Comparative studies on acetylcholinesterase characteristics between the aphids, Sitobion avenae and Rhopalosiphum padi

Termiticidal, biochemical, and morpho-histological effects of botanical based nanoemulsion against a subterranean termite, Odontotermes Formosanus Shiraki

Recently, the use of nanopesticides has shown significant efficacy in the control of many pests. However, the effect of nanopesticides, especially nanoemulsions, on suppressing termites, Odontotermes formosanus (Shiraki, 1909) (O. formosanus), has not been studied yet. Therefore, this study aimed to produce nanoemulsions of the essential oils of eucalyptus (Eucalyptus globulus Labill; E-EO) and nutmeg (Myristica fragrans Houtt; N-EO) to suppress O. formosanus. The analysis of eucalyptus nanoemulsion (E-NE) and nutmeg nanoemulsion (N-NE) was confirmed by using UV-Vis, dynamic light scattering, zeta potential, transmission electron microscopy, scanning electron microscopy, and energy dispersive spectroscopy. In addition, chemical analysis by Gas Chromatography with a mass spectrometer (GC-MS) exhibited the major constituents of E-NE and N-NE. The principal chemical components of E-NE included D-limonene, eucalyptol, 1,5-cyclooctadiene,3,4-dimethyl, benzene, and 1-methyl-3-(1 methylethyl)-, while the main constituents in N-NE were cyclohexane,1-methylene-4-(1 methylethenyl)-, eucalyptol, and L-. alpha. -terpineol. The mortality rates were 100% and 99.53%, respectively, after 24 hours of treatment with a concentration of 140 mg/mL, compared to 23.43% and 43.55%, respectively, from E-EO and N-EO treatment. These results refer to the essential oils' nanoemulsion as far more effective than the essential oils themselves. Furthermore, the effects of E-NE and N-NE on detoxification enzymes such as acetylcholinesterase, carboxylesterase, acid and alkaline phosphatase were investigated, as well as total protein concentrations, and the results have been found to be significantly increasing or decreasing in comparison with control. Besides, histological and morphological alterations found post exposure to E-NE and N-NE were shown. Overall, the results from this study clearly indicate that the nanopesticide-formulated nanoemulsions may have great potential to be used as novel, environmentally safe insecticides for controlling O. formosanus.

Cyperus rotundus L.: Invasive weed plant with insecticidal potential against Aphis craccivora Koch and Planococcus lilacinus (Cockerell)

Cyperus rotundus L. is a widely distributed invasive weed plant with vast traditional medicinal uses. Herein, the methanolic root extract of C. rotundus and its fractions (n-hexane, chloroform, n-butanol, and aqueous) were evaluated for insecticidal activity against nymphs of Aphis craccivora Koch and crawlers of Planococcus lilacinus (Cockerell) to find promising lead (s). In contact topical assay, among extract/fractions, n-hexane fraction exhibited more toxicity against A. craccivora (LD50 = 1.12 μg/insect) and P. lilacinus (LD50 = 0.94 μg/insect). The chemical analysis of n-hexane fraction revealed a volatile composition similar to that of the essential oil (EO) of C. rotundus roots. Hence, EO was extracted using water and deep eutectic solvents (DESs) as cosolvent, which revealed enhancement in EO yield (from 0.28 to 0.46% w/w) on implementing DESs. A total of 35 diverse volatile metabolites were identified in all EO samples, accounting for 85.0 to 91.8% of chemical composition, having cyperotundone, cyperene mustakone, isolongifolen-5-one, boronia butenal as major constituents. The EO obtained with DES-7 [choline chloride: ethylene glycol (1:4)] and DES-6 [choline chloride: lactic acid (1:3)] were found effective against A. craccivora (LD50 = 0.62-0.87 μg/insect) and P. lilacinus (LD50= 0.59-0.67 μg/insect) after 96 h. NMR analysis of EO revealed cyperotundone as a major compound, which was isolated along with cyperene and cyperene epoxide. All the molecules were found effective against P. lilacinus, whereas against A. craccivora cyperotundone, cyperene and cyperene epoxide showed promising toxicity (LD50 = 0.74-0.86 μg/insect). Extract/fractions, EO, and isolated molecules showed a significant reproductive inhibition rate of A. craccivora at higher concentrations. All the tested concentrations of cyperotundone showed significant inhibition of acetylcholinesterase (AChE) and glutathione-S-transferase (GST) in A. craccivora and P. lilacinus. Based upon the present study, C. rotundus can be recommended to control targeted insects in the greenhouse/field conditions after performing bio-efficacy and phytotoxicity studies.

Fitness Cost of the Field-Evolved Resistance to Sulfoxaflor and Multi-Insecticide Resistance of the Wheat Aphid Sitobion miscanthi (Takahashi)

Sulfoxaflor belongs to a new class of insecticides that is effective against many sap-feeding pests. In this study on Sitobion miscanthi (Takahashi) (i.e., the predominant wheat pest), a highly sulfoxaflor-resistant (SulR) population was obtained from a field. Its resistance to the other seven insecticides and its biological fitness were analyzed using a leaf-dip method and a two-sex life table approach, respectively. Compared with the relatively susceptible (SS) population, the SulR population was highly resistant to sulfoxaflor, with a relative insecticide resistance ratio (RR) of 199.8 and was moderately resistant to beta-cypermethrin (RR = 14.5) and bifenthrin (RR = 42.1) but exhibited low resistance to chlorpyrifos (RR = 5.7). Additionally, the SulR population had a relative fitness of 0.73, with a significantly prolonged developmental period as well as a lower survival rate and poorer reproductive performance than the SS population. In conclusion, our results suggest that S. miscanthi populations that are highly resistant to sulfoxaflor exist in the field. The possibility that insects may develop multi-resistance between sulfoxaflor and pyrethroids is a concern. Furthermore, the high sulfoxaflor resistance of S. miscanthi was accompanied by a considerable fitness cost. The study data may be useful for improving the rational use of insecticides and for exploring novel insecticide resistance mechanisms.

Effect of High Temperature and Natural Enemies on the Interspecies Competition Between Two Wheat Aphid Species, Rhopalosiphum padi and Sitobion miscanthi

Interspecies competition affects the distribution, quantity, and community structure of insects, especially among closely-related (congeners) species. Some ecological factors differentially affect the fitness of co-existing species, thus conferring an advantage on one competitor, and then the structure of communities. The present work evaluated the effects of high temperature and natural enemies on the interspecific competition between the grain aphid Sitobion miscanthi (Takahashi) (Aphididae: Hemiptera) and bird cherry-oat aphid Rhopalosiphum padi (L.) (Hemiptera: Aphididae), two key pests of wheat in China. Results showed that the population growth of R. padi was faster at 30°C, and the intrinsic rate of natural increase (rm) value was 5 times that of S. miscanthi, indicating that R. padi was more high-temperature resistant and has advantages in interspecific competition at high temperature. Moreover, compared to S. miscanthi, the population of R. padi was less affected by their predator, larvae of the multicolored Asian lady beetle, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), and aphid parasitoids, Aphidius avenae (Haliday) or Aphidius gifuensis (Ashmead) (Hymenoptera: Aphidiidae), which made them gain an advantage in the interspecific competition. Our results enrich the knowledge of phytophagous insect interspecific completion and implicate the ecological mechanism of R. padi may become the dominant species in wheat fields in China.

Chemical composition of essential oils from Thymus mongolicus, Cinnamomum verum, and Origanum vulgare and their acaricidal effects on Haemaphysalis longicornis (Acari: Ixodidae)

Chemical acaricides are mainly used in traditional tick control, which leads to the emergence of tick resistance and concurrently results in environmental pollution. In the present study, the chemical constituents of essential oils (EOs) from Thymus mongolicus, Cinnamomum verum, and Origanum vulgare was analyzed, and their potential application was evaluated to control the vector tick Haemaphysalis longicornis, which is widely distributed over vast areas of Eurasia, Australia, and New Zealand. Gas chromatography-mass spectrometry analysis revealed that the phenols thymol and carvacrol accounted for 34.66% and 75.72% of the EOs of T. mongolicus and O. vulgare, respectively, whereas trans-cinnamaldehyde (49.42%) was the main constituent of C. verum EO. Immersion tests showed that the EOs of C. verum and O. vulgare had significant acaricidal activity against larval H. longicornis, with the 50% lethal concentration (LC50) being 16.07 and 18.02 mg/mL, respectively, and the 95% lethal concentration (LC95) being 120.37 and 130.09 mg/mL, respectively. The EOs of O. vulgare and T. mongolicus showed significant acaricidal activity against unfed adult H. longicornis, with LC50 being 43.50 and 44.21 mg/mL, respectively, and LC95 being 113.66 and 137.99 mg/mL, respectively. The fumigant toxicity test showed significant acaricidal activity of the three EOs against both unfed and engorged nymphal and adult H. longicornis. Enzyme assays revealed that the EOs of both C. verum and O. vulgare significantly inhibited glutathione S-transferase activity (P < 0.05). In contrast, the activities of carboxylesterase and multifunction oxidases were significantly inhibited by EOs extracted from all three plants (P < 0.05). Taken together, these findings suggest that plant EOs may serve as an environment-friendly alternative for synthetic acaricides in future tick control.

Enhancement of Temephos and Deltamethrin Toxicity by Petroselinum crispum Oil and its Main Constituents Against Aedes aegypti (Diptera: Culicidae)

Previous work presented the profound antimosquito potential of Petroselinum crispum essential oil (PEO) against either the pyrethroid-susceptible or resistant strains of Aedes aegypti. This plant oil also inhibited the activity of acetylcholinesterase and mixed-function oxidases significantly, thus suggesting its potential as a synergist for improving mosquitocidal efficacy of insecticidal formulations. This study investigated the chemical composition, larvicidal activity, and potential synergism with synthetic insecticides of PEO and its main compounds for the purpose of interacting with insecticide resistance in mosquito vectors. The chemical profile of PEO, obtained by GC-MS analysis, showed a total of 17 bioactive compounds, accounting for 99.09% of the whole oil, with the most dominant constituents being thymol (74.57%), p-cymene (10.73%), and γ-terpinene (8.34%). All PEO constituents exhibited promising larvicidal effects, with LC50 values ranging from 19.47 to 59.75 ppm against Ae. aegypti, in both the pyrethroid-susceptible and resistant strains. Furthermore, combination-based bioassays revealed that PEO, thymol, p-cymene, and γ-terpinene enhanced the efficacy of temephos and deltamethrin significantly. The most effective synergist with temephos was PEO, which reduced LC50 values to 2.73, 4.94, and 3.28 ppb against MCM-S, PMD-R, and UPK-R, respectively, with synergism ratio (SR) values of 1.33, 1.38, and 2.12, respectively. The best synergist with deltamethrin also was PEO, which reduced LC50 values against MCM-S, PMD-R, and UPK-R to 0.008, 0.18, and 2.49 ppb, respectively, with SR values of 21.25, 9.00, and 4.06, respectively. This research promoted the potential for using essential oil and its principal constituents as not only alternative larvicides, but also attractive synergists for enhancing efficacy of existing conventional insecticides.

Field evolved resistance to pyrethroids, neonicotinoids, organophosphates and macrolides in Rhopalosiphum padi (Linnaeus) and Sitobion avenae (Fabricius) from China

Rhopalosiphum padi (Linnaeus) and Sitobion avenae (Fabricius) are the predominant pests coexisting on wheat plants. In this study, the susceptibilities of 29 R. padi and 30 S. avenae populations from 15 provinces in China to pyrethroids (beta-cypermethrin and bifenthrin), neonicotinoids (imidacloprid and thiamethoxam), organophosphates (omethoate and chlorpyrifos) and macrolides (avermectin) were determined during 2018-2019. The median lethal insecticide concentrations (LC₅₀) indicated that R. padi was more sensitive than S. avenae to most of the insecticides. Monitor results showed that most wheat aphid populations were moderately resistant to pyrethroids. Two R. padi populations were highly resistant to beta-cypermethrin with 127.3-fold and 442.8-fold resistance ratio (RR), and two were highly resistant to bifenthrin (RR of 293.9 and 320.6, respectively). One S. avenae population was highly resistant to beta-cypermethrin (RR of 136.8) and one was highly resistant to bifenthrin (RR of 313.4). All populations of two wheat aphids exhibited low to moderate resistance to neonicotinoids (RR < 100). But over half populations were sensitive or exhibited low resistance to organophosphates and macrolides. The pair-wise correlation coefficients for the insecticide LC₅₀ revealed a positive correlation between beta-cypermethrin and bifenthrin resistance, as well as between the resistance to bifenthrin and omethoate for the two-aphid species. Similarly, significant correlations were detected between the resistance to beta-cypermethrin and avermectin for R. padi. These results may be relevant for developing effective insecticide management strategies that prevent or delay the development of resistance among wheat aphids.

Susceptibility of Four Species of Aphids in Wheat to Seven Insecticides and Its Relationship to Detoxifying Enzymes

Sitobion avenae (Fabricius), Rhopalosiphum padi (Linnaeus), Schizaphis graminum (Rondani), and Metopolophium dirhodum (Walker) (Hemiptera: Aphididae) are important pests of wheat and other cereals worldwide. In this study, the susceptibilities of four wheat aphid species to seven insecticides were assessed. Furthermore, the activities of carboxylesterase (CarE), glutathione S-transferase (GSTs), and cytochrome P450 monooxygenase (P450s) were determined in imidacloprid treated and untreated aphids. The results showed that the susceptibilities of four wheat aphid species to tested insecticides are different and M. dirhodum has shown higher tolerance to most insecticides. Relatively higher CarE and GST activities were observed in M. dirhodum, and P450s activities increased significantly in response to imidacloprid treatment. Moreover, susceptibility to imidacloprid were increased by the oxidase inhibitor piperonyl butoxide in M. dirhodum (20-fold). The results we have obtained imply that P450s may play an important role in imidacloprid metabolic process in M. dirhodum. We suggest that a highly species-specific approach is essential for managing M. dirhodum.

Toxicity and physiological effects of ajwain (Carum copticum, Apiaceae) essential oil and its major constituents against Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

The effects of Ajwain essential oil and its major constituents were investigated against newly fourth instar larvae of Tuta absoluta, a major pest of tomato cropping in Iran and other parts of the world. The study included individual efficacy of each compound and determination of their synergistic or antagonistic efficacy. We also determined the activities of detoxifying enzymes in treated insects. According to the results, the Ajwain essential oil and thymol showed the highest toxicity against T. absoluta larvae (LC₅₀ = 6.14 and 7.72 μL/mL, respectively, and LC₃₀ = 3.05 and 3.57 μL/mL, respectively). The ɤ-terpinene and p-cymene had lesser toxicity compared to other compounds. The larvicidal potency increased by binary mixtures of active constituents, such as thymol + p-cymene and thymol + ɤ-terpinene. On the other hand, ɤ-terpinene + p-cymene mixture demonstrated antagonistic effect on the larvae. The esterases and glutathione S-transferases were significantly increased in all treatments. Finally, our results revealed a significant inhibition of AChE activity in the treated larvae by all treatments except for ɤ-terpinene + p-cymene. In vitro experiments representing AChE inhibition with IC₅₀ values were recorded 0.370, 0.457, 0.528, 1.094 and 1.323 μL/mL for thymol + p-cymene, thymol + ɤ-terpinene, thymol, ɤ-terpinene and p-cymene, respectively. Thymol demonstrated significant potential as a controlling agent of T. absoluta larvae, both larvicidal, compatible with other compounds with strong AChE inhibition properties. These finding could pave the way for development of new botanicals based on EO constituents which should be completed with preparation of an efficient formulation and field trials.

Propoxur resistance associated with insensitivity of acetylcholinesterase (AChE) in the housefly, Musca domestica (Diptera: Muscidae)

Two unique housefly strains, PSS and N-PRS (near-isogenic line with the PSS), were used to clarify the mechanisms associated with propoxur resistance in the housefly, Musca domestica. The propoxur-selected resistant (N-PRS) strain exhibited >1035-fold resistance to propoxur and 1.70-, 12.06-, 4.28-, 57.76-, and 57.54-fold cross-resistance to beta-cypermethrin, deltamethrin, bifenthrin, phoxim, and azamethiphos, respectively, compared to the susceptible (PSS) strain. We purified acetylcholinesterase (AChE) from the N-PRS and PSS strains using a procainamide affinity column and characterized the AChE. The sensitivity of AChE to propoxur based on the bimolecular rate constant (Ki) was approximately 100-fold higher in the PSS strain compared to the N-PRS strain. The cDNA encoding Mdace from both the N-PRS strain and the PSS strain were cloned and sequenced using RT-PCR. The cDNA was 2073 nucleotides long and encoded a protein of 691 amino acids. A total of four single nucleotide polymorphisms (SNPs), I162M, V260L, G342A, and F407Y, were present in the region of the active site of AChE from the N-PRS strain. The transcription level and DNA copy number of Mdace were significantly higher in the resistant strain than in the susceptible strain. These results indicated that mutations combined with the up-regulation of Mdace might be essential in the housefly resistance to propoxur.

Potential for insecticide-mediated shift in ecological dominance between two competing aphid species

Competition is a key structuring component of biological communities, which is affected by both biotic and abiotic environmental stressors. Among the latter, anthropic stressors and particularly pesticides are noteworthy due to their intrinsic toxicity and large use in agroecosystems. However this issue has been scarcely documented so far. In this context, we carried out experiments under laboratory conditions to evaluate stress imposed by the neonicotinoid insecticide imidacloprid on intra and interspecific competition among two major wheat pest aphids. The bird cherry-oat aphid Rhopalosiphum padi L. and the English grain aphid Sitobion avenae F. were subjected to competition on wheat seedlings under varying density combinations of both species and subjected or not to imidacloprid exposure. Intraspecific competition does take place without insecticide exposure, but so does interspecific competition between both aphid species with R. padi prevailing over S. avenae. Imidacloprid interfered with both intra and interspecific competition suppressing the former and even the latter for up to 14 days, but not afterwards when a shift in dominance takes place favoring S. avenae over R. padi, in contrast with the interspecific competition without imidacloprid exposure. These findings hinted that insecticides are indeed able to mediate species interaction and competition influencing community structure and raising management concerns for favoring potential secondary pest outbreaks.

Inheritance and stability of mevinphos-resistance in Plutella xylostella (L.), with special reference to mutations of acetylcholinesterase 1

Diamondback moth (Plutella xylostella L.) causes enormous damage on cruciferous vegetables and can rapidly develop resistance to all kinds of insecticides. To effectively manage the insecticide resistance of P. xylostella, an understanding of its inheritance and stability is essential. Here we investigated the phenotypic and genotypic basis of mevinphos resistance by crossing two genetically pure lines of P. xylostella, an SH_ggt wild-type strain and an SHM_TCN resistant strain carrying 892T/T, 971C/C, and 1156T/G (TCN) mutations of the acetylcholinesterase 1 gene (Pxace1). Similar median lethal concentrations and degrees of dominance in the reciprocal cross progeny, and no plateau on the log concentration-probit line of F1 backcross and self-cross progeny, suggest that the mevinphos-resistance in P. xylostella is inherited as an autosomal and incomplete dominant trait governed by more than one gene. In the absence of mevinphos exposure, the resistance ratio and Pxace1 mutation frequency declined concomitantly in the SHM_TCN strain. After 20-generation relaxation, the mevinphos resistance decreased from 52- to 6-fold and the Pxace1 mutation frequency of the TCN haplotype pair decreased from 100% to 0%. A good correlation was found between the resistance ratio and TCN frequency within the range of 12.5- to 25-fold resistance. Since there was no TCN haplotype pair detected below a resistance level of 12.5-fold, we speculate that resistance mechanisms other than target site insensitivity may exist. These observations are important for the prediction and management of mevinphos and related organophosphate resistance in field populations of P. xylostella.

TPS46, a Rice Terpene Synthase Conferring Natural Resistance to Bird Cherry-Oat Aphid, Rhopalosiphum padi (Linnaeus)

Plant terpene synthases (TPSs) are key enzymes responsible for terpene biosynthesis, and can play important roles in defense against herbivore attack. In rice, the protein sequence of TPS46 was most closely related to maize TPS10. However, unlike maize tps10, tps46 was also constitutively expressed in rice even in the absence of herbivore attack. Potential roles or constitutive emissions of specific volatiles may due to the constitutive expressions of tps46 in rice. Therefore, in the present study, RNA interference (Ri) and overexpression (Oe) rice lines were generated to investigate the potential function of TPS46 in Oryza sativa sp. japonica. Interestingly, the rice plants become more susceptible to Rhopalosiphum padi when expression of tps46 was silenced compared with Wt in greenhouse conditions. Artificial infestation bioassays further confirmed that Ri rice lines were susceptible to R. padi, whereas Oe rice lines were repellent to R. padi. Based on GC-MS and ToF-MS analysis, a total of eight volatile products catalyzed by TPS46 in rice were identified. Among them, only limonene and Eβf could be detected in all the Ri, Oe, and Wt lines, whereas other six volatiles were only found in the blend of volatiles from Oe lines. Moreover, the amount of constitutive limonene and Eβf in the Ri lines was significantly lower than in Wt lines, while the amounts of these two volatiles in the Oe line were obviously higher than in control rice. Our data suggested that the constitutive emissions of Eβf and limonene regulated by the constitutive expression of tps46 may play a crucial role in rice defense against R. padi. Consequently, tps46 could be a potential target gene to be employed for improving the resistance of plants to aphids.

Cloning, expression, and functional analysis of two acetylcholinesterase genes in Spodoptera litura (Lepidoptera: Noctuidae)

Two acetylcholinesterase genes (SlAce1 and SlAce2) were cloned from Spodoptera litura, which is an important pest that causes widespread economic damage to vegetables and ornamental plants. We analyzed their expression patterns and compared their biological functions by using RNA interference. Our results showed that SlAce1 and SlAce2 cDNA contains 2085bp and 1917bp nucleotides and encoding proteins of 694 and 638 amino acid residues, respectively. Phylogenic analysis indicated that the lineage of SlAce genes and SlAce1 was completely different from SlAce2. Although both genes were expressed in all developmental stages and majorly in the brain. The expression levels of the both genes were suppressed by inserting their related dsRNA in the 6th instar larvae, which led to 47.3% (SlAce1) and 37.9% (SlAce2) mortality. Interestingly, the suppression of the SlAce2 transcripts also led to significant reductions in the fecundity, hatching, and offspring in the parental generation of S. litura. It is concluded that SlAce2 is responsible for the hydrolysis of acetylcholine and also plays role in female breeding, embryo progress, and the development of progeny. Considerable larval mortality was observed after both AChE genes (i.e. Ace1 and Ace2) were silenced in S. litura confirms its insecticidal effectiveness, which provided a molecular basis in biological pest control approach.

Sublethal effects of imidacloprid on the fecundity, longevity, and enzyme activity of Sitobion avenae (Fabricius) and Rhopalosiphum padi (Linnaeus)

The aphid species Sitobion avenae and Rhopalosiphum padi are the most important pests in wheat growing regions of many countries. In this study, we investigated the sublethal effects of imidacloprid on fecundity, longevity, and enzyme activity in both aphid species by comparing 3-h exposure for one or three generations. Our results indicated that 3-h exposure to sublethal doses of imidacloprid for one generation had no discernible effect on the survival, fecundity, longevity, or enzyme activity levels of aphids. However, when pulse exposures to imidacloprid were sustained over three generations, both fecundity and longevity were significantly decreased in both S. avenae and R. padi. Interestingly, the fecundity of R. padi had almost recovered by the F5 generation, but its longevity was still deleteriously affected. These results indicated that R. padi laid eggs in shorter time lags and has a more fast resilience. The change in reproduction behavior may be a phenomenon of R. padi to compensate its early death. If this is stable for the next generation, it means that the next generation is more competitive than unexposed populations, which could be the reason underlying population outbreaks that occur after longer-term exposure to an insecticide. This laboratory-based study highlights the sublethal effects of imidacloprid on the longevity and fecundity of descendants and provides an empirical basis from which to consider management decisions for chemical control in the field.

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.

Gene silencing of two acetylcholinesterases reveals their cholinergic and non-cholinergic functions in Rhopalosiphum padi and Sitobion avenae

BACKGROUD

The function of acetylcholinesterase (AChE) is to terminate synaptic transmission by hydrolysing the neurotransmitter acetylcholine (ACh) in the synaptic cleft, and thus it is an effective target for organophosphate (OP) and carbamate (CB) insecticides.

RESULTS

The transcript levels of the four Ace genes were dramatically suppressed by injection of their respective dsRNA in Rhopalosiphum padi and Sitobion avenae. However, the AChE activity changes in the Ace1 knockdown aphids were consistent with the significant transcript level changes of Ace1 genes in these aphids, but not for Ace2. Bioassay results indicated that the suppression of RpAce1 increased its susceptibilities to pirimicarb and malathion, and SaAce1 silencing also increased susceptibility to pirimicarb in S. avenae, whereas the knockdowns of RpAce2 and SaAce2 had a slight effect on their susceptibilities. The knockdown of Ace1 genes also caused significant reductions in fecundity in the aphids of their parental generation.

CONCLUSIONS

These results suggest that AChE1 is a predominant cholinergic enzyme and is the target of anticholinesterase insecticides in both R. padi and S. avenae. It also plays a non-cholinergic role in fecundity of these aphids. AChE2 may also be important for the toxicological function, although its importance appeared to be lower than that of AChE1.

Genetic diversity of Sitobion avenae (Homoptera: Aphididae) populations from different geographic regions in China

Sitobion avenae is a major agricultural pest of wheat in China. Using microsatellite markers, we studied the potential gene flow, genetic diversity, genetic differentiation, and genetic structure of seven S. avenae populations from different regions of China (Beijing, Hebei, Henan, Hubei, Jiangsu, Shandong, and Shanxi provinces). The populations from Henan, Shandong, and Jiangsu showed high levels of genic and genotypic diversity. By contrast, the genic diversity in the Beijing and Hebei populations was much lower. Despite this low genic diversity, the genotypic diversity of the Beijing population was higher than that of all of the other populations, except those from Jiangsu and Shandong. Overall, the genetic divergence among the seven S. avenae populations tested was high, though there was almost no differentiation between the Shandong and Henan populations. We observed significant negative correlation between the strength of gene flow and the geographic distances among populations. Based on genetic analysis, the seven S. avenae populations studied can be divided into four distinct clusters; (i) Hubei, (ii) Shanxi, (iii) Beijing and Hebei, and (iv) Shandong, Henan, and Jiangsu. The present results provide a basis for potentially optimizing integrated pest management (IPM) programs in China, through adapting control methods that target biological traits shared by various populations of the same genotype.

Amino acid substitutions and intron polymorphism of acetylcholinesterase1 associated with mevinphos resistance in diamondback moth, Plutella xylostella (L.)

The diamondback moth, Plutella xylostella L., is the most destructive insect pest of Brassica crops in the world. It has developed resistance rapidly to almost every insecticide used for its control. Mevinphos, a fast degrading and slow resistance evocating organophosphorus insecticide, has been recommended for controlling P. xylostella in Taiwan for more than 40years. SHM strain of P. xylostella, with ca. 22-fold resistance to this chemical, has been established from a field SH strain by selecting with mevinphos since 1997. Three mutations, i.e., G892T, G971C, and T1156T/G leading to A298S, G324A, and F386F/V amino acid substitutions in acetylcholinesterase1 (AChE1), were identified in these two strains; along with three haplotype pairs and a polymorphic intron in AChE1 gene (ace1). Two genetically pure lines, i.e., an SHggt wild type with intron AS and an SHMTCN mutant carrying G892T, G971C, T1156T/G mutations and intron AR in ace1, were established by single pair mating and haplotype determination. The F1 of SHMTCN strain had 52-fold resistance to mevinphos in comparison with the F1 of SHggt strain. In addition, AChE1 of this SHMTCN population, which exhibited lower maximum velocity (Vmax) and affinity (Km), was less susceptible to the inhibition of mevinphos, with an I50 32-fold higher than that of the SHggt F1 population. These results imply that amino acid substitutions in AChE1 of SHMTCN strain are associated with mevinphos resistance in this insect pest, and this finding is important for insecticide resistance management of P. xylostella in the field.