Multiple insecticide resistance in onion thrips populations from Western USA.

Thrips tabaci is a key pest of onions, especially in the Pacific Northwestern USA. Management of T. tabaci is dominated by the application of various insecticides. However, T. tabaci is known to develop insecticide resistance which possibly leads to control failures, crop loss, and environmental concern. Here, we evaluated resistance status of T. tabaci populations from conventional and organic commercial onion fields to three widely used insecticides: oxamyl, methomyl, and abamectin with on-field concentration-mortality bioassays. The biochemistry and molecular mechanisms underlying resistance to these insecticides were also investigated by using enzymatic assays and detecting resistance-associated mutations. Field-evolved resistance to oxamyl, methomyl and abamectin were detected in most of the T. tabaci populations collected from conventional onion farms. At the labeled field rate, all the tested insecticides, particularly methomyl and oxamyl, had significantly reduced efficacy. Enzymatic assays of insecticide target and detoxification enzymes indicated that T. tabaci populations in Western USA onions harbor multiple mechanisms of resistance including enhanced activities of detoxification enzymes and target site insensitivity. Our results provide new information in understanding the dynamics of T. tabaci adaptation to multiple insecticides, which will help to design sustainable insecticide resistance management strategies for T. tabaci. Furthermore, this study provides the foundation for future research in identifying the biochemical and molecular markers associated with insecticide resistance in T. tabaci.

RNA interference of NADPH-cytochrome P450 reductase increases susceptibilities to multiple acaricides in Tetranychus urticae.

The two-spotted spider mite, Tetranychus urticae, is a polyphagous pest feeding on over 1100 plant species, including numerous highly valued economic crops. The control of T. urticae largely depends on the use of acaricides, which leads to pervasive development of acaricide resistance. Cytochrome P450-mediated metabolic detoxification is one of the major mechanisms of acaricide resistance in T. urticae. NADPH-cytochrome P450 reductase (CPR) plays as a crucial co-factor protein that donates electron(s) to microsomal cytochrome P450s to complete their catalytic cycle. This study seeks to understand the involvement of CPR/P450 in acaricide resistance in T. urticae. The full-length cDNA sequence of T. urticae's CPR (TuCPR) was cloned and characterized. TuCPR was ubiquitously transcribed in different life stages of T. urticae and the highest transcription was observed in the nymph and adult stages. TuCPR was constitutively over-expressed in six acaricide resistant populations compared to a susceptible one. TuCPR transcriptional expression was also induced by multiple acaricides in a time-dependent manner. Down-regulation of TuCPR via RNA interference (RNAi) in T. urticae led to reduced enzymatic activities of TuCPR and cytochrome P450s, as well as a reduction of resistance to multiple acaricides, abamectin, bifenthrin, and fenpyroximate. The outcome of this study highlights CPR as a potential novel target for eco-friendly control of T. urticae and other related plant-feeding pests.

Physiological resistance alters behavioral response of Tetranychus urticae to acaricides.

Multiple acaricide resistance in Tetranychus urticae continues to threaten crop production globally, justifying the need to adequately study resistance for sustainable pest management. Most studies on acaricide resistance have focused on the acute contact toxicity of acaricides with little or no information on the behavioral responses elicited after acaricide exposure. Furthermore, the impact of physiological resistance on these behavioral responses remains unknown in most pest species, including T. urticae. We tested the effect of acaricide resistance on contact toxicity, irritancy and repellency of mitochondrial electron transport inhibitor of complex I (MET-I) and mite growth inhibitor (MGI) acaricides on multiple T. urticae strains. We also tested whether acaricides with similar physiological target site/mode of action also elicit similar behavioral effects on T. urticae strains. MET-I acaricides (fenazaquin, fenpyroximate, and pyrabiden) and MGIs (clofentezine, hexythiazox and etoxazole) elicited a dose-dependent irritant and repellent effect on T. urticae. Selection of strains for physiological resistance to these acaricides affected the behavioral response of T. urticae, especially in MET-I resistant strains, that showed reduced irritancy and repellency to MET-I acaricides. Behavioral response also affected the oviposition of T. urticae, where strains generally showed preferential oviposition away from the acaricides. The outcome of this study highlights negative consequences of acaricide resistance that can potentially affect T. urticae management.

Phenotypic and Genotypic Plasticity of Acaricide Resistance in Populations of Tetranychus urticae (Acari: Tetranychidae) on Peppermint and Silage Corn in the Pacific Northwest.

Tetranychus urticae Koch is a generalist pest of economic crops and is notorious for its rapid development of acaricide resistance. This poses a significant threat to the sustainability of integrated pest management (IPM) in cropping systems plagued by T. urticae. It is critical to evaluate the resistance status of T. urticae populations on crops and identify any underlying resistance mechanisms. This study investigated the efficacy of five major acaricides on T. urticae populations on peppermint and silage corn in the Pacific Northwestern United States and identified the underlying resistance mechanisms. Significant variations in acaricide resistance status of T. urticae populations were identified to abamectin, bifenthrin, fenpyroximate, hexythiazox, and spirodiclofen. In most cases, T. urticae populations from silage corn exhibited greater levels of acaricide resistance relative to peppermint populations. We detected known target-site mutations: F1534S and F1538I (conferring resistance to bifenthrin), G126S (linked with resistance to bifenazate), and I1017 (conferring resistance to hexythiazox and etoxazole) in 10, 90, and 90% of the populations, respectively, from peppermint fields. These four mutations were identified in all the populations collected from silage corn fields. Significantly higher transcript levels of metabolic genes associated with resistance to abamectin, fenpyroximate, and spirodiclofen were observed in some T. urticae populations collected from both peppermint and silage corn fields. This study provides evidence of multiple resistance to diverse active ingredients in field populations of T. urticae and the reliability of known molecular markers for active acaricide resistance monitoring. The observed resistance pattern will help in designing a sustainable IPM program for T. urticae.

Geranium intoxication induces detoxification enzymes in the Japanese beetle, Popillia japonica Newman.

Popillia japonica is a generalist herbivore that feeds on >300 host plant species in at least 72 plant families. It is unknown why P. japonica, despite possessing active detoxification enzymes in its gut, is paralyzed when feeding on the petals of one of its preferred host plant, Pelargonium×hortorum, or on artificial diet containing quisqualic acid (QA), the active compound in zonal geranium. We hypothesized that Pelargonium×hortorum or QA do not induce activity of the cytochrome P450, glutathione S transferase (GST), and carboxylesterase (CoE) detoxification enzymes in P. japonica. In this study, P. japonica were fed petals of zonal geranium or agar plugs containing QA, or rose petals, another preferred but non-toxic host. Midgut enzyme activities of P450, GST, and CoE were then assayed after 6, 12, or 24h of feeding. In most cases, P450, GST, and CoE activities were significantly induced in P. japonica midguts by geranium petals and QA, though the induction was slower than with rose petals. Induced enzyme activity reached a peak at 24h after consumption, which coincides with the period of highest recovery from geranium and QA paralysis. This study shows that toxic geranium and QA induce detoxification enzyme activity, but the induced enzymes do not effectively protect P. japonica from paralysis by QA. Further investigation is required through in vitro studies to know if the enzymes induced by geranium are capable of metabolizing QA. This study highlights a rare physiological mismatch between the detoxification tool kit of a generalist and its preferred host.

Fungicides affect Japanese beetle Popillia japonica (Coleoptera: Scarabaeidae) egg hatch, larval survival and detoxification enzymes.

BACKGROUND: Larvae of the Japanese beetle, Popillia japonica (Coleoptera: Scarabaeidae), have a patchy distribution in soils, which complicates detection and management of this insect pest. Managed turf systems are frequently under pest pressure from fungal pathogens, necessitating frequent fungicide applications. It is possible that certain turfgrass fungicides may have lethal or sublethal adverse effects on eggs and larvae of P. japonica that inhabit managed turf systems. In this study, eggs and first-, second- and third-instar larvae were treated with the fungicides chlorothalonil and propiconazole, and survival was compared with that of untreated controls as well as positive controls treated with the insecticide trichlorfon. RESULTS: Chlorothalonil reduced survival of first-instar larvae treated directly and hatched from treated eggs. Propiconazole delayed egg hatch, reduced the proportion of eggs that successfully hatched and reduced survival of first-instar larvae treated directly and hatched from treated eggs. Sublethal doses of the fungicides lowered the activities of certain detoxification enzymes in third-instar grubs. CONCLUSIONS: Fungicide applications to turfgrass that coincide with oviposition and egg hatch of white grubs may have sublethal effects. This work is applicable both to high-maintenance turfgrass such as golf courses, where applications of pesticides are more frequent, and to home lawn services, where mixtures of multiple pesticides are commonly used.