Effect of Piperonyl Butoxide on the Toxicity of Four Classes of Insecticides to Navel Orangeworm (Amyelois transitella) (Lepidoptera: Pyralidae)

Desiccant Dusts, With and Without Bioactive Botanicals, Lethal to Rhipicephalus (Boophilus) microplus Canestrini (Ixodida: Ixodidae) in the Laboratory and on Cattle

The exotic southern cattle fever tick, Rhipicephalus (Boophilus) microplus (Canestrini) (Ixodida: Ixodidae), since its eradication from the United States in 1943, made a strong incursion into Texas, beginning 2016. The pest is arguably the most economically detrimental ectoparasite of cattle, Bos taurus L., worldwide. Current R. (B.) microplus control mostly relies on conventional synthetic acaricides to which the ixodid has been developing resistance. Our study demonstrates that commercially available desiccant dust products, with and without bioactive botanical additives, are strongly lethal, when applied dry, against larval R. (B.) microplus in the laboratory, and after being released on dust-treated cattle. Deadzone (renamed Celite 610, a diatomaceous earth product), Drione (silica gel + pyrethrins + piperonyl butoxide synergist), and EcoVia (silica gel + thyme oil), each prophylactically prevented larval R. (B.) microplus from attaching to and feeding on stanchioned calves. Desiccant dust-based products are less likely than conventional synthetic acaricides to decline in terms of efficacy as a result of ixodid resistance, and other desiccant dust advantages, including extended residual, flexibility in terms of application methods, environmental, animal, and human safety, and possible compatibility with organic, or 'green', production systems, are discussed. We anticipate that the desiccant dusts we evaluated, and others not included in this study (e.g., kaolin, perlite, and silica gel) will be effective when used with other control tactics in integrated pest management approaches for controlling R. (B.) microplus (and other ixodid species).

Lethal Effects of Commercial Kaolin Dust and Silica Aerogel Dust With and Without Botanical Compounds on Horn Fly Eggs, Larvae, Pupae, and Adults in the Laboratory

The horn fly, Haematobia irritans irritans (L.) (Diptera: Muscidae), is an important bloodsucking ectoparasite of cattle throughout much of the world. The fly is mostly controlled using conventional synthetic insecticides but as concerns about resistance increase, alternative tactics have come under heightened scrutiny. Four desiccant dust products: Surround WP, a kaolin clay-based wettable powder; CimeXa, comprised of silica aerogel; Drione, silica aerogel + pyrethrins; and EcoVia, silica aerogel + thyme oil, were assessed for their lethal effects against horn fly eggs, larvae, pupae, and adults, under laboratory conditions. Although Surround WP and CimeXa did not prevent egg hatching and (when mixed with manure substrate) pupal development, the two products were associated with moderate reductions of emerged adults, and with complete adult contact mortality within 6 hr and 24 hr, respectively. Drione and EcoVia eliminated egg hatching, pupal development, and adults within 15 min to 1 hr, respectively, whether the flies were exposed to treated filter paper substrate or exposed by immersion in the dusts. Implications for horn fly control and advantages of inert desiccant dust formulations are discussed.

Differential regulation of cytochrome P450 genes associated with biosynthesis and detoxification in bifenthrin-resistant populations of navel orangewom (Amyelois transitella)

Pyrethroid resistance was first reported in 2013 for the navel orangeworm, Amyelois transitella, but the genetic underpinnings of pyrethroid resistance are unknown. We investigated the role of cytochrome P450 monooxygenases (P450s) belonging to the CYP3 and CYP4 clans using colonies derived from individuals collected in 2016 from almond orchards in two counties. One colony (ALM) originated from an almond orchard in Madera County with no reported pyrethroid resistance and the second colony (R347) originated from the same Kern County orchard where pyrethroid resistance was first reported. We used high-throughput quantitative real-time PCR (qRT-PCR) analyses of 65 P450s in the CYP3 and CYP4 clans of A. transitella to identify P450s induced by bifenthrin and associated with pyrethroid resistance. Nine P450s were constitutively overexpressed in R347 compared to ALM, including CYP6AE54 (11.7-fold), belonging to a subfamily associated with metabolic pesticide detoxification in Lepidoptera and CYP4G89 (33-fold) belonging to a subfamily associated with cuticular hydrocarbon (CHC) synthesis and resistance via reduced pesticide penetrance. Cuticular hydrocarbons analysis revealed that R347 produced twice as many total CHCs in the egg and adult stages as ALM. Topical toxicity bioassays for R347 determined that egg mortality was reduced at low bifenthrin concentrations and larval mortality was reduced at high concentrations of bifenthrin compared to ALM. Our discovery of both changes in metabolism and production of CHCs for R347 have implications for the possible decreased efficacy of other classes of insecticide used to control this insect. The threat of widespread pyrethroid resistance combined with the potential for cross-resistance to develop through the mechanism of reduced penetrance warrants developing management strategies that facilitate insecticide passage across the cuticle.

Lethal Effects of a Silica Gel + Pyrethrins (Drione) on Amblyomma americanum (Ixodida: Ixodidae) Larvae and Nymphs

Ixodids (hard ticks) ingest blood from host animals, and they can transmit pathogenic organisms that induce medical and veterinary diseases. As resistance to synthetic conventional acaricides becomes more common, alternative tactics are coming under heightened scrutiny. Laboratory bioassays were used to assess the efficacy of CimeXa, a commercially available silica gel desiccant dust product, and Drione, a commercial product containing silica gel + pyrethrins and a synergist, piperonyl butoxide, against lone star tick, Amblyomma americanum (L.) (Ixodida: Ixodidae), larvae and nymphs. Both life stages were completely killed by CimeXa by 24 h, and Drione caused total larval mortality within 1 h when they were briefly immersed in the dusts and when they crawled across dust-treated substrate; nymphs were completely killed by 4 h after the same kinds of exposure. Mortality of A. americanum larvae and nymphs occurred after the pests crawled across dried aqueous suspensions of the products, but this was not as efficient and fast-acting as when the immature life stages were exposed to dry dusts. Further, dried aqueous suspensions of Drione were not substantially more lethal than dried aqueous suspensions of CimeXa. CimeXa and Drione will provide prophylactic control on vegetation and animals for as long as the silica gel remains without being physically removed. Both of the dust-based products will likely also be effective against other problematic ixodid species. Advantages and disadvantages, and potential uses, of desiccant dust-based acaricides are discussed.

Effects of Silica-Based CimeXa and Drione Dusts Against Lone Star Tick (Ixodida: Ixodidae) on Cattle

Ixodid ticks vector pathogens of humans, livestock, and wildlife, and occur in tropical and temperate regions worldwide. We used the lone star tick, Amblyomma americanum (L.) (Ixodida: Ixodidae), as a model for other ixodid species, including the southern cattle fever tick, Rhipicephalus (Boophilus) microplus (Canestrini) (Ixodida: Ixodidae). We assessed the lethality of CimeXa, an inert silica-based desiccant dust, and Drione, a silica-based desiccant dust with pyrethrin and piperonyl butoxide (PBO), against A. americanum larvae and nymphs on calves. CimeXa was highly lethal to larvae released onto shaved calf skin 72-96 h previously, and moderately toxic to nymphs. It did not affect immature life stages treated after they began feeding because desiccation was offset by ingestion of blood. Being inert, CimeXa can protect cattle from infestation before feeding occurs for a potentially indefinite period of time. Drione killed immatures within 24 h when released on treated skin and when applied to feeding ixodids because of the pyrethrin and PBO (PBO will also reduce resistance to pyrethrin if it develops). When CimeXa was applied to calves with the hair intact, ≈90% and ≈70% of A. americanum larvae and nymphs, respectively, were killed within 24 h and Drione killed 100% of each life stage. Drione also prevented and drastically reduced egg production and hatching. While CimeXa protects against infestation by immatures, Drione will also control ticks that are feeding. When pyrethrin and PBO in Drione eventually degrade, the silica aerogel carrier can continue to protect against infestation indefinitely.

Amidase, a novel detoxifying enzyme, is involved in cyflumetofen resistance in Tetranychus cinnabarinus (Boisduval)

Amidase is an important hydrolytic enzyme in detoxification metabolism. Amidase hydrolyzes a wide variety of nonpeptide carbon‑nitrogen bonds by attacking a cyano group or carbonyl carbon. However, little is known about the relationship between amidase and insecticides. In this study, the amidase activity was significantly higher in cyflumetofen-resistant strain (CyR) than in the susceptible strain (SS) of Tetranychus cinnabarinus, and diethyl-phosphoramidate (an amidase inhibitor) significantly decreased cyflumetofen resistance in T. cinnabarinus. More importantly, an amidase gene, TcAmidase01, was identified in T. cinnabarinus, and the TcAmidase01 overexpression was detected in both two cyflumetofen-resistant strains (CyR and YN-CyR), indicating that it is involved in cyflumetofen resistance in mites. A phylogenetic analysis showed that TcAmidase01 was clustered with deaminated glutathione amidases, which possess hydrolytic activity. The recombinant TcAmidase01 protein showed amidase activity toward succinamate, and the activity could be inhibited by cyflumetofen. High-performance liquid chromatography-mass spectrometry (HPLC-MS) analysis provided evidence that recombinant TcAmidase01 could decompose cyflumetofen by hydrolysis, and the potential metabolites (2-(4-(tert-butyl) phenyl)-2-cyanoacetate and 2-(trifluoromethyl) benzoic acid) were identified. These results show that TcAmidase01 contribute to cyflumetofen-resistance in T. cinnabarinus by hydrolyzing cyflumetofen, and this is the first study to suggest that amidase has a role in insecticides resistance in arthropods.

Factors Associated with Variation in Cuticular Hydrocarbon Profiles in the Navel Orangeworm, Amyelois transitella (Lepidoptera: Pyralidae)

Cuticular hydrocarbons (CHCs) are the main components of the epicuticular wax layer that in many insects functions as a barrier against desiccation. CHCs also play many other roles, including serving as sex pheromones, kairomones, primer pheromones, and colony-, caste-, species- and sex-recognition signals. In insects, CHC profiles can vary depending upon age, species, sex, and strain. Understanding factors associated with variation in hydrocarbon profiles is important for identifying potential vulnerabilities relating to pest ecology and life histories and for developing tools for pest monitoring and management strategies. In this study, we assessed potential sources of variation in CHC profiles in the navel orangeworm Amyelois transitella (Walker) (Lepidoptera: Pyralidae), an economically important pest of nut crops in California. Using coupled gas chromatography-mass spectrometry, we characterized and compared CHC profiles between adults of pyrethroid-resistant (R347) and susceptible (ALMOND) strains. We further compared CHC profiles from adults differing in age (1, 3, 5, and 7 d post-eclosion) and sex. Hydrocarbon profiles comprised 47 different CHCs in detectable quantities that ranged from C₁₇ to C₄₃ in chain length and included straight-chain alkanes and a variety of mono-, di-, and tri-methylalkanes. Adults from resistant populations had greater quantities of CHCs in total than those from susceptible strains, but relative quantities of individual components were similar. The six most abundant compounds were n-pentacosane, n-heptacosane, n-nonacosane, n-hentriacontane, 11,25 + 13,23 + 15,21-dimethylpentatriacontane, and 13,23 + 11,25 + 9,17-dimethylheptatriacontane. Post-eclosion, total CHCs increased with adult age, with males producing greater quantities than females at all ages. Our results show that CHC profiles vary depending on age, sex, and strain and suggest that CHC profiles may be useful as biomarkers to differentiate between insecticide- resistant and susceptible populations.

Accelerated Development and Toxin Tolerance of the Navel Orangeworm Amyelois transitella (Lepidoptera: Pyralidae) in the Presence of Aspergillus flavus

The navel orangeworm (Amyelois transitella) and the fungus Aspergillus flavus constitute a facultative mutualism and pest complex in tree nut and fruit orchards in California. The possibility exists that the broad detoxification capabilities of A. flavus benefit its insect associate by metabolizing toxicants, including hostplant phytochemicals and pesticides. We examined this hypothesis by conducting laboratory bioassays to assess growth rates and survivorship of pyrethroid-resistant (R347) and susceptible (CPQ) larval strains on potato dextrose agar diet containing almond meal with and without two furanocoumarins, xanthotoxin and bergapten, found in several hostplants, and with and without two insecticides, bifenthrin and spinetoram, used in almond and pistachio orchards. Additionally, fungi were incubated in liquid diets containing the test chemicals, and extracts of these diets were added to almond potato dextrose agar (PDA) diets and fed to larvae to evaluate the ability of the fungus to metabolize these chemicals. Larvae consuming furanocoumarin-containing diet experienced higher mortality than individuals on unamended diets, but adding A. flavus resulted in up to 61.7% greater survival. Aspergillus flavus in the diet increased development rate > two-fold when furanocoumarins were present, demonstrating fungal enhancement of diet quality. Adding extracts of liquid diets containing xanthotoxin and fungus decreased mortality compared to xanthotoxin alone. On diets containing bifenthrin and spinetoram, however, mortality increased. These results support the hypothesis that A. flavus enhances navel orangeworm performance and contributes to detoxification of xenobiotics. Among practical implications of our findings, this mutualistic association should be considered in designing chemical management strategies for these pests.

Monitoring and biochemical characterization of beta-cypermethrin resistance in Spodoptera exigua (Lepidoptera: Noctuidae) in Sichuan Province, China

The beet armyworm Spodoptera exigua, a major pest affecting numerous cultivated crops in China, has developed a serious resistance to many traditional chemical insecticides. The resistance levels of the field-collected populations from different districts in Sichuan Province, China, to nine insecticides were detected with a diet-incorporation bioassay. Compared to the Lab-ZN strain, five (in 2014) and three (in 2016) field populations displayed either high or extremely high levels of resistance to beta-cypermethrin. All the field populations collected in 2014 were susceptible to emamectin benzoate, hexaflumuron, methoxyfenozide, chlorantraniliprole, cyantraniliprole and indoxacarb but exhibited low or moderate levels of resistance to abamectin. The resistances of field populations collected in 2016 were significantly higher than two years earlier, especial for chlorantraniliprole and cyantraniliprole with RRs rising from 173.4- to 582.6-fold and 175.3- to 287.6-fold, respectively, even though the field populations had retained moderate or low levels of resistance to chlorpyrifos and hexaflumuron. The synergism experiment revealed that the resistance of the LS16 population to beta-cypermethrin may be mainly related to cytochrome P450 monooxygenases (P450s), which was responsible for the highest increase ratio of 37.97-fold, for piperonyl butoxide, rather than either carboxylesterase (CarE) or glutathione S-transferase (GST). The cytochrome P450 ethoxycoumarin O-deethylase activity of the LS16 population was also the strongest among the treatments (P < 0.05). Non-denaturing polyacrylamide gel electrophoresis (native PAGE) indicated that enhanced E11, E13 and E15-E16 bands in the LS16 population likely contribute to the development of resistance to beta-cypermethrin.

Can Toxicokinetic and Toxicodynamic Modeling Be Used to Understand and Predict Synergistic Interactions between Chemicals?

Some chemicals are known to enhance the effect of other chemicals beyond what can be predicted with standard mixture models, such as concentration addition and independent action. These chemicals are called synergists. Up until now, no models exist that can predict the joint effect of mixtures including synergists. The aim of the present study is to develop a mechanistic toxicokinetic (TK) and toxicodynamic (TD) model for the synergistic mixture of the azole fungicide, propiconazole (the synergist), and the insecticide, α-cypermethrin, on the mortality of the crustacean Daphnia magna. The study tests the hypothesis that the mechanism of synergy is the azole decreasing the biotransformation rate of α-cypermethrin and validates the predictive ability of the model on another azole with a different potency: prochloraz. The study showed that the synergistic potential of azoles could be explained by their effect on the biotransformation rate but that this effect could only partly be explained by the effect of the two azoles on cytochrome P450 activity, measured on D. magna in vivo. TKTD models of interacting mixtures seem to be a promising tool to test mechanisms of interactions between chemicals. Their predictive ability is, however, still uncertain.

Over-expression of UDP-glycosyltransferase gene UGT2B17 is involved in chlorantraniliprole resistance in Plutella xylostella (L.)

BACKGROUND

UDP-glycosyltransferases (UGTs) are phase II detoxification enzymes widely distributed within living organisms. Their involvement in the biotransformation of various lipophilic endogenous compounds and phytoalexins in insects has been documented. However, the roles of this enzyme family in insecticide resistance have rarely been reported. Here, the functions of UGTs in chlorantraniliprole resistance in Plutella xylostella were investigated.

RESULTS

Treatment with sulfinpyrazone and 5-nitrouracil (both inhibitors of UGT enzymes) significantly increased the toxicity of chlorantraniliprole against the third instar larvae of P. xylostella. Among the 23 UGT transcripts examined, only UGT2B17 was found to be over-expressed (with a range from 30.7- to 77.3-fold) in all four chlorantraniliprole-resistant populations compared to the susceptible one (CHS). The knock-down of UGT2B17 by RNA interference (RNAi) dramatically increased the toxicity of chlorantraniliprole by 27.4% and 29.8% in the CHS and CHR (resistant) populations, respectively. In contrast, exposure to phenobarbital significantly increased the relative expression of UGT2B17 while decreasing the toxicity of chlorantraniliprole to the larvae by 14.0%.

CONCLUSION

UGT2B17 is involved in the detoxification of chlorantraniliprole, and its over-expression may play an important role in chlorantraniliprole resistance in P. xylostella. These results shed some light upon and further our understanding of the mechanisms of diamide insecticide resistance in insects. © 2016 Society of Chemical Industry.

Variable Pyrethroid Susceptibility Among the Sweet Corn-Infesting Ulidiidae (Diptera) in Florida and New Baseline Susceptibilities

Florida sweet corn is intensively treated to prevent infestation from the corn-infesting picture-winged fly complex (hereafter referred to as silk flies, Diptera: Ulidiidae). Previous bioassays performed on Euxesta stigmatias Loew demonstrated some pyrethroids performed weakly, while others were more efficacious and with longer-lasting residual activity. Since the last published bioassays, new active ingredients have been made available, other species in the complex discovered, and label restrictions increased for some products. For these reasons, topical bioassays were performed on the three most common species to assess insecticide efficacy of current commercial products labeled for either silk fly or fall armyworm ( Spodoptera frugiperda (J.E. Smith), Lepidoptera: Noctuidae) control. Bioassays were conducted using formulated product mixed in water and applied using a Generation III Research Spray Booth. The median lethal concentration ratio (LC 50 ) of beta-cyfluthrin with and without the pyrethroid synergist piperonyl butoxide was investigated. Acetamiprid, chlorantraniliprole, carbaryl, and flubendiamide did not result in high mortality to any species tested. Euxesta eluta Loew was susceptible to all other insecticides tested, and exhibited the lowest LC 50 to beta-cyfluthrin. Both Chaetopsis massyla Walker and E. stigmatias recovered from several pyrethroid treatments. Euxesta stigmatias also had the highest beta-cyfluthrin LC 50 , and piperonyl butoxide restored beta-cyfluthrin efficacy and lowered the LC 50 s of all three species.

Evidence for Metabolic Pyrethroid Resistance in the Common Bed Bug (Hemiptera: Cimicidae)

Resistance to insecticides, especially the pyrethroids, in the common bed bug, Cimex lectularius L., has been well-documented. However, the presence and relative contribution of metabolic detoxifying microsomal oxidases and hydrolytic esterases to the observed resistance has yet to be fully elucidated. This is due, in part, to the absence of a simple bioassay procedure that appropriately isolates esterases from potentially competing oxidases. Recently, an analogue of piperonyl butoxide (PBO) was developed, EN16/5-1 (6-[2-(2-butoxyethoxy)ethoxymethyl]-5-propyl-2,3-dihydrobenzofuranby), which inhibits esterases but has limited efficacy against the oxidases, whereas PBO inhibits both. The opportunity is now available to use both synergists via established bioassay methodologies and to screen for the potential presence of oxidase- or esterase-derived pyrethroid resistance in insecticide-resistant insects, including bed bugs. In the present study, EN16/5-1 and PBO were assayed in conjunction with deltamethrin against four field strains of C. lectularius collected from independent geographic locations across Australia. All strains expressed a high degree of resistance to deltamethrin and significant inhibition of the observed resistance with preexposure to PBO. Nonsignificant differences between the cumulative mortality values for PBO and EN16/5-1 were then observed in two of the four bed bug strains, which indicate that detoxifying esterases are conferring substantially to the observed resistance in those strains. This study is the first to provide evidence that metabolic detoxification in the form of both hydrolytic esterases and microsomal oxidases is a major contributing factor to pyrethroid resistance in C. lectularius.

Impact of Pesticide Resistance on Toxicity and Tolerance of Hostplant Phytochemicals in Amyelois Transitella (Lepidoptera: Pyralidae)

For some polyphagous insects, adaptation to phytochemically novel plants can enhance resistance to certain pesticides, but whether pesticide resistance expands tolerance to phytochemicals has not been examined. Amyelois transitella Walker (navel orangeworm) is an important polyphagous pest of nut and fruit tree crops in California. Bifenthrin resistance, partially attributable to enhanced cytochrome P450 (P450)-mediated detoxification, has been reported in an almond-infesting population exposed to intense pesticide selection. We compared the toxicity of bifenthrin and three phytochemicals-chlorogenic acid, and the furanocoumarins xanthotoxin and bergapten-to three strains of A. transitella: pyrethroid-resistant R347 (maintained in the laboratory for ∼10 generations), fig-derived FIG (in the laboratory for ∼25 generations), and CPQ-a laboratory strain derived from almonds ∼40 years ago). Whereas both Ficus carica (fig) and Prunus dulcis (almond) contain chlorogenic acid, furanocoumarins occur only in figs. Both R347 and FIG exhibited 2-fold greater resistance to the three phytochemicals compared with CPQ; surprisingly, bifenthrin resistance was highest in FIG. Piperonyl butoxide, a P450 synergist, increased toxicity of all three phytochemicals only in CPQ, implicating alternate tolerance mechanisms in R347 and FIG. To test the ability of the strains to utilize novel hostplants directly, we compared survival on diets containing seeds of Wisteria sinensis and Prosopis pallida, two non-host Fabaceae species; survival of FIG was highest and survival of R347 was lowest. Our results suggest that, while P450-mediated pesticide resistance enhances tolerance of certain phytochemicals in this species, it is only one of multiple biochemical adaptations associated with acquiring novel hostplants.