Effects of host plants on insecticide susceptibility and carboxylesterase activity in Bemisia tabaci biotype B and greenhouse whitefly, Trialeurodes vaporariorum

Variation in Pesticide Toxicity in the Western Honey Bee (Apis mellifera) Associated with Consuming Phytochemically Different Monofloral Honeys

Insecticide toxicity to insect herbivores has long been known to vary across different host plants; this phenomenon has been widely documented in both foliage-feeders and sap-feeders. Species-specific phytochemical content of hostplant tissues is assumed to determine the pattern of induction of insect enzymes that detoxify insecticides, but specific phytochemicals have rarely been linked to host plant-associated variation in pesticide toxicity. Moreover, no studies to date have examined the effects of nectar source identity and phytochemical composition on the toxicity of insecticides to pollinators. In this study, we compared LD50 values for the insecticide bifenthrin, a frequent contaminant of nectar and pollen in agroecosystems, in the western honey bee, Apis mellifera, consuming three phytochemically different monofloral honeys: Nyssa ogeche (tupelo), Robinia pseudoacacia (black locust), and Fagopyrum esculentum (buckwheat). We found that bifenthrin toxicity (LD50) values for honey bees across different honey diets is linked to their species-specific phytochemical content. The profiles of phenolic acids and flavonoids of buckwheat and locust honeys are richer than is the profile of tupelo honey, with buckwheat honey containing the highest total content of phytochemicals and associated with the highest bifenthrin LD50 in honey bees. The vector fitting in the ordination analysis revealed positive correlations between LD50 values and two honey phytochemical richness estimates, Chao1 and Abundance-based Coverage Estimator (ACE). These findings suggest unequal effects among different phytochemicals, consistent with the interpretation that certain compounds, including ones that are rare, may have a more pronounced effect in mitigating pesticide toxicity.

Long-term impact of rearing substrates on susceptibility to insecticides and metabolic enzyme activities in the house fly Musca domestica

Musca domestica Linnaeus is a devastating insect pest of medical and veterinary importance with reports of resistance development to commonly used insecticides worldwide. Rearing substrates usually play a crucial role in determining susceptibility to insecticides and control of insect pests. The aim of the present study was to investigate the effect of five rearing substrates of M. domestica on its susceptibility to different insecticides and activities of metabolic enzymes. After 30 generations of rearing, susceptibility of M. domestica to tested insecticides, viz., malathion, pirimiphos-methyl, alpha-cypermethrin, deltamethrin, methomyl, propoxur, spinetoram, and chlorfenapyr had evident differences. Musca domestica reared on hen liver exhibited reduced susceptibility to all insecticides followed by the strain reared on poultry manure. However, M. domestica reared on milk-based diet showed the highest susceptibility to tested insecticides followed by the strain reared on manures of buffalo and horse. In addition, M. domestica reared on different substrates exhibited significant differences (p < 0.01) in the activities of glutathione S-transferase (GST), cytochrome P450-dependent monooxygenase, and carboxylesterase (CarE). Overall, hen liver and poultry manure strains exhibited higher activities of metabolic enzymes than those of the milk-based diet, buffalo, and horse manure strains. In conclusion, the data of the present study exhibited a significant effect of rearing substrates on the susceptibility to insecticides and activities of metabolic enzymes in M. domestica. These results could be helpful for the sustainable management of M. domestica on different hosts by selecting appropriate insecticides.

The host plant strongly modulates acaricide resistance levels to mitochondrial complex II inhibitors in a multi-resistant field population of Tetranychus urticae

The two-spotted spider mite Tetranychus urticae is a polyphagous pest with an extraordinary ability to develop acaricide resistance. Here, we characterize the resistance mechanisms in a T. urticae population (VR-BE) collected from a Belgian tomato greenhouse, where the grower was unsuccessful in chemically controlling the mite population resulting in crop loss. Upon arrival in the laboratory, the VR-BE population was established both on bean and tomato plants as hosts. Toxicity bioassays on both populations confirmed that the population was highly multi-resistant, recording resistance to 12 out of 13 compounds tested from various mode of action groups. DNA sequencing revealed the presence of multiple target-site resistance mutations, but these could not explain resistance to all compounds. In addition, striking differences in toxicity for six acaricides were observed between the populations on bean and tomato. The highest difference was recorded for the complex II inhibitors cyenopyrafen and cyflumetofen, which were 4.4 and 3.3-fold less toxic for VR-BE mites on tomato versus bean. PBO synergism bioassays suggested increased P450 based detoxification contribute to the host-dependent toxicity. Given the involvement of increased detoxification, we subsequently determined genome-wide gene expression levels of VR-BE on both hosts, in comparison to a reference susceptible population, revealing overexpression of a large set of detoxification genes in VR-BE on both hosts compared to the reference. In addition, a number of mainly detoxification genes with higher expression in VR-BE on tomato compared to bean was identified, including several cytochrome P450s. Together, our work suggests that multi-resistant field populations can accumulate a striking number of target-site resistance mutations. We also show that the host plant can have a profound effect on the P450-associated resistance levels to cyenopyrafen and cyflumetofen.

Differential insecticide resistance in Bemisia tabaci (Hemiptera: Aleyrodidae) field populations in the Punjab Province of Pakistan

The cotton whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) has a propensity for developing high-level resistance to insecticides. Management of B. tabaci in cotton grown in Pakistan depends on insecticide use, resistance monitoring has become essential to minimize the development of resistance. In this study, resistance was monitored in adult whiteflies collected from cotton fields in the Bahawalpur, Faisalabad, Lodhran, Multan, and Vehari districts of the Punjab Province, Pakistan during 2017, 2018, and 2019. Resistance monitoring was carried out for two insect growth regulators (pyriproxyfen and buprofezin) four neonicotinoids acetamiprid, imidacloprid, thiamethoxam, thiacloprid, and the historically used pyrethroid, bifenthrin and organophosphate, chlorpyrifos. Results based on resistance ratio (RR) showed that moderate to high level of resistance against noenicitinoids insecticides have been observed in all four districts while whiteflies exhibited very low to low resistance to pyriproxyfen and buprofezin. The RRs for acetamiprid, imidacloprid, thiamethoxam, thiacloprid varied from 7.60 to 50.99, 19.32 to 65.72, 17.18 to 54.65 and 6.49-47.49-fold, respectively. Bifenthrin and chlorpyrifos showed very low toxicity against whiteflies in all districts except Faisalabad, with RRs of 12.28-50.56-fold and 7.94-26.24-fold, respectively. The results will facilitate 'smart' selection and guide rates of insecticide applications for whitefly management in cotton for effective whitefly management while also delaying the development of resistance.

Transcriptomic profile of the predatory mite Amblyseius swirskii (Acari: Phytoseiidae) on different host plants

Amblyseius swirskii Athias-Henriot (Acari: Phytoseiidae) is a predatory mite, effective at controlling whiteflies and thrips in protected crops. However, on tomato its efficacy as a biocontrol agent is hindered, most probably by the plant trichomes and their exudates. Our aim was to characterize the response of A. swirskii to the tomato trichome exudates and identify three major detoxification gene sets in this species: cytochromes P450 (CYPs), glutathione S-transferases (GSTs) and carboxyl/cholinesterases (CCEs). Mites were exposed separately to tomato and pepper, a favourable host plant for A. swirskii, after which their transcriptional responses were analysed and compared. The de novo transcriptome assembly resulted in 71,336 unigenes with 66.1% of them annotated. Thirty-nine A. swirskii genes were differentially expressed after transfer on tomato leaves when compared to pepper leaves; some of the expressed genes were associated with the metabolism of tomato exudates. Our results illustrate that the detoxification gene sets CYPs, GSTs and CCEs are abundant in A. swirskii, but do not play a significant role when in contact with the tomato exudates.

Plant flavonoids enhance the tolerance to thiamethoxam and flupyradifurone in whitefly Bemisia tabaci (Hemiptera: Aleyrodidae)

The sweetpotato whitefly Bemisia tabaci is a polyphagous crop pest distributed worldwide and frequent exposure to many different defensive secondary metabolites in its host plants. To counteract these defensive plant secondary metabolites, B. tabaci elevate their production of detoxification enzymes, including cytochrome P450 monooxygenases. Besides their tolerance to phytotoxin, B. tabaci have quickly developed resistance to various insecticides in the field. However, the relationship between host plant secondary metabolites and insecticide resistance in B. tabaci is not fully understood. In this study, the influence of plant flavonoid ingestion on B. tabaci tolerance to thiamethoxam and flupyradifurone insecticides and its possible mechanism were examined. Eight plant flavonoids were screened to evaluate their effects on B. tabaci adult sensitivity to thiamethoxam and flupyradifurone. Of which rutin, quercetin, kaempferol, myricetin and catechin significantly reduced adult sensitivity to thiamethoxam and flupyradifurone. Application of cytochrome P450 inhibitor piperonyl butoxide significantly increased the mortality of B. tabaci adults treated with thiamethoxam and flupyradifurone. Moreover, flavonoid ingestion predominantly enhanced the activity of cytochrome P450 enzyme in B. tabaci adults. Meanwhile, the expression level of three cytochrome P450 genes, CYP6CM1, CYP6CX4 and CYP4C64 were induced by the flavonoids in B. tabaci adults. In conclusion, plant flavonoids enhanced the tolerance to thiamethoxam and flupyradifurone in B. tabaci and cytochrome P450s may contribute the flavonoid adaptation. The reduced sensitivity of thiamethoxam and flupyradifurone in flavonoid-fed B. tabaci adults suggested that previous exposure to the host plant-derived flavonoids is likely to compromise the efficacy of insecticides.

Micronutrient Fertilization of Greenhouse Cucumbers Mitigates Pirimicarb Resistance in Aphis gossypii (Hemiptera: Aphididae)

The nutritional status of host plants can have direct impacts on herbivore physiology and insect-plant interactions. We investigated the effect of micronutrients, including manganese, iron, zinc, and copper, on cucumber plant physiology, and on the biology and physiology of a strain of Aphis gossypii Glover selected over 12 generations to be resistant to pirimicarb. The micronutrient treatment increased the activity of superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase in cucumber plants, and also increased levels of total phenolics, hydrogen peroxide, salicylic acid, and total chlorophyl, whereas malondialdehyde levels were unaffected. Pirimicarb-resistant cotton aphids that fed on micronutritient-amended cucumber plants expressed significantly decreased levels of acetylcholinesterase and detoxifying enzymes, specifically glutathione S-transferase, and carboxylesterase. Analysis of energy reserves in resistant A. gossypii fed on micronutritient-amended plants revealed decreases in the lipid and protein contents of aphids, whereas glycogen and carbohydrate contents showed no response. Resistant cotton aphids fed on micronutritient-amended plants showed significantly reduced fecundity, longevity, and reproductive periods, and a 1.7-fold reduction in pirimicarb LC50 compared with those fed on control plants. We conclude that micronutrient amendment negatively impacts the biological performance of insecticide-resistant cotton aphids, and diminishes their resistance to pirimicarb. Both direct effects on plant health, such as enhanced inducible defenses, and indirect effects on aphid fitness, such as reduced biological performance and detoxification abilities, were implicated. Therefore, optimization of micronutrient amendments could be a useful complement to other tactics for managing insecticide-resistant A. gossypii on cucumbers, and warrants exploration in other contexts.

Physiological responses of plants and mites to salicylic acid improve the efficacy of spirodiclofen for controlling Tetranychus urticae (Acari: Tetranychidae) on greenhouse tomatoes

Salicylic acid (SA) is a signaling molecule that can induce plant resistance to certain herbivores. Although the role of jasmonic acid in mediating mite-tomato plant interactions has been well studied, the role of salicylic acid has not. This study examined how the application of exogenous SA, via its effects on tomato plant physiology, alters the activity of mite digestive enzymes, mite energy reserves, and mite susceptibility to spirodiclofen. Enzymatic activity-including superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase-along with contents of total phenolic, hydrogen peroxide, and total chlorophyll significantly increased in plants 24 h after treatment with 2 mM of SA. In contrast, catalase activity significantly decreased in treated plants, and malondialdehyde content was unaffected. Mites fed on tomato plants treated with SA had significantly lower glutathione S-transferase, esterase, α-amylase, and aminopeptidase activities than those fed on control plants. Energy reserve analyses demonstrated a significant decrease in contents of lipid, protein, and glycogen in mites fed on SA-treated plants, whereas carbohydrate content significantly increased. The LC₅₀ of spirodiclofen was decreased 1.8-fold for Tetranychus urticae fed on SA-treated tomato plants compared to controls. Treatment of adult mites with 2 mM SA on leaf discs did not cause any direct mortality after 24 h. Finally, a greenhouse bioassay confirmed that spider mite mortality following exposure to spirodiclofen was significantly higher on SA plants than on control plants. Mortality of mites exposed to half of the recommended rate of spirodiclofen was similar to those exposed to the recommended rate when they were held on treated plants. These results have valuable implications for T. urticae management programs in tomato production.

Nitric Oxide Boosts Bemisia tabaci Performance Through the Suppression of Jasmonic Acid Signaling Pathway in Tobacco Plants

The intimate relationships between plants and insects start with herbivory, which can be traced back to approximately 420 million year ago. Like many other relationships, a plant–insect interaction can be mutualistic, commensalistic, or antagonistic. Within antagonistic relationships, plants deploy inducible defense to insect phytophagy. Insects, however, can evade/suppress effectual plant defenses by manipulating plant defense signaling. Previously, we showed that the sweet potato whitefly, Bemisia tabaci, a global invasive insect pest, can suppress jasmonic acid (JA)-dependent defenses, thereby enhancing their performance on host plants. Given that nitric oxide (NO), a multifunctional signaling molecule, interacts closely with JA signaling pathway, we hypothesized that NO is involved in the suppression of JA defensive responses. Equipped with an integrated approach, we comprehensively examined this overarching hypothesis. We showed that: (1) tobacco plants responded to B. tabaci infestation by accumulating high levels of NO, (2) the exogenous application of sodium nitroprusside, a NO donor, in tobacco plants attracted B. tabaci adults and accelerated nymphal development, whereas plants treated with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a NO scavenger, repelled B. tabaci adults and prolonged nymphal development, and, more importantly, (3) silencing of NO-associated protein 1, a gene associated with NO accumulation, and cPTIO application disrupted the B. tabaci-mediated suppression of JA in plants. Collectively, these results suggest that: (1) NO signaling is activated by B. tabaci infestation, (2) NO is involved in the suppression of JA-dependent plant defense, and, consequently, (3) NO improves B. tabaci performance on host plants. Our study reflects the remarkable arm race that co-evolved for millions of years between plants and insects and offers a potential novel target (nitric oxide) for the long-term sustainable management of this global invasive pest.

Host plant adaptation in the polyphagous whitefly, Trialeurodes vaporariorum, is associated with transcriptional plasticity and altered sensitivity to insecticides

BACKGROUND

The glasshouse whitefly, Trialeurodes vaporariorum, is a damaging crop pest and an invasive generalist capable of feeding on a broad range of host plants. As such this species has evolved mechanisms to circumvent the wide spectrum of anti-herbivore allelochemicals produced by its host range. T. vaporariorum has also demonstrated a remarkable ability to evolve resistance to many of the synthetic insecticides used for control.

RESULTS

To gain insight into the molecular mechanisms that underpin the polyphagy of T. vaporariorum and its resistance to natural and synthetic xenobiotics, we sequenced and assembled a reference genome for this species. Curation of genes putatively involved in the detoxification of natural and synthetic xenobiotics revealed a marked reduction in specific gene families between this species and another generalist whitefly, Bemisia tabaci. Transcriptome profiling of T. vaporariorum upon transfer to a range of different host plants revealed profound differences in the transcriptional response to more or less challenging hosts. Large scale changes in gene expression (> 20% of genes) were observed during adaptation to challenging hosts with a range of genes involved in gene regulation, signalling, and detoxification differentially expressed. Remarkably, these changes in gene expression were associated with significant shifts in the tolerance of host-adapted T. vaporariorum lines to natural and synthetic insecticides.

CONCLUSIONS

Our findings provide further insights into the ability of polyphagous insects to extensively reprogram gene expression during host adaptation and illustrate the potential implications of this on their sensitivity to synthetic insecticides.

Plant Defense Responses Induced by Two Herbivores and Consequences for Whitefly Bemisia tabaci

Diverse herbivores are known to induce various plant defenses. The plant defenses may detrimentally affect the performance and preference to subsequent herbivores on the same plant, such as affecting another insect’s feeding, settling, growth or oviposition. Here, we report two herbivores (mealybug Phenacoccus solenopsis and carmine spider mite Tetranychus cinnabarinus) which were used to pre-infest the cucumber to explore the impact on the plants and the later-colonizing species, whitefly Bemisia tabaci. The results showed that the whiteflies tended to select the treatments pre-infested by the mites, rather than the uninfected treatments. However, the result of treatments pre-infested by the mealybugs was opposite. Total number of eggs laid of whiteflies was related to their feeding preference. The results also showed that T. cinnabarinus were more likely to activate plant jasmonic acid (JA) regulated genes, while mealybugs were more likely to activate key genes regulated by salicylic acid (SA). The different plant defense activities on cucumbers may be one of the essential factors that affects the preference of B. tabaci. Moreover, the digestive enzymes and protective enzymes of the whitefly might play a substantial regulatory role in its settling and oviposition ability.

Patterns of Tarnished Plant Bug (Hemiptera: Miridae) Resistance to Pyrethroid Insecticides in the Lower Mississippi Delta for 2008-2015: Linkage to Pyrethroid Use and Cotton Insect Management

Populations of tarnished plant bug, Lygus lineolaris (Palisot de Beauvois) (Hemiptera: Miridae), from the Lower Mississippi Delta regions of Arkansas, Louisiana, and Mississippi were evaluated from 2008 through 2015 for susceptibility to pyrethroid insecticides using a diagnostic-dose assay with permethrin. Resulting data add to the compilation of pyrethroid susceptibility data carefully tracked in this pest since 1994 and provide continuing evidence of high frequencies of pyrethroid resistance in field populations of the tarnished plant bug. Resistance levels are variable, and some populations remain susceptible suggesting practical value in the continued use of the diagnostic-dose assays prior to pyrethroid treatments. Recent studies with dose-response models suggest that levels of pyrethroid resistance in some populations may still be evolving, with some populations requiring higher doses to reach levels of control comparable to those observed 10 yr ago. Concerns for frequent use of multiple classes of insecticides and possible selection for tarnished plant bugs with metabolic resistance mechanisms capable of detoxifying available insecticide chemistries warrant continued efforts to manage resistance in this important crop pest. Associations among measured pyrethroid resistance levels, published data on annual use of pyrethroid insecticides, and annual estimates of cotton insect losses and control costs were explored and summarized for the 8 yr of this investigation. Mortality of tarnished plant bugs at the diagnostic-dose of permethrin was negatively correlated with kilograms of pyrethroids applied per acre of harvested cropland.

Identification and Expression Patterns of Putative Diversified Carboxylesterases in the Tea Geometrid Ectropis obliqua Prout

Carboxylesterases (CXEs) belong to a family of metabolic enzymes. Some CXEs act as odorant-degrading enzymes (ODEs), which are reportedly highly expressed in insect olfactory organs and participate in the rapid deactivation of ester pheromone components and plant volatiles. The tea geometrid Ectropis obliqua Prout produces sex pheromones consisting of non-ester functional compounds but relies heavily on acetic ester plant volatiles to search for host plants and locate oviposition sites. However, studies characterizing putative candidate ODEs in this important tea plant pest are still relatively scarce. In the present study, we identified 35 candidate EoblCXE genes from E. obliqua chemosensory organs based on previously obtained transcriptomic data. The deduced amino acid sequences possessed the typical characteristics of the insect CXE family, including oxyanion hole residues, the Ser-Glu-His catalytic triad, and the Ser active included in the conserved pentapeptide characteristic of esterases, Gly-X-Ser-X-Gly. Phylogenetic analyses revealed that the EoblCXEs were diverse, belonging to several different insect esterase clades. Tissue- and sex-related expression patterns were studied via reverse-transcription and quantitative real-time polymerase chain reaction analyses (RT- and qRT-PCR). The results showed that 35 EoblCXE genes presented a diversified expression profile; among these, 12 EoblCXEs appeared to be antenna-biased, two EoblCXEs were non-chemosensory organ-biased, 12 EoblCXEs were ubiquitous, and nine EoblCXEs showed heterogeneous expression levels among different tissues. Intriguingly, two EoblCXE genes, EoblCXE7 and EoblCXE13, were not only strongly localized to antennal sensilla tuned to odorants, such as the sensilla trichodea (Str I and II) and sensilla basiconica (Sba), but were also expressed in the putative gustatory sensilla styloconica (Sst), indicating that these two CXEs might play multiple physiological roles in the E. obliqua chemosensory processing system. This study provides the first elucidation of CXEs in the chemosensory system of a geometrid moth species and will enable a more comprehensive understanding of the functions of insect CXEs across lepidopteran species.

Effects of Feeding on Different Host Plants and Diets on Bradysia Odoriphaga Population Parameters and Tolerance to Heat and Insecticides

Bradysia odoriphaga Yang et Zhang, the Chinese chive root maggot, is a devastating pest of agricultural plants causing significant losses in vegetable crops, edible mushrooms, and humus. To determine the effects of feeding on different host plants and diets on the life parameters of B. odoriphaga and its tolerance to stressful conditions, we analyzed the life-table data of B. odoriphaga reared on Chinese chive (Allium hookeri), Welsh onion (Allium fistulosum), garlic (Allium sativum), oyster mushroom (Pleurotus ostreatus), and humus and assayed its tolerance to heat and insecticides. Furthermore, we assayed the physiological responses of B. odoriphaga reared on different hosts. Development from egg to adult was successfully completed on five hosts. Life table indicated that when reared on Chinese chive and oyster mushroom, B. odoriphaga showed better life parameters (shorter development time, higher survival rate, and fecundity) than on humus and garlic. However, B. odoriphaga larvae fed on humus and garlic better tolerated heat and insecticides than those that were fed Chinese chive and oyster mushroom; larvae survived longer at 38 and 40°C heat shock and at higher insecticide doses. Activities of antioxidant and detoxification enzymes varied significantly in larvae fed on different hosts. Catalase, superoxide dismutase, glutathione S-transferase, and carboxylesterase activities were higher in larva reared on garlic and humus than on Chinese chive and oyster mushroom. These findings indicated that B. odoriphaga preferred liliaceous vegetables and mushroom, and their tolerance against stress increased when reared on humus and garlic. Such basic information can promote targeted pest management in different agricultural fields and allow better understanding of the acclimatization strategy of B. odoriphaga.

Longitudinal Measurements of Tarnished Plant Bug (Hemiptera: Miridae) Susceptibility to Insecticides in Arkansas, Louisiana, and Mississippi: Associations with Insecticide Use and Insect Control Recommendations

Concentration-response assays were conducted from 2008 through 2015 to measure the susceptibility of field populations of Lygus lineolaris (Palisot de Beauvois) from the Delta regions of Arkansas, Louisiana, and Mississippi to acephate, imidacloprid, thiamethoxam, permethrin, and sulfoxaflor. A total of 229 field populations were examined for susceptibility to acephate, 145 for susceptibility to imidacloprid, and 208 for susceptibility to thiamethoxam. Permethrin assays were conducted in 2014 and 2015 to measure levels of pyrethroid resistance in 44 field populations, and sulfoxaflor assays were conducted against 24 field populations in 2015. Resistance to acephate and permethrin is as high or higher than that previously reported, although some populations, especially those exposed to permethrin, appear to be susceptible. Variable assay responses were measured for populations exposed to imidacloprid and thiamethoxam. Average response metrics suggest that populations are generally susceptible to the neonicotinoids, but a few populations from cotton fields experiencing control problems exhibited elevated LC50s. Efforts to associate variability in LC50s with recorded use of insecticides and estimated cotton insect losses and control costs suggest that intensive use of insecticides over several decades may have elevated general detoxifying enzymes in L. lineolaris and some field populations may be exhibiting resistance to multiple classes of insecticide. These results suggest that efforts should be made to manage these pests more efficiently with a reduced use of insecticides and alternative controls.

Transcriptional responses of detoxification genes to four plant allelochemicals in Aphis gossypii

Aphis gossypii Glover (Hemiptera: Aphididae) can damage a variety of agricultural crops, so it is very important for cotton aphids to evolve adaptive mechanisms to various allelochemicals from host plants. Our results aim to provide a fundamental and rich resource for exploring aphid functional genes in A. gossypii. A transcriptome data set and five expression profile data sets of A. gossypii samples were analyzed by Illumina sequencing platform. In total, 53,763,866 reads were assembled into 1,963,516 contigs and 28,555 unigenes. Compared with the control, 619 genes were significantly up- or downregulated in the treatment group by 2-tridecanone. There were 516, 509, and 717 of differential expression genes in tannic acid, quercetin, and gossypol treatment groups, respectively. Furthermore, there were 4 of 54 putative cytochrome P450 genes and 1 of 7 putative carboxylesterases downregulated in all treatment groups by four plant allelochemicals. When aphids fed on 2-tridecanone, tannic acid, and quercetin, only one P450 gene was upregulated. These results show that plant allelochemical stress can induce differential gene expression in A. gossypii. The differential response information of gene expression based on a large-scale sequence would be useful to reveal molecular mechanisms of adaptation for A. gossypii to plant allelochemicals.

Resistance Monitoring for Eight Insecticides on the Sweetpotato Whitefly (Hemiptera: Aleyrodidae) in China

The sweetpotato whitefly, Bemisia tabaci (Gennadius), is an important pest of many crops worldwide. Because control of B. tabaci still depends on the application of insecticides in China, monitoring the insecticide resistance of B. tabaci populations is essential for achieving control and for managing resistance. In this study, field populations of B. tabaci on vegetables were collected in three regions of China in 2011, 2012, and 2013. The resistance of these populations (all of which were determined to belong to biotype Q) to eight insecticides (abamectin, spinetoram, imidacloprid, thiamethoxam, acetamiprid, nitenpyram, chlorpyrifos, and bifenthrin) was assessed by the leaf-dip method. No resistance to abamectin and spinetoram was detected. All of the B. tabaci populations exhibited resistance to neonicotinoid insecticides; the resistance was 3.6- to 125.0-fold greater than that of a susceptible reference strain. The traditional insecticides chlorpyrifos and bifenthrin had very low toxicity. Bemisia tabaci specimens in some regions exhibited annual differences in resistance to some of the insecticides. The data presented will be helpful for making decisions on the proper insecticide usage in the field.

Health Status of Sand Flathead (Platycephalus bassensis), Inhabiting an Industrialised and Urbanised Embayment, Port Phillip Bay, Victoria as Measured by Biomarkers of Exposure and Effects

Port Phillip Bay, Australia, is a large semi-closed bay with over four million people living in its catchment basin. The Bay receives waters from the Yarra River which drains the city of Melbourne, as well as receiving the discharges of sewage treatment plants and petrochemical and agricultural chemicals. A 1999 study demonstrated that fish inhabiting Port Phillip Bay showed signs of effects related to pollutant exposure despite pollution management practices having been implemented for over a decade. To assess the current health status of the fish inhabiting the Bay, a follow up survey was conducted in 2015. A suite of biomarkers of exposure and effects were measured to determine the health status of Port Phillip Bay sand flathead (Platycephalus bassensis), namely ethoxyresorufin-O-deethylase (EROD) activity, polycyclic aromatic hydrocarbons (PAH) biliary metabolites, carboxylesterase activity (CbE) and DNA damage (8-oxo-dG). The reduction in EROD activity in the present study suggests a decline in the presence of EROD activity-inducing chemicals within the Bay since the 1990s. Fish collected in the most industrialised/urbanised sites did not display higher PAH metabolite levels than those in less developed areas of the Bay. Ratios of PAH biliary metabolite types were used to indicate PAH contaminant origin. Ratios indicated fish collected at Corio Bay and Hobsons Bay were subjected to increased low molecular weight hydrocarbons of petrogenic origin, likely attributed to the close proximity of these sites to oil refineries, compared to PAH biliary metabolites in fish from Geelong Arm and Mordialloc. Quantification of DNA damage indicated a localised effect of exposure to pollutants, with a 10-fold higher DNA damage level in fish sampled from the industrial site of Corio Bay relative to the less developed site of Sorrento. Overall, integration of biomarkers by multivariate analysis indicated that the health of fish collected in industrialised areas was compromised, with biologically significant biomarkers of effects (LSI, CF and DNA damage) discriminating between individuals collected in industrialised areas from observations made in fish collected in less developed areas of the Bay.

Effects of Host Plant Factors on the Bacterial Communities Associated with Two Whitefly Sibling Species

Background

Although discrepancy in the specific traits and ecological characteristics of Bemisia tabaci between species are partially attributed to the B. tabaci-associated bacteria, the factors that affect the diversity of B. tabaci-associated bacteria are not well-understood. We used the metagenomic approach to characterize the B. tabaci-associated bacterial community because the approach is an effective tool to identify the bacteria.

Methodology and Results

To investigate the effects of the host plant and a virus, tomato yellow leaf curl virus (TYLCV), on the bacterial communities of B. tabaci sibling species B and Q, we analyzed the bacterial communities associated with whitefly B and Q collected from healthy cotton, healthy tomato, and TYLCV-infected tomato. The analysis used miseq-based sequencing of a variable region of the bacterial 16S rDNA gene. For the bacteria associated with B. tabaci, we found that the influence of the host plant species was greater than that of the whitefly cryptic species. With further analysis of host plants infected with the TYLCV, the virus had no significant effects on the B. tabaci-associated bacterial community.

Conclusions

The effects of different plant hosts and TYLCV-infection on the diversity of B. tabaci-associated bacterial communities were successfully analyzed in this study. To explain why B. tabaci sibling species with different host ranges differ in performance, the analysis of the bacterial community may be essential to the explanation.

Invasion and Management of Agricultural Alien Insects in China

China is the world's fourth-largest country in terms of landmass. Its highly diverse biogeography presents opportunities for many invasive alien insects. However, physical and climate barriers sometimes prevent locally occurring species from spreading. China has 560 confirmed invasive alien species; 125 are insect pests, and 92 of these damage the agricultural ecosystem. The estimated annual economic loss due to alien invasive species is more than $18.9 billion. The most harmful invasive insects exhibit some common characteristics, such as high reproduction, competitive dominance, and high tolerance, and benefit from mutualist facilitation interactions. Regional cropping system structure adjustments have resulted in mono-agricultural ecosystems in cotton and other staple crops, providing opportunities for monophagous insect pests. Furthermore, human dietary shifts to fruits and vegetables and smallholder-based farming systems result in highly diverse agricultural ecosystems, which provide resource opportunities for polyphagous insects. Multiple cropping and widespread use of greenhouses provide continuous food and winter habitats for insect pests, greatly extending their geographic range. The current management system consists of early-warning, monitoring, eradication, and spread blocking technologies. This review provides valuable new synthetic information on integrated management practices based mainly on biological control for a number of invasive species. We encourage farmers and extension workers to be more involved in training and further research for novel protection methods that takes into consideration end users' needs.

Generating Within-Plant Spatial Distributions of an Insect Herbivore Based on Aggregation Patterns and Per-Node Infestation Probabilities

Most predator-prey models extrapolate functional responses from small-scale experiments assuming spatially uniform within-plant predator-prey interactions. However, some predators focus their search in certain plant regions, and herbivores tend to select leaves to balance their nutrient uptake and exposure to plant defenses. Individual-based models that account for heterogeneous within-plant predator-prey interactions can be used to scale-up functional responses, but they would require the generation of explicit prey spatial distributions within-plant architecture models. The silverleaf whitefly, Bemisia tabaci biotype B (Gennadius) (Hemiptera: Aleyrodidae), is a significant pest of tomato crops worldwide that exhibits highly aggregated populations at several spatial scales, including within the plant. As part of an analytical framework to understand predator-silverleaf whitefly interactions, the objective of this research was to develop an algorithm to generate explicit spatial counts of silverleaf whitefly nymphs within tomato plants. The algorithm requires the plant size and the number of silverleaf whitefly individuals to distribute as inputs, and includes models that describe infestation probabilities per leaf nodal position and the aggregation pattern of the silverleaf whitefly within tomato plants and leaves. The output is a simulated number of silverleaf whitefly individuals for each leaf and leaflet on one or more plants. Parameter estimation was performed using nymph counts per leaflet censused from 30 artificially infested tomato plants. Validation revealed a substantial agreement between algorithm outputs and independent data that included the distribution of counts of both eggs and nymphs. This algorithm can be used in simulation models that explore the effect of local heterogeneity on whitefly-predator dynamics.

Transcriptome analysis of host-associated differentiation in Bemisia tabaci (Hemiptera: Aleyrodidae)

Host-associated differentiation is one of the driving forces behind the diversification of phytophagous insects. In this study, host induced transcriptomic differences were investigated in the sweetpotato whitefly Bemisia tabaci, an invasive agricultural pest worldwide. Comparative transcriptomic analyses using coding sequence (CDS), 5′ and 3′ untranslated regions (UTR) showed that sequence divergences between the original host plant, cabbage, and the derived hosts, including cotton, cucumber and tomato, were 0.11–0.14%, 0.19–0.26%, and 0.15–0.21%, respectively. In comparison to the derived hosts, 418 female and 303 male transcripts, respectively, were up-regulated in the original cabbage strain. Among them, 17 transcripts were consistently up-regulated in both female and male whiteflies originated from the cabbage host. Specifically, two ESTs annotated as Cathepsin B or Cathepsin B-like genes were significantly up-regulated in the original cabbage strain, representing a transcriptomic response to the dietary challenges imposed by the host shifting. Results from our transcriptome analysis, in conjunction with previous reports documenting the minor changes in their reproductive capacity, insecticide susceptibility, symbiotic composition and feeding behavior, suggest that the impact of host-associated differentiation in whiteflies is limited. Furthermore, it is unlikely the major factor contributing to their rapid range expansion/invasiveness.

Variation in mortality among populations is higher for pymetrozine than for imidacloprid and spiromesifen in Trialeurodes vaporariorum in greenhouses in Finland

BACKGROUND

Insecticide resistance in Trialeurodes vaporariorum W. is unknown in the species' northern distribution range where it inhabits mainly commercial greenhouses. Resistance development in whiteflies feeding on year-round crops in greenhouses is possible owing to the use of chemical treatments to back up biocontrol. The authors tested the response levels to spiromesifen, pymetrozine and imidacloprid in whiteflies collected from seven greenhouses within a 35 km radius in western Finland.

RESULTS

All except one (PR) population had LC50 values below the recommended concentrations for the tested compounds. However, some populations showed reduced susceptibility to pymetrozine in comparison with the reference susceptible population. Resistance ratios to pymetrozine were highly variable (resistance ratio 0.5-39.7), even among closely located greenhouses, and higher than those for imidacloprid (resistance ratio 1.05-10.5) and spiromesifen (resistance ratio 0.8-11.5). LC50 values and application frequencies of pymetrozine correlated positively among the sampled populations.

CONCLUSION

High variation in resistance levels to pymetrozine among populations within natural whitefly dispersal limits reflects variation in the usage of this compound among individual greenhouse crop producers. Thus, resistance management is recommended at the individual greenhouse crop producer level, even in a dense production cluster. © 2014 Society of Chemical Industry.

Effects of Fraxinellone on the midgut enzyme activities of the 5th Instar Larvae of Oriental Armyworm, Mythimna separata walker

Isolated from Dictamnus dasycarpus Turcz., fraxinellone exhibited multiple bioactivities against insects. In the present paper, the changes of digestive enzymes and detoxification enzymes of Mythimna separata Walker (5th instar larvae), treated with fraxinellone, were investigated. Compared with those of the control, the α-amylase activity of the fraxinellone-treated 5th instar larvae was inhibited, whereas the level of their protease activity was increased. Based upon further studies on the specific proteases, the levels of the active alkaline trypsin-like enzyme (BApNA as the substrate) and the chymotrypsin-like enzyme (BTEE as the substrate) activities of the treated larvae were declined; however, the level of activity of the weak alkaline trypsin-like enzyme (TAME as the substrate) of the treated ones was increased. Meanwhile, the activities of two detoxification enzymes, such as carboxylesterase (CarE) and glutathione S-transferase (GST), of the treated larvae were increased to some extent, but the activities of NADPH-P450 reductase and O-demethylase of the treated ones declined. Therefore, protease (especially the weak alkaline trypsin-like enzyme), CarE and GST played important roles in the metabolism of fraxinellone in the midgut of Mythimna separata (M. separata).

Short-term and transgenerational effects of the neonicotinoid nitenpyram on susceptibility to insecticides in two whitefly species

The cosmopolitan silverleaf whitefly, Bemisia tabaci which had coexisted with Trialeurodes vaporariorum in Northern China for many years, has become the dominant species in the last years. Recent reports show that it is gradually displacing the other greenhouse whitefly species. Neonicotinoid, which includes nitenpyram, is a major group of insecticides used against whiteflies in various crops. When exposed to low doses of insecticides, insects may develop resistance by adapting physiologically. The short- and long-term effects of nitenpyram on insecticide sensitivity in B. tabaci biotype B and T. vaporariorum adult populations have been compared in the present study. After being exposed to LC(25) of nitenpyram for 24 h, the B. tabaci biotype B adults showed no significant change in susceptibility to nitenpyram or to five other insecticides: imidacloprid, acetamiprid, abamectin, chlorpyrifos and beta-cypermethrin. By contrast, exposure to the LC(25) of nitenpyram for 24 h led to a significant increase in the susceptibility of T. vaporariorum to nitenpyram and imidacloprid, by 1.8- and 2-fold, respectively. When exposed for seven generations to the LC(25) of nitenpyram, B. tabaci developed 6-fold resistance to nitenpyram, and 3.1- and 5-fold cross-resistance to imidacloprid and acetamiprid, respectively, whereas T. vaporariorum developed lower resistance (3.7-fold) to the nitenpyram and very low cross-resistance to imidacloprid (2.5-fold). The higher adaptable nature of B. tabaci (demonstrated here in the case of nitenpyram) when exposed to low doses of insecticides may provide a selective advantage when competing with T. vaporariorum in crops.

Variation in the sensitivity of Callosobruchus (Coleoptera: Bruchidae) acetylcholinesterase to the organophosphate insecticide malaoxon: effect of species, geographical strain and food type

BACKGROUND

Bruchid beetles, Callosobruchus species, are serious pests of economically important grain legumes; their activity in stores is often controlled by the use of synthetic insecticides. Esterases are known to be involved in insecticide resistance in insects. However, there is a dearth of information on esterase activity in the genus Callosobruchus. In this study, the effect of species, geographical strain and food type on the variation in acetylcholinesterase (AChE) activity and its inhibition by malaoxon (malathion metabolite) was investigated using an in vitro spectrophotometric method.

RESULT

AChE activity varied significantly among species and strains and also among legume type used for rearing them. Generally, irrespective of species, strain or food type, the higher the AChE activity of a population, the higher is its inhibition by malaoxon. C. chinensis had the highest AChE activity of the species studied, and in the presence of malaoxon it had the lowest remaining AChE activity, while C. rhodesianus retained the highest activity.

CONCLUSION

A first-hand knowledge of AChE activity in regional Callosobruchus in line with the prevailing food types should be of utmost importance to grain legume breeders, researchers on plant materials for bruchid control and pesticide manufacturer/applicators for a robust integrated management of these bruchids.

Transcriptome analysis of the citrus red mite, Panonychus citri, and its gene expression by exposure to insecticide/acaricide

The citrus red mite, Panonychus citri, is known for its ability rapidly to evolve resistance to insecticides/acaricides and to adapt to hosts that produce toxins. In this study, we constructed an unprecedented four gigabase pair transcriptome of P. citri, which was assembled into 64 149 unique transcripts, the functions of which were annotated by five public databases. A total of 116 unique transcripts were identified as representatives of potential involvement in the detoxification of xenobiotics. Genes recorded to encoding insecticide/acaricide target proteins were also obtained from the P. citri transcriptome. In order to explore novel candidate genes potentially involved in the pesticide detoxification of P. citri, we also constructed digital gene expression libraries of short-term transcriptome responses of P. citri to pesticides, which resulted in the identification of 120 unique transcripts potentially associated with insecticide/acaricide detoxification. Our study will facilitate molecular research on pesticide resistance in citrus red mites, as well as in other phytophagous mites.

Host plant effects on alkaline phosphatase activity in the whiteflies, Bemisia tabaci Biotype B and Trialeurodes vaporariorum

Bemisia tabaci (Gennadius) B-biotype and Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) often coexist on greenhouse-grown vegetable crops in northern China. The recent spread of B. tabaci B-biotype has largely replaced T. vaporariorum, and B-biotype now overlaps with T. vaporariorum where common hosts occur in most invaded areas. The impact of the B-biotype on the agro eco system appears to be widespread, and involves the ability to compete with and perhaps replace other phytophages like T. vaporariorum. An emerging hypothesis is that the B-biotype is physiologically superior due at least in part to an improved ability to metabolically utilize the alkaline phosphatase pathway. To test this hypothesis, alkaline phosphatase activity was studied in the B-biotype and T. vaporariorum after feeding on a number of different hosts for a range of durations, with and without host switching. Alkaline phosphatase activity in T. vaporariorum was 1.45 to 2.53-fold higher than that of the B-biotype when fed on tomato for 4 and 24 h, or switched from tomato to cotton and cabbage for the same durations. However, alkaline phosphatase activity in the B-biotype was 1.40 to 3.35-fold higher than that of T. vaporariorum when the host switching time was ∼72 and ∼120 h on the same plant. Both short-term (4 h) and long-term (72 h) switching of plant hosts can significantly affect the alkaline phosphatase activity in the two species. After ∼120 h, feeding on tomato and cotton alkaline phosphatase activity in the B-biotype was significantly higher than that of T. vaporariorum. It was shown that alkaline phosphatase aids the species feeding on different plant species, and that the B-biotype is physiologically superior to T. vaporariorum in utilizing the enzyme compared to T. vaporariorum over longer periods of feeding.

Induction effects of host plants on insecticide susceptibility and detoxification enzymes of Bemisia tabaci (Hemiptera: Aleyrodidae)

BACKGROUND

The polyphagous B-biotype Bemisia tabaci (Gennadius) has developed a high resistance to commonly used insecticides in China. To illustrate the induced changes by host plant, bioassay and biochemical research on five different host populations were investigated.

RESULTS

Except for bifenthrin, all tested insecticides showed lower toxicity to the B. tabaci poinsettia population compared with other host populations. Moreover, four insecticides, the exceptions being abamectin and fipronil, showed highest toxicity towards the tomato population. The LC(50) values of the poinsettia population, particularly towards acetamiprid, were 14.8-, 10.3- and 7.29-fold higher than those of tomato, cucumber and cabbage respectively. The CarE activities of B. tabaci cabbage and cucumber populations were all significantly higher than those of poinsettia, cotton and tomato populations. The ratio of the cabbage population was 1.97-, 1.79- and 1.30-fold higher than that of poinsettia, cotton and tomato respectively. The frequency profiles for this activity also have obvious differences. The GST and P450 activities of the cucumber population were the lowest in the five host populations.

CONCLUSION

Long-term induction of host plants for B-biotype B. tabaci could influence their susceptibilities to several insecticides. Rational selection and usage of insecticides for particular hosts will be helpful for resistance management and control of this species.

Effects of four host plants on susceptibility of Spodoptera litura (Lepidoptera: Noctuidae) larvae to five insecticides and activities of detoxification esterases

BACKGROUND

The tobacco cutworm, Spodoptera litura (F.), is one of the most destructive polyphagous pests worldwide. The susceptibility of S. litura larvae reared on tobacco, Chinese cabbage, cowpea and sweet potato to phoxim, chlorfenapyr, methomyl, fenvalerate and emamectin benzoate under laboratory conditions was determined.

RESULTS

Spodoptera litura larvae reared on tobacco were most tolerant to all insecticides, whereas those that fed on sweet potato were most susceptible. When larvae were reared on each host plant for three generations, the susceptibilities to phoxim of larvae that fed on Chinese cabbage and cowpea were similar, whereas the susceptibility of larvae that fed on sweet potato decreased by the third generation, and on tobacco the susceptibility decreased in each consecutive generation. When nicotine was added to their diet for three consecutive generations, the tolerance of larvae to phoxim increased twofold, and to emamectin benzoate 3.1-fold, but the tolerance of larvae to fenvalerate and chlorfenapyr did not change. The acetylcholinesterase activities of the larvae that fed on sweet potato and cowpea were greater than the activities of those that fed on Chinese cabbage and tobacco. In contrast, the carboxylesterase activities of the larvae that fed on tobacco and Chinese cabbage were greater than the activities of those that fed on sweet potato and cowpea. The glutathione S-transferase activities of larvae were highest when they fed on tobacco, followed by Chinese cabbage and cowpea, and the lowest activities were observed when larvae fed on sweet potato.

CONCLUSION

Feeding on tobacco or with nicotine added to the diet, the larvae became more tolerant to insecticides, especially to phoxim and emamectin benzoate.

Host plant influence on susceptibility of Bemisia tabaci (Hemiptera: Aleyrodidae) to insecticides

A resistance monitoring program conducted for the polyphagous whitefly, Bemisia tabaci (Gennadius), in Imperial Valley, CA, USA generated a large set of LC50s for adults collected from broccoli, cantaloupe and cotton crops over a four-year period. A vial bioassay and, subsequently, a yellow-sticky card bioassay produced similar temporal profiles of relative susceptibilities to the pyrethroid insecticide bifenthrin. Both bioassays revealed that whiteflies collected from broccoli were significantly less susceptible to bifenthrin compared to the other two crops. A similar finding was observed for endosulfan and the mixture of bifenthrin+endosulfan in the yellow-sticky card bioassay. The possibility that seasonal differences contributed to the observed differences in susceptibility provided the impetus to conduct a reciprocal transfer experiment using broccoli (or kale) and cantaloupe grown simultaneously in the field and greenhouse. Whitefly adults collected from an organic farm over three consecutive weeks had significantly higher LC50s on kale than those collected the same day on cantaloupe. After culturing in the greenhouse on broccoli or cantaloupe and testing again, LC50s remained significantly higher on broccoli after one week and again at the F1 generation. In contrast, whiteflies originating on kale in the field and transferred to cantaloupes in the greenhouse had significantly reduced LC50s at the F1 generation. When tested against the bifenthrin+endosulfan mixture, significantly higher LC50s were generated for whiteflies reared on broccoli in the greenhouse at one week and the F1 compared to the field source from cantaloupes. The consistently higher LC50s for whiteflies on broccoli and other Brassica spp. crops, compared to cantaloupes or cotton, point to statistically significant host-plant influences that are expressed in both field-collected and greenhouse-reared populations of whiteflies.

Invasive mechanism and management strategy of Bemisia tabaci (Gennadius) biotype B: progress report of 973 Program on invasive alien species in China

Bemisia tabaci (Gennadius) biotype B, called a "superbug", is one of the most harmful biotypes of this species complex worldwide. In this report, the invasive mechanism and management of B. tabaci biotype B, based on our 5-year studies, are presented. Six B. tabaci biotypes, B, Q, ZHJ1, ZHJ2, ZHJ3 and FJ1, have been identified in China. Biotype B dominates the other biotypes in many regions of the country. Genetic diversity in biotype B might be induced by host plant, geographical conditions, and/or insecticidal application. The activities of CarE (carboxylesterase) and GSTs (glutathione-S-transferase) in biotype B reared on cucumber and squash were greater than on other host plants, which might have increased its resistance to insecticides. The higher activities of detoxification enzymes in biotype B might be induced by the secondary metabolites in host plants. Higher adaptive ability of biotype B adults to adverse conditions might be linked to the expression of heat shock protein genes. The indigenous B. tabaci biotypes were displaced by the biotype B within 225 d. The asymmetric mating interactions and mutualism between biotype B and begomoviruses via its host plants speed up widespread invasion and displacement of other biotypes. B. tabaci biotype B displaced Trialeurodes vaporariorum (Westwood) after 4-7 generations under glasshouse conditions. Greater adaptive ability of the biotype B to adverse conditions and its rapid population increase might be the reasons of its successful displacement of T. vaporariorum. Greater ability of the biotype B to switch to different host plants may enrich its host plants, which might enable it to better compete with T. vaporariorum. Native predatory natural enemies possess greater ability to suppress B. tabaci under field conditions. The kairomones in the 3rd and 4th instars of biotype B may provide an important stimulus in host searching and location by its parasitoids. The present results provide useful information in explaining the mechanisms of genetic diversity, evolution and molecular eco-adaptation of biotype B. Furthermore, it provides a base for sustainable management of B. tabaci using biological and ecological measures.