A strain of beet armyworm,
Spodoptera exigua (Hubner), designated ICI-DOW is proposed as a standard susceptible strain to all insecticides. Strains were combined from ICI (now Syngenta) and DOW. Discriminating doses and resistance ratios can be established for each insecticide when LD50s and consistent sizes of 95% confidence intervals are determined for the susceptible strain.
Range of
LD50s or an LD50 was also determined for strains considered to be susceptible obtained from USDA-ARS laboratories in
Phoenix, AZ,
Stoneville, MS and Orange County,
CA. Range of LD50s or an LD50 for ICI-DOW strain for methomyl, methyl parathion, chlorpyrifos, emamectin benzoate, fenvalerate, permethrin, cypermethrin, profenofos, bifenthrin, cyfluthrin,
lambda cyhalothrin and deltamethrin were 0.00094 to 0.8
µg/larva, 0.12 to 0.16 µg/larva, 0.00002 to 0.48 µg/larva, 0.008 to 0.064 µg/larva, 0.037 µg/larva, 0.0086 µg/larva, 0.00021 µg/larva, 0.00029 µg/larva, 0.0013 µg/larva, 0.0044 µg/larva, 0.0091 µg/larva and 0.0034 µg/larva for this strain, respectively. Chlorpyrifos showed a 24,000 difference while methomyl showed a 5,511 difference. The strain is not homozygous for susceptibility to chlorpyrifos and methomyl because of these large differences in LD50s, but the strain was still susceptible. In 1990 the strain from Phoenix, AZ, maintained in the laboratory for >20y, showed an LD50 for methomyl of 11.23 µg/larva; in 1995 an LD50 of 0.15 µg/larva was determined, a 75 fold difference. This strain was resistant to methomyl in 1990, but susceptible in 1996. LD50s for a strain from Orange County, CA were 0.053, 9.37, 0.55 and 0.97 µg/larva for fenvalerate, methomyl, methyl parathion and permethrin, respectively. Strain was resistant to methomyl but susceptible to the other insecticides. When reared from individual egg masses 0% to 86% mortality with four doses of methomyl and 0% to 100% mortality with three doses of fenvalerate and esfenvalerate were determined. Heterozygosity was exhibited in the ICI-DOW from larvae reared from individual egg masses.
