Minnesota populations of Diabrotica virgifera virgifera LeConte, the western corn rootworm, surviving Cry3Bb1-expressing corn in the field and western corn rootworm populations assumed to be susceptible to all Bt proteins were evaluated for susceptibility to Cry3Bb1, mCry3A, eCry3.1Ab, and Cry34/35Ab1 in diet assays and three different plant-based assays. Rootworm populations originating from Cry3Bb1 fields and that consistently experienced greater than expected damage had increased survival and larval growth compared to control populations assayed on Cry3Bb1 as well as mCry3a and eCry3.1Ab. Cross resistance was documented between Cry3Bb1 and both mCry3A and eCry3.1Ab as single toxins. Despite very high resistance ratios in some comparisons, cross resistance was not complete and also varied with the population being evaluated, the trait measured, and the susceptible rootworm population used for comparison. Regardless of resistance and cross resistance, all proteins, even Cry3Bb1, retained some efficacy in terms of either reducing rootworm larval growth, protecting plants from damage, or both, for all rootworm populations evaluated. For one Cry3Bb1-selected population, a resistance ratio of 9.1-fold was found to Cry34/35Ab1 when evaluating EC values relative to a susceptible control population; however, resistance to Cry34/35Ab1 was not evident in all assays in this population. The United States Environmental Protection Agency recently suggested eliminating diet assays as part of the Bt resistance monitoring process. However, given the variability of responses of western corn rootworm populations to different proteins in different assays, both plant and diet assays are needed as options for detecting and fully characterizing resistance.

Pyrethroid insecticides have been used to control larvae or adults of the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, a key pest of field corn in the United States. In response to reports of reduced efficacy of pyrethroids in WCR management programs in southwestern areas of Nebraska and Kansas the present research was designed to establish a baseline of susceptibility to the pyrethroid insecticide, bifenthrin, using susceptible laboratory populations and to compare this baseline with susceptibility of field populations. Concentration-response bioassays were performed to estimate the baseline susceptibility. From the baseline data, a diagnostic concentration (LC99) was determined and used to test adults of both laboratory and field populations. Larval susceptibility was also tested using both laboratory and field populations. Significant differences were recorded in adult and larval susceptibility among WCR field and laboratory populations. The highest LC50 for WCR adults was observed in populations from Keith 2 and Chase Counties, NE, with LC50s of 2.2 and 1.38 μg/vial, respectively, and Finney County 1, KS, with 1.43 μg/vial, as compared to a laboratory non-diapause population (0.24 μg/vial). For larvae, significant differences between WCR field and laboratory populations were also recorded. Significant differences in mortalities at the diagnostic bifenthrin concentration (LC99) were observed among WCR adult populations with western Corn Belt populations exhibiting lower susceptibility to bifenthrin, especially in southwestern Nebraska and southwestern Kansas. This study provides evidence that resistance to bifenthrin is evolving in field populations that have been exposed for multiple years to pyrethroid insecticides. Implications to sustainable rootworm management are discussed.

Recently, resistance to the pyrethroid bifenthrin was detected and confirmed in field populations of western corn rootworm, Diabrotica virgifera virgifera LeConte from southwestern areas of Nebraska and Kansas. As a first step to understand potential mechanisms of resistance, the objectives of this study were i) to assess adult mortality at diagnostic concentration-LC99 to the pyrethroids bifenthrin and tefluthrin as well as DDT, ii) estimate adult and larval susceptibility to the same compounds as well as the organophosphate methyl-parathion, and iii) perform synergism experiments with piperonyl butoxide (PBO) (P450 inhibitor) and S,S,S-tributyl-phosphorotrithioate (DEF) (esterase inhibitor) in field populations. Most of the adult field populations exhibiting some level of bifenthrin resistance exhibited significantly lower mortality to both pyrethroids and DDT than susceptible control populations at the estimated LC99 of susceptible populations. Results of adult dose-mortality bioassays also revealed elevated LC50 values for bifenthrin resistant populations compared to the susceptible control population with resistance ratios ranging from 2.5 to 5.5-fold for bifenthrin, 28 to 54.8-fold for tefluthrin, and 16.3 to 33.0 for DDT. These bioassay results collectively suggest some level of cross-resistance between the pyrethroids and DDT. In addition, both PBO and DEF reduced the resistance ratios for resistant populations although there was a higher reduction in susceptibility of adults exposed to PBO versus DEF. Susceptibility in larvae varied among insecticides and did not correlate with adult susceptibility to tefluthrin and DDT, as most resistance ratios were < 5-fold when compared to the susceptible population. These results suggest that both detoxifying enzymes and target site insensitivity might be involved as resistance mechanisms.