Esterase-based resistance in the tobacco-adapted form of the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae) in the eastern United States

Insecticide-contaminated honeydew: risks for beneficial insects

Honeydew is the sugar-rich excretion of phloem-feeding hemipteran insects such as aphids, mealybugs, whiteflies, and psyllids, and can be a main carbohydrate source for beneficial insects in some ecosystems. Recent research has revealed that water-soluble, systemic insecticides contaminate honeydew excreted by hemipterans that feed on plants treated with these insecticides. This contaminated honeydew can be toxic to beneficial insects, such as pollinators, parasitic wasps and generalist predators that feed on it. This route of exposure has now been demonstrated in three plant species, for five systemic insecticides and four hemipteran species; therefore, we expect this route to be widely available in some ecosystems. In this perspective paper, we highlight the importance of this route of exposure by exploring: (i) potential pathways through which honeydew might be contaminated with insecticides; (ii) hemipteran families that are more likely to excrete contaminated honeydew; and (iii) systemic insecticides with different modes of action that might contaminate honeydew through the plant. Furthermore, we analyse several model scenarios in Europe and/or the USA where contaminated honeydew could be problematic for beneficial organisms that feed on this ubiquitous carbohydrate source. Finally, we explain why this route of exposure might be important when exotic, invasive, honeydew-producing species are treated with systemic insecticides. Overall, this review opens a new area of research in the field of ecotoxicology to understand how insecticides can reach non-target beneficial insects. In addition, we aim to shed light on potential undescribed causes of insect declines in ecosystems where honeydew is an important carbohydrate source for insects, and advocate for this route of exposure to be included in future environmental risk assessments.

Transcriptome Responses to Defined Insecticide Selection Pressures in the German Cockroach (Blattella germanica L.)

Cockroaches are important global urban pests from aesthetic and health perspectives. Insecticides represent the most cost-effective way to control cockroaches and limit their impacts on human health. However, cockroaches readily develop insecticide resistance, which can quickly limit efficacy of even the newest and most effective insecticide products. The goal of this research was to understand whole-body physiological responses in German cockroaches, at the metatranscriptome level, to defined insecticide selection pressures. We used the insecticide indoxacarb as the selecting insecticide, which is an important bait active ingredient for cockroach control. Six generations of selection with indoxacarb bait produced a strain with substantial (>20×) resistance relative to inbred control lines originating from the same parental stock. Metatranscriptome sequencing revealed 1,123 significantly differentially expressed (DE) genes in ≥two of three statistical models (81 upregulated and 1,042 downregulated; FDR P < 0.001; log2FC of ±1). Upregulated DE genes represented many detoxification enzyme families including cytochrome-P450 oxidative enzymes, hydrolases and glutathione-S-transferases. Interestingly, the majority of downregulated DE genes were from microbial and viral origins, indicating that selection for resistance is also associated with elimination of commensal, pathogenic and/or parasitic microbes. These microbial impacts could result from: (i) direct effects of indoxacarb, (ii) indirect effects of antimicrobial preservatives included in the selecting bait matrix, or (iii) selection for general stress response mechanisms that confer both xenobiotic resistance and immunity. These results provide novel physiological insights into insecticide resistance evolution and mechanisms, as well as novel insights into parallel fitness benefits associated with selection for insecticide resistance.

Changes in the genetic composition of Myzus persicae nicotianae populations in Chile and frequency of insecticide resistance mutations

Myzus persicae is a cosmopolitan aphid that is highly polyphagous and an important agricultural pest. The subspecies M. persicae nicotianae has been described for highly specialized phenotypes adapted to tobacco (Nicotiana tabacum). In Chile, the population of M. persicae nicotianae was originally composed of a single red genotype that did not possess insecticide resistance mutations. However, in the last decade, variation in the colour of tobacco aphids has been observed in the field. To determine whether this variation stems from the presence of new genotypes, sampling was carried out across the entire distribution of tobacco cultivation regions in Chile. The aphids collected were genotyped, and the frequency of kdr (L1014F), super-kdr (M918T), modification of acetylcholinesterase (MACE) and nicotinic acetylcholine receptor β subunit (nAChRβ) mutations associated with insecticide resistance was determined. A total of 16 new genotypes of M. persicae nicotianae were detected in Chile: four of them possessed the MACE mutation, and none of them possessed the kdr, super-kdr or nAChRβ mutation. The previously described red genotype was not detected in any of the sampled fields over two seasons. These results raise questions about the mechanisms underlying changes in the genetic structure of M. persicae nicotianae populations in Chile. Future research aimed at addressing these questions could provide new insight into aphid evolution and agricultural practices.

Differences in susceptibility to insecticides among color morphs of the Asian citrus psyllid

Insect color morphs are known to vary in detoxification enzyme activity associated with susceptibility to insecticides. Diaphorina citri occurs in three distinct color morphs, and little data are available addressing this question. In this study, susceptibility to nine insecticides was compared among orange/yellow, gray/brown and blue/green morphs of adult D. citri. Imidacloprid and lambda-cyhalothrin resulted in higher mortality in the orange/yellow morph than in the blue/green morph. The orange/yellow morph was significantly more susceptible to acetamiprid, bifenthrin and chlorfenapyr than the blue/green and gray/brown morphs. For clothianidin and chlorpyriphos, the orange/yellow morph was significantly more susceptible than the gray/brown morphs. Susceptibility to dinotefuran and thiamethoxam was not significantly different between the three color morphs of D. citri. Biochemical enzyme assays revealed that the esterase, glutathione S-transferase and cytochrome P450 activities were significantly higher in the gray/brown and blue/green morphs than in the orange/yellow morph; however, the activities of three detoxification enzymes were not significantly different between gray/brown and blue/green morphs. We analyzed the relative expression among the color morphs of six detoxification genes involved in insecticide resistance. GSTS1 and EST-6 were expressed at significantly higher levels in the blue/green morph than in the orange/yellow and gray/brown morphs. The expression levels of CYP4g15, CYP303A1, CYP4C62 and CYP6BD5 were significantly greater in the gray/brown and blue/green morphs than in the orange/yellow morph. These results suggest that insecticide susceptibility in D. citri is associated with color morphs. Detoxification enzyme activity and the relative expression of six detoxification genes may partially explain these differences. These results may facilitate further work to develop a management strategy for control of D. citri.

Insecticide toxicity associated with detoxification enzymes and genes related to transcription of cuticular melanization among color morphs of Asian citrus psyllid

The Asian citrus psyllid (Diaphorina citri Kuwayama) is known to exhibit abdominal color polymorphisms. In the current study, susceptibility to four insecticides was compared among orange/yellow, blue/green and gray/brown color morphs of field collected D. citri. The LD₅₀ values and 95% fiducial limits were quantified for each insecticide and color morph combination and ranged between 0.10 ng/μL (0.06-0.10) and 6.16 ng/μL (3.30-12.50). Second, we measured the detoxification enzyme activity levels of orange/yellow, blue/green and gray/brown color morphs for cytochrome P450, glutathione S-transferase, and general esterase. The mean P450 activity (equivalent units) was significantly lower in gray/brown (0.152 ± 0.006) and blue/green morphs (0.149 ± 0.005) than in the orange/yellow morphs (0.179 ± 0.008). GST activity (μmol/min/mg protein) was significantly lower in the orange/yellow morph (299.70 ±1.24) than gray/brown (350.86 ± 1.19) and blue/green (412.25 ± 1.37) morphs. The mean EST activity (μmol/min/mg protein) was significantly higher in blue/green (416.72 ± 5.12) and gray/brown morphs (362.19 ± 4.69) than in the orange/yellow morphs (282.56 ± 2.93). Additionally, we analyzed the relative expression of assortment genes involved in cuticular melanization and basal immunity. The transcripts of Dopa Decarboxylase and Tyrosine Hydroxylase were expressed higher in blue/green and gray/brown than orange/yellow morphs. The transcription results paralleled the susceptibility of D. citri to organophosphate, neonicotinoid and pyrethroid insecticides. GST and EST activity may also be correlated with low levels of insecticide susceptibility. Cuticular melanization could be a factor for the development of resistance to insecticides among different color morphs.

Detection and biochemical characterization of insecticide resistance in field populations of Asian citrus psyllid in Guangdong of China

The Asian citrus psyllid, Diaphorina citri Kuwayama, is one of the most damaging pests of citrus-producing regions throughout the world. The use of insecticides is the main strategy for controlling psyllid and has increased year by year. In this study, four field populations of D. citri were evaluated for resistance to nine different insecticides using the leaf-dip method. The results showed that the highest level of resistance for D. citri was found in imidacloprid with a resistance ratio of 15.12 in the Zengcheng population compared with the laboratory susceptible population. This was followed by chlorpyriphos (6.47), dinotefuran (6.16), thiamethoxam (6.04), lambda-cyhalothrin (4.78), and bifenthrin (4.16). Piperonyl butoxide (PBO) and triphenyl phosphate (TPP) showed significant synergism on imidacloprid effects in the Zengcheng population (3.84- and 2.46-fold, respectively). Nevertheless, diethyl maleate (DEM) had no synergism on imidacloprid. Biochemical enzyme assays suggested that general esterase, glutathione S-transferase and cytochrome P450 monooxygenase activities were higher in the field-collected populations than in the laboratory susceptible population. However, glutathione S-transferase may play a minor role in the resistance of adult D. citri to insecticides. At the molecular level, resistance of D. citri to imidacloprid is mainly related to the increased expression of CYP4C68 and CYP4G70 (>5-fold).

Molecular Signatures of Reduced Nerve Toxicity by CeCl3 in Phoxim-exposed Silkworm Brains

CeCl3 can reduce the damage caused by OP pesticides, in this study we used the brain of silkworms to investigate the mechanism of CeCl3 effects on pesticide resistance. The results showed that phoxim treatments led to brain damages, swelling and death of neurons, chromatin condensation, and mitochondrial damage. Normal nerve conduction was severely affected by phoxim treatments, as revealed by: increases in the contents of neurotransmitters Glu, NO, and ACh by 63.65%, 61.14%, and 98.54%, respectively; decreases in the contents of 5-HT and DA by 53.19% and 43.71%, respectively; reductions in the activities of Na(+)/K(+)-ATPase, Ca(2+)/Mg(2+)-ATPase, and AChE by 85.27%, 85.63%, and 85.63%, respectively; and increase in the activity of TNOS by 22.33%. CeCl3 pretreatment can significantly reduce such damages. Results of DGE and qRT-PCR indicated that CeCl3 treatments significantly upregulated the expression levels of CYP4G23, cyt-b5, GSTs-σ1, ace1, esterase-FE4, and β-esterase 2. Overall, phoxim treatments cause nerve tissue lesions, neuron death, and nerve conduction hindrance, but CeCl3 pretreatments can promote the expression of phoxim resistance-related genes in silkworm brains to reduce phoxim-induced damages. Our study provides a potential new method to improve the resistance of silkworms against OP pesticides.

Elevated expression of esterase and cytochrome P450 are related with lambda-cyhalothrin resistance and lead to cross resistance in Aphis glycines Matsumura

A resistant strain of the Aphis glycines Matsumura (CRR) has developed 76.67-fold resistance to lambda-cyhalothrin compared with the susceptible (CSS) strain. Synergists piperonyl butoxide (PBO), S,S,S-Tributyltrithiophosphate (DEF) and triphenyl phosphate (TPP) dramatically increased the toxicity of lambda-cyhalothrin to the resistant strain. Bioassay results indicated that the CRR strain had developed high levels of cross-resistance to chlorpyrifos (11.66-fold), acephate (8.20-fold), cypermethrin (53.24-fold), esfenvalerate (13.83-fold), cyfluthrin (9.64-fold), carbofuran (14.60-fold), methomyl (9.32-fold) and bifenthrin (4.81-fold), but did not have cross-resistance to chlorfenapyr, imidacloprid, diafenthiuron, abamectin. The transcriptional levels of CYP6A2-like, CYP6A14-like and cytochrome b-c1 complex subunit 9-like increased significantly in the resistant strain than that in the susceptible. Similar trend were observed in the transcripts and DNA copy number of CarE and E4 esterase. Overall, these results demonstrate that increased esterase hydrolysis activity, combined with elevated cytochrome P450 monooxygenase detoxicatication, plays an important role in the high levels of lambda-cyhalothrin resistance and can cause cross-resistance to other insecticides in the CRR strain.

Abdominal color of the Asian citrus psyllid, Diaphorina citri, is associated with susceptibility to various insecticides

BACKGROUND

Color morphs of an insect species are known to vary in activities of detoxifying enzymes and associated susceptibility to insecticides. In Diaphorina citri, three color morphs are known to occur. In the present study, susceptibility to four insecticides was compared among gray/brown, blue/green and orange/yellow color morphs of field-collected D. citri.

RESULTS

The orange/yellow morph was significantly more susceptible to fenpropathrin than the blue/green morph, and imidacloprid and chlorpyrifos caused higher mortality in the orange/yellow morph than in the blue/green and gray/brown morphs. To confirm the genetic basis of variable levels of susceptibility, the relative expression of five CYP4 genes was compared among the color morphs. CYP4C67 was expressed at significantly higher levels in the blue/green than in the orange/yellow and gray/brown morphs. CYP4DA1, CYP4C68, CYP4G70 and CYP4DB1 were expressed at significantly higher levels in the blue/green and gray/brown morphs than in the orange/yellow morph. Lower expression of CYP4 genes in the orange/yellow morph as compared with the others was correlated with reduced signal of 45 kD cytochrome P450 proteins, as determined by the western blot.

CONCLUSIONS

The results indicate differential susceptibility of D. citri color morphs to insecticides, which will need to be accounted for in future insecticide monitoring programs and may affect management programs.

Correlation between pesticide resistance and enzyme activity in the diamondback moth, Plutella xylostella

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is one of the most important pests that has developed high pesticide resistance. The resistances of five Chinese populations of this moth, four resistant strains (from Beijing, Henan, Fujian, and Guangdong) and one susceptible strain, to five pesticides were determined, and the activities of carboxylesterase, glutathione S-transferase, and acetylcholine esterase were tested in all five populations. The correlations between pesticide resistance and enzyme activity were analyzed. The results showed that the resistance status to the five pesticides was different among the five populations. The resistance ratios of the Beijing and Henan populations to spinosad were 5.84 and 8.22, respectively, and those to beta-cypermethrin were 4.91 and 4.98, respectively. These ratios were higher than those for the Fujian and Guangdong populations. The Fujian population was more sensitive to abamectin and chlorpyrifos than the susceptible population (the resistance ratios were 0.14 and 0.91, respectively); in fact, the median lethal concentration for P. xylostella was significantly higher for chlorpyrifos than that for any of the other four pesticides. The carboxylesterase activity in P. xylostella showed positive correlations with the resistance to spinosad, beta-cypermethrin, chlorpyrifos, and abamectin, but no correlation was observed between the carboxylesterase activity and resistance to emamectin benzoate, between glutathione S-transferase activity and resistance to any of the five pesticides tested, or between acetylcholine esterase activity and any of the pesticides except for emamectin benzoate.

Aliphatic esters as targets of esterase activity in the parsnip webworm (Depressaria pastinacella)

As a specialist on the reproductive structures of Pastinaca sativa and species in the related genus Heracleum, the parsnip webworm (Depressaria pastinacella) routinely encounters a distinctive suite of phytochemicals in hostplant tissues. Little is known, however, about the detoxification mechanisms upon which this species relies to metabolize these compounds. In this study, larval guts containing hostplant tissues were homogenized, and metabolism was determined by incubating reactions with and without NADPH and analyzing for substrate disappearance and product appearance by gas chromatography-mass spectrometry. Using this approach, we found indications of carboxylesterase activity, in the form of appropriate alcohol metabolites for three aliphatic esters in hostplant tissues-octyl acetate, octyl butyrate, and hexyl butyrate. Involvement of webworm esterases in hostplant detoxification subsequently was confirmed with metabolism assays with pure compounds. This study is the first to implicate esterases in lepidopteran larval midgut metabolism of aliphatic esters, ubiquitous constituents of flowers and fruits. In addition, this method confirmed that webworms detoxify furanocoumarins and myristicin in their hostplants via cytochrome P450-mediated metabolism, and demonstrated that these enzymes also metabolize the coumarin osthol and the fatty acid derivative palmitolactone.

Insecticide resistance in field populations of Asian citrus psyllid in Florida

BACKGROUND

Asian citrus psyllid (ACP), Diaphorina citri, is a major pest of citrus because it vectors the putative causal agent of huanglongbing disease. Insecticides are currently the basis of psyllid management programs, and the number of annual insecticide applications has increased significantly. In this paper, a series of investigations of insecticide resistance among field populations of adult and immature ACP in Florida is described.

RESULTS

In 2009, the highest level of resistance for adult ACP, as compared with the laboratory susceptible (LS) population, was found with imidacloprid with an LD(50) resistance ratio (RR(50) ) of 35 in one population. This was followed by chlorpyriphos (RR(50) = 17.9, 13.3, 11.8 and 6.9), thiamethoxam (RR(50) = 15 and 13), malathion (RR(50) = 5.4 and 5.0) and fenpropathrin (RR(50) = 4.8). In 2010, mortality of adults from all five sites sampled was lower than with the LS population at three diagnostic concentrations of each insecticide tested. Among nymph populations, indications of resistance were observed with carbaryl (RR(50) = 2.9), chlorpyriphos (RR(50) = 3.2), imidacloprid (RR(50) = 2.3 and 3.9) and spinetoram (RR(50) = 4.8 and 5.9). General esterase, glutathione S-transferase and monooxygenase levels were also elevated in field-collected adult and nymph ACP as compared with the LS population.

CONCLUSION

The present results suggest that varying levels of insecticide susceptibility exist in ACP populations across the citrus-growing areas of Florida. Increased levels of detoxifying enzymes in these populations may partially explain these differences. The present results indicate that insecticide resistance may become an emerging problem for ACP control if effective resistance management is not practiced.

Effect of Candidatus Liberibacter asiaticus infection on susceptibility of Asian citrus psyllid, Diaphorina citri, to selected insecticides

BACKGROUND

In the present investigation, the effect of Candidatus Liberibacter asiaticus (Las), a bacterium considered to be responsible for causing huanglongbing (HLB) disease in citrus, on the physiology of its vector, the Asian citrus psyllid (ACP) Diaphorina citri Kuwayama, was determined. Specifically, the effects of Las infection on the susceptibility of ACP to selected insecticides were determined. Furthermore, total protein content and general esterase activity were quantified in Las-infected and uninfected ACP to gain insight into the possible mechanism(s) responsible for altered susceptibility to insecticides owing to Las infection.

RESULTS

LC(50) values were significantly lower in Las-infected than in uninfected ACP adults for chlorpyrifos and spinetoram. Furthermore, there was a general trend towards lower LC(50) values for three other insecticides for Las-infected ACP; however, the differences were not statistically significant. Total protein content (µg mL(-1)) was significantly lower in Las-infected (23.5 ± 1.3 in head + thorax; 27.7 ± 1.9 in abdomen) than in uninfected (29.7 ± 2.1 in head + thorax; 35.0 ± 2.3 in abdomen) ACP. Likewise, general esterase enzyme activity (nmol min(-1) mg(-1) protein) was significantly lower in Las-infected (111.6 ± 4.5 in head + thorax; 109.5 ± 3.7 in abdomen) than in uninfected (135.9 ± 7.5 in head + thorax; 206.1 ± 23.7 in abdomen) ACP.

CONCLUSION

Susceptibility of ACP to selected insecticides from five major chemistries was greater in Las-infected than in uninfected ACP. The lower total protein content and reduced general esterase activity in Las-infected than in uninfected ACP may partly explain the observed higher insecticide susceptibility of Las-infected ACP.