Resistance management of the western corn rootworm (Diabrotica virgifera virgifera) : behavior, survival and the potential for cross resistance on Bt corn in the field, greenhouse and laboratory

The Environmental Protection Agency recently registered seed blend refuges for two of the transgenic Bt corn products targeting the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte. Larval movement between Bt and isoline plants can be detrimental to resistance management for high dose Bt products because the insect larvae will potentially be exposed to sublethal amounts of the Bt, however, the effect of this movement on low to moderate dose products is unknown. All current rootworm products are low dose. The main criteria for whether movement by WCR larvae between isoline and Bt corn plants will influence the development of resistance is whether or not selection for resistance is taking place. We found that movement between isoline and SmartStax® hybrid plants did occur in seed blend scenarios in our field study. The majority of plant damage to the SmartStax plants occurred when the larvae moved from surrounding infested isoline plants moved late in their development. These older, larger larvae are all able to tolerate the Bt in the plants, therefore resistance will likely not develop in these larvae. In a similar experiment, movement also occurred between Agrisure® Duracad[trademark] and isoline plants in seed blend scenarios, however the damage was low for all treatments. With isoline plants being mixed with Bt plants in seed blend refuges, host recognition behavior of the western corn rootworm on Bt and isoline plants is also important to understand. There were no differences between the host recognition behavior of WCR larvae after exposure to mCry3A, Cry3Bb1, Cry34/35Ab1, or their isoline corn hybrids, therefore all hybrids were perceived as hosts by WCR larvae. With all the hybrids on the currently registered being pyramided by different companies to control rootworms, the potential for cross resistance between these hybrids was evaluated using field resistant and susceptible populations. Based on the data from laboratory and greenhouse assays, the potential for cross resist

Bifenthrin Baseline Susceptibility and Evaluation of Simulated Aerial Applications in \u3ci\u3eStriacosta albicosta\u3c/i\u3e (Lepidoptera: Noctuidae)

Striacosta albicosta (Smith) is a maize pest that has recently expanded its geographical range into the eastern United States and southeastern Canada. Aerial application of pyrethroids, such as bifenthrin, has been a major practice adopted to manage this pest. Reports of field failure of pyrethroids have increased since 2013. Striacosta albicosta populations were collected in 2016 and 2017 from maize fields in Nebraska, Kansas, and Canada and screened with bifenthrin active ingredient in larval contact dose-response bioassays. Resistance ratios estimated were generally low in 2016 (1.04- to 1.32-fold) with the highest LC50 in North Platte, NE (66.10 ng/cm2) and lowest in Scottsbluff, NE (50.10 ng/cm2). In 2017, O’Neill, NE showed the highest LC50 (100.66 ng/cm2) and Delhi, Canada exhibited the lowest (6.33 ng/cm2), resulting in a resistance ratio variation of 6.02- to 15.90-fold. Implications of bifenthrin resistance levels were further investigated by aerial application simulations. Experiments were conducted with a spray chamber where representative S. albicosta populations were exposed to labeled rates of a commercial bifenthrin formulation. Experiments resulted in 100% mortality for all populations, instars, insecticide rates, and carrier volumes, suggesting that levels of resistance estimated for bifenthrin active ingredient did not seem to impact the efficacy of the correspondent commercial product under controlled conditions. Results obtained from this research indicate that control failures reported in Nebraska could be associated with factors other than insecticide resistance, such as issues with the application technique, environmental conditions during and/or after application, or the insect’s natural behavior. Data generated will assist future S. albicosta resistance management programs

Cross-resistance and synergism bioassays suggest multiple mechanisms of pyrethroid resistance in western corn rootworm populations

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 \u3c 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

Evidence of Field-Evolved Resistance to Bifenthrin in Western Corn Rootworm (\u3ci\u3eDiabrotica virgifera virgifera\u3c/i\u3e LeConte) Populations in Western Nebraska and Kansas

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

WCR larval susceptibility and RRs of field and non-diapause laboratory populations to the insecticides bifenthrin, tefluthrin, DDT, and methyl parathion.

<p>Diapause eggs were collected from field populations in 2014 and 2015 (Keith populations) and bioassays performed in 2015 and 2016 (Keith populations).</p

WCR adult susceptibility of field populations to tefluthrin with and without synergists PBO (cytochrome P-450 monooxigenases inhibitor) and DEF (esterases inhibitor) and respective RR.

<p>WCR adult susceptibility of field populations to tefluthrin with and without synergists PBO (cytochrome P-450 monooxigenases inhibitor) and DEF (esterases inhibitor) and respective RR.</p