Response of the Pacific Coast wireworm, Limonius canus, and the dusky wireworm, Agriotes obscurus (Coleoptera: Elateridae), to insecticide-treated wheat seeds in a soil bioassay

Abstract

Larvae of the dusky wireworm, Agriotes obscurus, and the Pacific Coast wireworm, Limonius canus, were exposed to germinated, insecticide-treated wheat seeds in a soil-filled bioassay in 2005 and 2006. Position in the bioassay and contact and/or repellence behaviour towards the seeds were recorded every 5 min for 3 or 5 h. Wireworm health was recorded for 70 or 126 d after exposure. Seeds were treated with the fungicides Dividend XLRTA (difenoconazole, mefenoxam) or Raxil MD (tebuconazole, metalaxyl), and/or the insecticides Vitavax Dual (lindane), Poncho 600F (clothianidin), Cruiser 350FS (thiamethoxam), Admire 240FS (imidacloprid), Gaucho 480FL (imidacloprid), Tefluthrin 20CS (tefluthrin), or Tefluthrin-Cruiser combinations. Most wireworms (> 80%) came into contact with the seeds in all treatments. Wireworms generally remained in contact throughout the observation period in the control treatments (Dividend, Raxil, untreated seeds). Unless moribund, wireworms were repelled after brief (< 20 min) contact in all Tefluthrin treatments. Most wireworms recovered from contact-induced morbidity within 21 d and did not relapse, except L. canus exposed to Cruiser and some A. obscurus exposed to Gaucho and Admire. Wireworm mortality was low (< 50%) in all treatments except L. canus exposed to Cruiser at 15 and 30 g a.i. 100 kg-1 seed (60 and 75%, respectively). Mortality was significantly less important when L. canus larvae were exposed to Tefluthrin-Cruiser combinations than when exposed to Cruiser alone. We suggest that efficacy assessments of insecticides for wireworm control require direct observation of their behaviour and long-term post-exposure health assessments, and discuss the impact of repellence and/or morbidity elicited by insecticides on wireworm control in the field.

Contact behaviour and mortality of wireworms exposed to six classes of insecticide applied to wheat seed

“…The proportion of L. canus wireworms placed in bioassay arenas that contacted seeds within 3 h (range 0.75-0.85; mean: 0.82; control treatment: 0.86; Table 2), and the time required to first contact seeds (range 38.4-59.3 min) did not differ significantly among treatments, and was similar to that observed in previous studies (van Herk et al 2008aHerk et al , 2010. There was no significant difference between treatments in the proportion of wireworms that had contacted seeds within 24 h (Table 2; Chi = 0.94, df = 4, P = 0.92).…”

“…A small proportion (0.11-0.18) of larvae exposed to seeds treated with bifenthrin at 20 g and tefluthrin at both 10 and 20 g were moribund at 1 DAE, but these all had recovered by 7 DAE and there was no subsequent mortality. This rapid recovery from pyrethroid intoxication was previously observed for both A. obscurus and L. canus exposed to tefluthrin at 10 or 20 g (van Herk and Vernon 2007a;van Herk et al 2008a).…”

“…The recovery from morbidity, and subsequent low incidence of mortality after exposure to thiamethoxam at 10 g alone is similar to previous results reported for both A. obscurus and L. canus larvae after exposure to thiamethoxam ? fungicide (van Herk et al 2008a).…”

“…The considerations listed above have prompted us to develop several laboratory bioassays for determining the efficacy of insecticides on various wireworm pest species, enabling us to screen candidate insecticides before using them in labour-intensive field studies and to interpret results obtained in such studies. Among these bioassays, our soil window arenas are particularly useful, in that they permit the direct observation of wireworm behaviour in response to insecticides in a soil environment, and enable us to determine their response to these insecticides when applied in the field (van Herk and Vernon 2007a;van Herk et al 2008a). In this paper, we report on the behaviour of the dusky wireworm, Agriotes obscurus L., an important pest species in Europe, Asia, and North America, and of the Pacific Coast wireworm, Limonius canus LeConte, an important pest in British Columbia and the Pacific Northwest, to a range of candidate insecticides and insecticide combinations proposed for their control.…”

Control methods and monitoring of Agriotes wireworms (Coleoptera: Elateridae)

“…Vernon et al (2008) found mortalities of A. obscurus and A. sputator dermally exposed to clothianidin and imidacloprid comparable to chlorpyrifos. However, in a soil-filled bioassay wireworm mortality was low (< 50%) when being exposed to insecticide-treated wheat seeds (clothianidin, thiamethoxam, imidacloprid) (van Herk et al 2008a). In contrast, van Herk et al (2008b) observed neither a toxic nor a repellent effect of those particular neonicotinoids in a soil-less bioassay.…”