Scott-Dupree et al. 



JESO Volume 139, 2008 



Discussion 



In 2006, the July adult peaks noted in Norfolk County likely represented migratory 

 adults, while that in August comprised the 1st generation arising from those adults (Fig. 1). 

 McClanahan and Elliott (1976) reported that VCW had 3 peaks in Essex County in July, 

 August and September suggesting that it had 2 generations in that area. Although adult 

 collections were low at the Essex County sites in 2006, results were similar (Fig. 1). It 

 appears that the warmer climatic conditions in Essex as compared to Norfolk County allow 

 2nd generation VCW to complete development. 



The 2 pyrethroid insecticides, lambda-cyhalothrin and permethrin were the most 

 toxic of the insecticides commonly used for cutworm control. Of the 3 reduced risk 

 insecticides tested, chlorantraniliprole was at least as toxic as lambda-cyhalothrin (Table 1) 

 suggesting that it might have a role in VCW IPM programs. Spinosad and metaflumizone 

 were less toxic by direct contact, however it is known that both insecticides act primarily 

 as stomach poisons. Results obtained with methomyl, chlorpyrifos and permethrin were 

 compared with some obtained under identical bioassay conditions by Harris et al. (1977) 

 (Table 2). VCW appears to have developed a relatively low level of resistance to methomyl, 

 possibly due to exposure of the migrant population to use of that chemical on numerous 

 host crops in North America. Results obtained with chlorpyrifos in the 2 years showed no 

 indication of significant resistance development to that chemical. 



While it has been suggested that unsatisfactory VCW control with permethrin on 

 tomatoes could be due to resistance, results obtained with the 2006 Ontario strain were 

 identical to those reported 30 years earlier (Harris et al., 1977). An explanation for less 

 than acceptable control with permethrin can be found in past research which has shown 

 that several factors influence the effectiveness of insecticides applied for VCW control 

 including stage of larval development, climatic conditions and application method. Stage 

 of larval development is very important. Similar to the results of this study, Harris et 

 al. (1977) showed that, while 1st instar VCW were very susceptible, 3rd and 5th instar 

 larvae were 2.9x and 3.7x more tolerant to permethrin, respectively. Harris and Kinoshita 

 (1977) demonstrated that, like DDT, pyrethroid toxicity can be negatively correlated with 

 temperature - permethrin was 7.4x more toxic to 3rd instar VCW at 15° as compared to 

 32°C. Finally application method has a major role. For example, VCW spends the larval 

 stage on, or very close to, the soil surface hidden under dense foliage. While soil surface 

 applications with permethrin were effective, foliar applications were more so (Harris and 

 Svec 1968; Harris et al. 1977). 



Results of this study show that the pyrethroid insecticides remain highly toxic to 

 the Ontario VCW populations found in southern Ontario and that there is no evidence of 

 resistance to permethrin. Lack of insecticide effectiveness in the field is undoubtedly due 

 to limited mobility of the cutworms once they become established under the plant canopy. 

 To achieve adequate control, insecticides should be applied when early instar larvae are 

 present, under moderate to cool climatic conditions in a manner resulting in the most 

 effective penetration of the plant canopy as possible. 



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