Larval susceptibility of an insecticide-resistant western corn rootworm (Coleoptera : Chrysomelidae) population to soil insecticides: Laboratory bioassays, assays of detoxification enzymes, and field performance

Soil insecticides were evaluated in laboratory and field studies against la rvae of an insecticide resistant population (Phelps County, NE) of western corn rootworm, Diabrotica virgifera virgifera LeConte. Insecticide toxicity was evaluated by topical application of technical insecticides to 3rd inst ars from Saunders County, NE (susceptible) and Phelps County populations. R esistance ratios (LD50 Phelps County/LD50 Saunders County) for the insectic ides methyl parathion, tefluthrin, carbofuran, terbufos, and chlorpyrifos w ere 28.0, 9.3, 8.7, 2.6 and 1.3, respectively. Biochemical investigation of suspected enzymatic resistance mechanisms in 3rd instars identified signif icant elevation of esterase activity (alpha and, beta naphthyl acetate hydr olysis [3.8- and 3.9-fold]). Examination of 3rd instar esterases by native PAGE identified increased intensity of several isoenzymes in the resistant population. Assays of cytochrome P-450 activity (4-CNMA demethylation and a ldrin epoxidation) did not identify elevated. activity in resistant 3rd ins tars. Granular soil insecticides were applied at planting to corn, Zen mays L., in replicated field trials in 1997 and 1998 at the same Phelps County site as the source of resistant rootworms for the laboratory studies. In 19 97, planting time applications of Counter 20CR, Counter 15 C (terbufos), an d Lorsban 15 G (chlorpyrifos) resulted in the lowest root injury ratings (1 -6 Iowa scale); 2.50, 2.55, 2.65, respectively (untreated check root rating of 4.55). In 1998, all insecticides performed similarly against a lower ro otworm density (untreated check root rating of 3.72). These studies suggest that resistance previously documented in adults also is present in 3rd ins tars, esterases are possibly involved as resistance mechanisms, and resista nce to methyl parathion in adults is also evident in larvae, but does not c onfer cross-resistance in larvae to all organophosphate insecticides.