Extended Sentinel Monitoring of Helicoverpa zea Resistance to Cry and Vip3Aa Toxins in Bt Sweet Corn: Assessing Changes in Phenotypic and Allele Frequencies of Resistance.

Transgenic corn and cotton that produce Cry and Vip3Aa toxins derived from Bacillus thuringiensis (Bt) are widely planted in the United States to control lepidopteran pests. The sustainability of these Bt crops is threatened because the corn earworm/bollworm, Helicoverpa zea (Boddie), is evolving a resistance to these toxins. Using Bt sweet corn as a sentinel plant to monitor the evolution of resistance, collaborators established 146 trials in twenty-five states and five Canadian provinces during 2020-2022. The study evaluated overall changes in the phenotypic frequency of resistance (the ratio of larval densities in Bt ears relative to densities in non-Bt ears) in H. zea populations and the range of resistance allele frequencies for Cry1Ab and Vip3Aa. The results revealed a widespread resistance to Cry1Ab, Cry2Ab2, and Cry1A.105 Cry toxins, with higher numbers of larvae surviving in Bt ears than in non-Bt ears at many trial locations. Depending on assumptions about the inheritance of resistance, allele frequencies for Cry1Ab ranged from 0.465 (dominant resistance) to 0.995 (recessive resistance). Although Vip3Aa provided high control efficacy against H. zea, the results show a notable increase in ear damage and a number of surviving older larvae, particularly at southern locations. Assuming recessive resistance, the estimated resistance allele frequencies for Vip3Aa ranged from 0.115 in the Gulf states to 0.032 at more northern locations. These findings indicate that better resistance management practices are urgently needed to sustain efficacy the of corn and cotton that produce Vip3Aa.

Impact of insecticide programs on pests, the predatory mite Phytoseiulus persimilis, and staked tomato profitability.

BACKGROUND: Various insecticides are available to manage diverse pest complexes in commercial vegetable production, but knowledge gaps exist regarding their overall performance in pest suppression, profitability, and compatibility with biological control. We conducted trials in staked tomatoes in western North Carolina in 2017-2018 to compare how different insecticide programs managed key pests and their interactions with Phytoseiulus persimilis Athias-Henriot, a predator of the twospotted spider mite (TSSM, Tetranychus urticae Koch). Treatments compared no insecticides to broad-spectrum ('hard') foliar applications, selective ('soft') foliar applications, and to chemigation of selective systemic insecticides. Treatment efficacy was based on pest control, net profitability, and Environmental Impact Quotient (EIQ) ratings comparing environmental and human health risks. RESULTS: All programs similarly suppressed aphids, while flower thrips and flea beetle populations were low and unaffected by program. Only the 'hard' foliar program suppressed TSSM (including P. persimilis), which quickly rebounded. This program inhibited biological control, and eventual TSSM suppression was likely caused by P. persimilis immigrating from nearby plots. All programs were generally equivalent in reducing fruit damage. Net profits were similar among insecticide programs, which were significantly greater than in untreated plots. Yield and profit trends were similar both years, but impacted by record flooding in 2018. CONCLUSION: Safer and more selective chemigation and 'soft' foliar programs were as effective and economical as the 'hard' foliar program, while also conserving biological control. These results support existing research demonstrating that improved insecticide use can be integrated with biological control for more profitable and environmentally sustainable vegetable production. © 2022 Society of Chemical Industry.

Comparison of chemigation versus foliar insecticide use: management of lepidopteran larvae and stink bugs in North Carolina field tomatoes with environmental and farmworker benefits.

BACKGROUND: Commercial vegetable production in the United States of America (USA) often relies on foliar insecticide sprays for managing key insect pests. However, foliar applications of insecticides have a number of drawbacks to the health of consumers, farmworkers and the environment. Drip chemigation is the application of pesticides to the soil through trickle (drip) irrigation systems, and can overcome a number of the drawbacks typical of foliar insecticide applications. RESULTS: We conducted a two-year study in five commercial fields of staked tomatoes in western North Carolina to compare the efficacy, economics and environmental impact of drip chemigation versus foliar sprays. Drip chemigation significantly reduced insecticide inputs, utilized more selective and environmentally compatible insecticides, and reduced the time lost to reentry intervals, while maintaining comparable efficacy and economic returns. CONCLUSIONS: Drip chemigation was an effective tool for managing key insect pests, provided a broad range of human and environmental health benefits, and will likely become increasingly cost-effective in the future as insecticide patents expire and more insecticide options become available.

Compatibility of Bifenazate and Phytoseiulus persimilis for Management of Twospotted Spider Mites in North Carolina Staked Tomatoes.

The twospotted spider mite (TSSM, Tetranychus urticae Koch) is a key pest of tomatoes (Solanum lycopersicon L. [Solanales: Solanaceae]) in North Carolina, and its management has relied principally on synthetic acaricides. Augmentative biological control of TSSM is a commonplace and effective management strategy in greenhouses worldwide, but in field-grown vegetable crops biocontrol of TSSM is poorly developed. We conducted small-plot field experiments in 2016 and 2019 to test the ability of the predatory mite, Phytoseiulus persimilis Athias-Henriot, to establish, disperse, and reduce TSSM in staked tomatoes, as well as test their compatibility with a commonly used reduced-risk acaricide, bifenazate (Acramite). Across both years, the most effective treatment for reducing TSSM was the combination of P. persimilis + Acramite. Phytoseiulus persimilis successfully established both years, but its population growth and dispersal were greater in 2016 than 2019. Acramite alone significantly reduced TSSM pressure in 2019 and P. persimilis alone did not reduce TSSM pressure below the control in either year. However, results in 2016 were influenced by the eventual dispersal of P. persimilis into all experimental plots, despite efforts to hinder their movement with corn barriers. Results in 2019 were influenced by the late-season invasion of TSSM into the trial, which delayed P. persimilis releases and influenced their establishment and growth. This study is the first to demonstrate the utility and limitations of P. persimilis in staked field tomatoes, which is a promising option for growers to manage TSSM in tomatoes in the southeast United States.

Evaluation of Insecticide Thresholds in Late-Planted Bt and Non-Bt Corn for Management of Fall Armyworm (Lepidoptera: Noctuidae).

The fall armyworm, Spodoptera frugiperda (J. E. Smith), is a major pest of corn in North and South America. It is managed primarily with transgenic corn-producing insecticidal proteins of Bacillus thuringiensis (Bt), but the development of resistance threatens their durability and necessitates the use of alternative management strategies. We conducted late-planted field trials during 2016 and 2017 in South Carolina using natural infestations. We evaluated the use of Bt and non-Bt corn hybrids in combination with foliar applications of chlorantraniliprole at varying infestation thresholds to protect field corn from infestation and damage and determine effects on grain yield. All Bt hybrids were more effective at reducing fall armyworm infestation rates and leaf injury than multiple insecticide sprays, and no Bt hybrid reached the lowest infestation threshold (20%) to require supplemental insecticide treatments, despite infestations in non-Bt corn reaching >68% in each year. The only Bt and/or insecticide treatment to significantly reduce ear feeding or the proportion of ears injured (mainly by Helicoverpa zea [Boddie]) was the Bt hybrid pyramid producing Vip3A. However, significant protection of yield was detected only in the Bt hybrids producing Cry1A.105 + Cry2Ab2. All Bt traits tested in this study were effective in reducing infestation and feeding damage from fall armyworm, although this did not always result in significant protection of yield. Our results demonstrate the potential and limitations of using chlorantraniliprole with Bt (when resistance is present) and non-Bt corn to manage this pest.

Development, survival, and feeding behavior of Helicoverpa zea (Lepidoptera: Noctuidae) relative to Bt protein concentrations in corn ear tissues.

The corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), preferentially oviposits and feeds on ears of corn (Zea mays L.) and can be managed using transgenic hybrids that produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt). Concentrations of Bt proteins can vary spatially and temporally in plant tissues, creating a heterogeneous environment that can increase the risk of resistance development. We planted small-plot trials of nine Bt and non-Bt corn hybrids in South Carolina in 2016 and 2017 and investigated the development, survival, feeding injury, and feeding behavior in corn ear tissues. ELISA was used to quantify the concentrations of Cry1F and Cry2Ab2 in young silk, old silk, maternal tip tissue, kernels, and husk. Cry1F and Cry2Ab2 significantly varied with silk age and both proteins were generally highest in the silk and tip tissue. Hybrids with pyramided proteins significantly reduced feeding injury to the silk, tip, and kernel ear tissues, which was less apparent with single Bt protein hybrids. The pyramided hybrid expressing Vip3A incurred no injury to either the ear tip or kernels, and only eight 1st instar larvae were collected in the silk of 520 sampled ears. Age of larvae significantly varied among ear tissues but not between hybrids. Depending on hybrid family, mean larval instar in the silk, tip, and kernels was 1st or 2nd, 3rd, and 5th, respectively. Instar-specific feeding penetrance into corn ears increased with age but did not differ between hybrids. We characterized the instar- and tissue-specific feeding behavior of H. zea larvae but did not detect differences in feeding behavior between Bt and non-Bt hybrids. Implications for resistance management strategies such as seed mixtures are discussed.

Susceptibility of Corn Earworm (Lepidoptera: Noctuidae) to Cry1A.105 and Cry2Ab2 in North and South Carolina.

The corn earworm, Helicoverpa zea (Boddie), is managed in corn and cotton in the United States primarily using transgenic cultivars that produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt). However, increasing reports of resistance to one or more Bt proteins threaten the continued efficacy of Bt traits. To better understand the development of resistance of H. zea to Bt corn and cotton in the southeastern United States, we monitored for resistance to Cry1A.105 and Cry2Ab2 among 22 field populations of H. zea collected in non-Bt and Bt corn expressing Cry1A.105 + Cry2Ab2 during 2017 and 2018. Colonies were established in the laboratory and progeny were screened in diet-overlay bioassays to purified Cry1A.105 and Cry2Ab2 proteins. Compared with two susceptible laboratory colonies, all 14 field colonies tested with Cry1A.105 were highly resistant, with resistance ratios (RRs) ranging from 13.5 to >4,000. For Cry2Ab2, 19 colonies were tested and RRs ranged from 0.26 to 33.7. Field populations were significantly more susceptible to Cry2Ab2 than Cry1A.105. We documented variability in F0 and F1 pupal weight and developmental rates of natural populations of H. zea, but observed no significant correlation with susceptibility to either Cry1A.105 or Cry2Ab2. Our results expand on the recent reports of H. zea resistance to Cry1A and Cry2A proteins and will aid in the design and deployment of future pyramided crops in the United States.

Effects of Bt Corn on the Development and Fecundity of Corn Earworm (Lepidoptera: Noctuidae).

The corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), is only moderately susceptible to most toxins from the bacterium Bacillus thuringiensis (Bt) expressed in transgenic corn. To better understand the impact of Bt corn on the life cycle of H. zea, we collected pupae of H. zea during 2014-2016 in North Carolina, South Carolina, and Mississippi from corn hybrids expressing Cry1Ab, Cry1F, Cry1F + Cry1Ab, Cry1F + Cry1Ab + Vip3Aa20, Cry1A.105 + Cry2Ab2, and Cry1A.105 + Cry2Ab2 + Cry1F, as well as from non-Bt near-isolines. We investigated the effect of Bt corn on pupal weight, fecundity, and egg viability of H. zea. Pupal weights were significantly reduced for males and females from all Bt hybrids compared with non-Bt near-isolines. Female pupae from the hybrid expressing Cry1F + Cry1Ab were also significantly lighter relative to those from the near-isolines expressing only Cry1F. Reductions in pupal weight did not result in any detectable effects on fecundity or egg viability. The reduction in pupal weight in the hybrids expressing Cry1F and Cry1F + Cry1Ab significantly declined over time in South Carolina, possibly indicating developing resistance to these Bt toxins. These data can be incorporated into insect resistance management models used to improve risk management decisions regarding H. zea in Bt crops in the complex landscapes of the southern United States.