Activity of Selected Insecticides Against the Soybean Looper, Pseudoplusia Includens (Walker), Using an Artificial Diet Overlay Bioassay Technique: Dosage Mortality, Discriminating Concentrations, and Resistance Monitoring.

Abstract

Diet overlay bioassays were conducted, evaluating insecticides requiring ingestion, on 3rd instar soybean loopers, Pseudoplusia includens (Walker). Laboratory reference and field strains were exposed to several insecticide concentrations, and LC$\sb{50}$s were compared. Field strains had significantly greater LC$\sb{50}$s than the reference strain when evaluating standard insecticides. Bioassays with experimental insecticides revealed few differences among strains. The diet overlay technique appears to be a viable method for monitoring insecticide resistance to these insecticides. Insecticide discriminating concentrations were determined using diet overlay bioassays to evaluate susceptibility of field and F1 generation, laboratory and field soybean looper strains. Field strains, exposed to permethrin (Ambush$\sp\circler )$ and thiodicarb (Larvin$\sp\circler ),$ exhibited significantly greater survival compared to laboratory insects. However, survival of field strains to chlorfenapry (Pirate$\sp\circler ),$ emamectin benzoate (Proclaim$\sp\circler ),$ and spinosad (Tracer$\sp\circler ),$ was not significantly different than the reference strain. Toxicity of Bacillus thuringiensis (Bt) insecticides against a reference strain was evaluated using dosage-mortality bioassays. Costas$\sp\circler$ and Design WSP$\sp\circler$ had the lowest LC$\sb{50}$s, followed by Dipel ES$\sp\circler ,$ Condor XL$\sp\circler ,$ and Javelin WG$\sp\circler .$ LC$\sb{50}$s were highest for MVP II$\sp\circler .$ Mattch$\sp\circler ,$ MVP II$\sp\circler ,$ and Xentari$\sp\circler$ required significantly more time to affect mortality than the other Bt insecticides. Dosage-mortality bioassays were conducted on soybean loopers collected from soybean and transgenic Bt-cotton ('NuCOTN 33B', 'Hartz 1220 BG'). Field strains had LC$\sb{50}$s significantly higher than the susceptible strain, Two Bt-cotton strains (Pointe Coupee Parish and Franklin Parish, Louisiana) had LC$\sb{50}$s significantly higher than soybean strains. Soybean and Bt-cotton strains were significantly less susceptible than the reference strain, when exposed to the discriminating concentration of Condor XL$\sp\circler .$ Bt-cotton strains had significantly greater survival, indicating increased tolerance of Bt-cotton strains to Bt insecticide Condor XL$\sp\circler .$. Insecticide field trials in central and northeast Louisiana documented that Condor XL$\sp\circler$ and Larvin$\sp\circler$ provided adequate soybean looper control, although laboratory bioassays revealed differences occurring in field strains compared to the reference strain in response to these currently recommended insecticides. Experimental insecticides, Intrepid$\sp\circler ,$ Pirate$\sp\circler ,$ Proclaim$\sp\circler ,$ and Tracer$\sp\circler ,$ were effective in the field against soybean looper populations and show promise as alternatives for management if resistance develops to Larvin$\sp\circler$ or the B. thuringiensis insecticides.