations. In this study, we assessed the effects of a low-lethal (0.20 mg L(-1)) and two sublethal (0.05 and 0.10 mg L(-1)) imidacloprid concentrations on various A. glycines life history traits. Aphid exposure to 0.20 mg L(-1) imidacloprid caused slower juvenile development, shorter reproductive period, and reduced adult longevity, fecundity and total lifespan. Stimulatory effects, i.e. hormesis, on reproduction and immature development duration were observed in aphids exposed to the lower sublethal imidacloprid concentrations. Consequently, the net reproduction rate (R 0) was significantly higher than in the control aphids. These findings stress the importance of the actual imidacloprid concentration in its toxicological properties on A. glycines. Therefore, our results would be useful for assessing the overall effects of imidacloprid on A. glycines and for optimizing integrated pest management programs targeting this pest.

The genetic basis of population fecundity prediction across multiple field populations of Nilaparvata lugens

Identifying the molecular markers for complex quantitative traits in natural populations promises to provide novel insight into genetic mechanisms of adaptation and to aid in forecasting population dynamics. In this study, we investigated single nucleotide polymorphisms (SNPs) using candidate gene approach from high- and low-fecundity populations of the brown planthopper (BPH) Nilaparvata lugens Stål (Hemiptera: Delphacidae) divergently selected for fecundity. We also tested whether the population fecundity can be predicted by a few SNPs. Seven genes (ACE, fizzy, HMGCR, LpR, Sxl, Vg and VgR) were inspected for SNPs in N. lugens, which is a serious insect pest of rice. By direct sequencing of the complementary DNA and promoter sequences of these candidate genes, 1033 SNPs were discovered within high- and low-fecundity BPH populations. A panel of 121 candidate SNPs were selected and genotyped in 215 individuals from 2 laboratory populations (HFP and LFP) and 3 field populations (GZP, SGP and ZSP). Prior to association tests, population structure and linkage disequilibrium (LD) among the 3 field populations were analysed. The association results showed that 7 SNPs were significantly associated with population fecundity in BPH. These significant SNPs were used for constructing general liner models with stepwise regression. The best predictive model was composed of 2 SNPs (ACE-862 and VgR-816 ) with very good fitting degree. We found that 29% of the phenotypic variation in fecundity could be accounted for by only two markers. Using two laboratory populations and a complete independent field population, the predictive accuracy was 84.35-92.39%. The predictive model provides an efficient molecular method to predict BPH fecundity of field populations and provides novel insights for insect population management.

Unexpected effects of low doses of a neonicotinoid insecticide on behavioral responses to sex pheromone in a pest insect

In moths, which include many agricultural pest species, males are attracted by female-emitted sex pheromones. Although integrated pest management strategies are increasingly developed, most insect pest treatments rely on widespread use of neurotoxic chemicals, including neonicotinoid insecticides. Residual accumulation of low concentrations of these insecticides in the environment is known to be harmful to beneficial insects such as honey bees. This environmental stress probably acts as an “info-disruptor” by modifying the chemical communication system, and therefore decreases chances of reproduction in target insects that largely rely on olfactory communication. However, low doses of pollutants could on the contrary induce adaptive processes in the olfactory pathway, thus enhancing reproduction. Here we tested the effects of acute oral treatments with different low doses of the neonicotinoid clothianidin on the behavioral responses to sex pheromone in the moth Agrotis ipsilon using wind tunnel experiments. We show that low doses of clothianidin induce a biphasic effect on pheromone-guided behavior. Surprisingly, we found a hormetic-like effect, improving orientation behavior at the LD20 dose corresponding to 10 ng clothianidin. On the contrary, a negative effect, disturbing orientation behavior, was elicited by a treatment with a dose below the LD0 dose corresponding to 0.25 ng clothianidin. No clothianidin effect was observed on behavioral responses to plant odor. Our results indicate that risk assessment has to include unexpected effects of residues on the life history traits of pest insects, which could then lead to their adaptation to environmental stress.

Increased long-flight activity triggered in beet armyworm by larval feeding on diet containing Cry1Ac protoxin

Evaluating ecological safety and conducting pest risk analysis for transgenic crops are vitally important before their commercial planting. The beet armyworm, Spodoptera exigua, a long-distance migratory insect pest, is not a direct target of transgenic Cry1Ac-expressing cotton in China, but nevertheless it has recently become an important pest. Migrants leaving their natal field arrive in other appropriate habitat far away in a short time, often followed by larval outbreaks. S. exigua has low susceptibility to Cry1Ac. However, our results from laboratory experiments identified (i) sublethal effects of Cry1Ac protoxin on larval development rate, larval and pupal weight, and adult lifetime fecundity, and (ii) increased long-flight behavior triggered by Cry1Ac which may contribute to larval outbreaks elsewhere. No significant differences in larval mortality, pupation rate, adult emergence rate, longevity, pre-oviposition period, or oviposition period were observed between controls and larvae fed on artificial diet incorporating a low concentration of Cry1Ac protoxin. The negative sublethal effects on some developmental and reproductive traits and lack of effect on others suggest they do not contribute to the observed severity of S. exigua outbreaks after feeding on Cry1Ac cotton. Interestingly, the percentage of long fliers increased significantly when larvae were reared on diet containing either of two low-dose treatments of Cry1Ac, suggesting a possible increased propensity to disperse long distances triggered by Cry1Ac. We hypothesize that negative effects on development and reproduction caused by Cry1Ac in the diet are offset by increased flight propensity triggered by the poor food conditions, thereby improving the chances of escaping adverse local conditions before oviposition. Increased long-flight propensity in turn may amplify the area damaged by outbreak populations. This phenomenon might be common in other migratory insect pests receiving sublethal doses of Bt toxins and warrants further study.

Imidacloprid as a contact arrestant for larvae of the European chafer, Amphimallon majale

BACKGROUND

Manipulative studies of the behavioral response of soil-dwelling insects to insecticides and other antagonists are stymied by the difficulties of observing and interpreting interactions played out below ground. Six experiments were carried out using X-ray radiography to quantify the movement of the European chafer, Amphimallon majale (Razoumowsky), larvae in response to imidacloprid and how this was affected by host plant cues and cold temperature.

RESULTS

The movement of third instars was arrested in imidacloprid-treated soil at ≥ 0.6 ppm concentration. At ≥ 0.8 ppm, the arrestant effect of imidacloprid was stronger than the attraction cue posed by germinating grass seed. There was a less disruptive effect on dispersal distance in vertical versus lateral panels. In vertical panels, there was a less disruptive effect on downward movement under a cold temperature treatment that simulated overwintering conditions.

CONCLUSION

Larvae of A. majale do not remotely detect imidacloprid in the soil; they neither evade contact, nor are repelled after contact. Imidacloprid thereby acts as a contact arrestant to disrupt grub movement. This finding might help to explain the synergistic effect of imidacloprid in combination with other biological agents for white grub control, and its effects on grub overwintering behavior.

Assessment of potential sublethal effects of various insecticides on key biological traits of the tobacco whitefly, Bemisia tabaci

The tobacco whitefly Bemisia tabaci is one of the most devastating pests worldwide. Current management of B. tabaci relies upon the frequent applications of insecticides. In addition to direct mortality by typical acute toxicity (lethal effect), insecticides may also impair various key biological traits of the exposed insects through physiological and behavioral sublethal effects. Identifying and characterizing such effects could be crucial for understanding the global effects of insecticides on the pest and therefore for optimizing its management in the crops. We assessed the effects of sublethal and low-lethal concentrations of four widely used insecticides on the fecundity, honeydew excretion and feeding behavior of B. tabaci adults. The probing activity of the whiteflies feeding on treated cotton seedlings was recorded by an Electrical Penetration Graph (EPG). The results showed that imidacloprid and bifenthrin caused a reduction in phloem feeding even at sublethal concentrations. In addition, the honeydew excretions and fecundity levels of adults feeding on leaf discs treated with these concentrations were significantly lower than the untreated ones. While, sublethal concentrations of chlorpyrifos and carbosulfan did not affect feeding behavior, honeydew excretion and fecundity of the whitefly. We demonstrated an antifeedant effect of the imidacloprid and bifenthrin on B. tabaci, whereas behavioral changes in adults feeding on leaves treated with chlorpyrifos and carbosulfan were more likely caused by the direct effects of the insecticides on the insects' nervous system itself. Our results show that aside from the lethal effect, the sublethal concentration of imidacloprid and bifenthrin impairs the phloem feeding, i.e. the most important feeding trait in a plant protection perspective. Indeed, this antifeedant property would give these insecticides potential to control insect pests indirectly. Therefore, the behavioral effects of sublethal concentrations of imidacloprid and bifenthrin may play an important role in the control of whitefly pests by increasing the toxicity persistence in treated crops.

Assessment of physiological sublethal effects of imidacloprid on the mirid bug Apolygus lucorum (Meyer-Dür)

Apolygus lucorum (Meyer-Dür) (Hemiptera: Miridae) is currently one of major mirid pests in the Yangtze River and the Yellow River regions in China. Imidacloprid (neonicotinoid) is widely used against pierce-sucking pest insects, including against A. lucorum. In addition to its direct lethal effect, multiple negative sublethal effects may also occur in exposed insects. We assessed potential sublethal effects of imidacloprid on some biological characteristics of A. lucorum with the aim of increasing rational use of imidacloprid against that cotton pest. The lethal toxicity of imidacloprid on adults of A. lucorum was determined in laboratory conditions by a topical application bioassay (LD(50) = 6.70 ng a.i. [active ingredient]/A. lucorum adult). We also estimated a sublethal dose, LD(5) (0.38 ng a.i./adult), a low lethal dose, LD(25) (1.96 ng a.i./adult), and moderate lethal dose, LD(40) (3.97 ng a.i./adult). The sublethal dose of imidacloprid (LD(5)) shortened the pre-oviposition period of females but increased the time required for eggs to develop (i.e. longer embryogenesis). The low lethal dose (LD(25)) also reduced the pre-oviposition period. Females exposed to the LD(40) laid eggs that developed faster but overall percentage of eggs hatching was reduced. LD(25) and LD(40) reduced longevity of males but not of females. In addition, the susceptibility to seven insecticides generally used on Chinese crops was not modified in A. lucorum previously exposed to the LD(25) of imidacloprid. Our results demonstrate sublethal effects of low doses of imidacloprid on A. lucorum (notably on pre-oviposition period and egg development) which may have an impact on population dynamics of this pest.

Insects, insecticides and hormesis: evidence and considerations for study

Insects are ubiquitous, crucial components of almost all terrestrial and fresh water ecosystems. In agricultural settings they are subjected to, intentionally or unintentionally, an array of synthetic pesticides and other chemical stressors. These ecological underpinnings, the amenability of insects to laboratory and field experiments, and our strong knowledgebase in insecticide toxicology, make the insect-insecticide model an excellent one to study many questions surrounding hormesis. Moreover, there is practical importance for agriculture with evidence of pest population growth being accelerated by insecticide hormesis. Nevertheless, insects have been underutilized in studies of hormesis. Where hormesis hypotheses have been tested, results clearly demonstrate stimulatory effects on multiple taxa as measured through several biological endpoints, both at individual and population levels. However, many basic questions are outstanding given the myriad of chemicals, responses, and ecological interactions that are likely to occur.

Pest insect olfaction in an insecticide-contaminated environment: info-disruption or hormesis effect

Most animals, including pest insects, live in an “odor world” and depend strongly on chemical stimuli to get information on their biotic and abiotic environment. Although integrated pest management strategies including the use of insect growth regulators (IGRs) are increasingly developed, most insect pest treatments rely on neurotoxic chemicals. These molecules are known to disrupt synaptic transmission, affecting therefore sensory systems. The wide-spread use of neurotoxic insecticides and the growing use of IGRs result in residual accumulation of low concentrations in the environment. These insecticide residues could act as an “info-disruptor” by modifying the chemical communication system, and therefore decrease chances of reproduction in target insects. However, residues can also induce a non-expected hormesis effect by enhancing reproduction abilities. Low insecticide doses might thus induce adaptive processes in the olfactory pathway of target insects, favoring the development of resistance. The effect of sublethal doses of insecticides has mainly been studied in beneficial insects such as honeybees. We review here what is known on the effects of sublethal doses of insecticides on the olfactory system of insect pests.

Sublethal effects of imidacloprid on Bemisia tabaci (Hemiptera: Aleyrodidae) under laboratory conditions

The sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), is one of the most important pests in tropical and subtropical agriculture and is a key pest in greenhouse production worldwide. Current management of B. tabaci relies upon frequent applications of insecticides. Insecticide use not only directly affects pest populations through acute toxicity but also has indirect (sublethal) effects on pest physiology or behavior. In this study, we described sublethal effects of imidacloprid on adult feeding, immature development, adult fecundity, and F1 development of B. tabaci. Honeydew excretion of adults feeding on leaves treated with LC20 and LC40 concentration was significantly lower than that on untreated leaf discs. Egg production of B. tabaci adults subject to LC20 and LC40 concentrations also was less than untreated individuals. Upon transfer to untreated leaves, honeydew excretion and egg production recovered well within 24 and 48 h, respectively. Exposure to LC20 and LC40 concentrations significantly affected developmental time of B. tabaci eggs and nymphs, whereas it did not affect adult molting rate. We did not find sublethal effects on longevity and fecundity of B. tabaci adults when exposed to LC90 and LC40 concentrations for 24 h, and on egg hatching rate, nymphal mortality, and molting rate of the subsequent F1 generation. Exposure to imidacloprid at LC40 concentration significantly decreased the number of females in the F1 generation. Imidacloprid negatively affects development and reproduction of exposed individuals, and sex ratio of subsequent (F1) generation of B. tabaci, which probably disrupts B. tabaci population dynamics, slows population increase, and reduces infestation levels. Therefore, it is necessary to consider potential impact from imidacloprid for integrated management of the pest.