Temporal efficacy of neonicotinoid seed treatments against Frankliniella fusca on cotton

Longevity of neonicotinoid seed treatments in cotton seedlings under various deficit irrigation levels

Neonicotinoids are one of the most widely used classes of insecticides in agriculture. They are systemic insecticides mainly used as seed treatments to manage sucking insect pests in a wide range of field crops. Environment and growing conditions can impact the efficacy of neonicotinoid seed treatments. To evaluate this, an experiment was conducted to study the interactive effect of soil, water levels on the neonicotinoid concentration and plant growth in cotton. The experiment was laid out in a split-plot design, with three water levels, 30%, 60%, and 100% (control) of recommended irrigation and four seed treatments, including clothianidin, thiamethoxam, imidacloprid and an untreated control. Cotton seedlings were collected and analyzed for physiological changes and neonicotinoid concentrations at 15, 30 and 45 days after germination (DAG). Data were collected on plant height, fresh biomass and leaf area. The neonicotinoid concentrations were analyzed in leaf tissues using LC-MS/MS. The 30% and 60% of recommended irrigation treatments resulted in significantly taller seedlings with greater biomass and leaf area compared to the 100% water saturation (control) treatment. Thiamethoxam-treated seedlings had greater plant height and shoot fresh mass compared to clothianidin and the untreated control, whereas imidacloprid-treated seedlings had greater leaf area than thiamethoxam, clothianidin, and control at all water levels. 30% recommended irrigation increased neonicotinoid concentrations in leaf tissues, with clothianidin showing higher levels compared to other neonicotinoids at 45 DAG. Neonicotinoid seed treatment efficacy may vary with environmental factors, impacting sustainable pest control.

Cotton thrips infestation predictor: a practical tool for predicting tobacco thrips (Frankliniella fusca) infestation of cotton seedlings in the south-eastern United States

BACKGROUND

Thrips (order Thysanoptera) infestations of cotton seedlings result in plant injury, increasing the detrimental consequences of other challenges to production agriculture, such as abiotic stress or infestation by other pests. Using Frankliniella fusca as a thrips species of focus, we empirically developed a composite model of thrips phenology and cotton seedling susceptibility to predict site-specific infestation risk so that monitoring and other resources can be allocated efficiently, to optimize the timing of thrips control measures to maximize effectiveness, and to inform stakeholders about the dynamics of thrips infestation and cotton seedling injury at a time when thrips are evolving resistance to commonly-used pesticides.

RESULTS

A mixture distribution model of thrips infestation potential, fit to data describing F. fusca adult dispersal in time, proved best for predicting infestations of F. fusca on cotton seedlings. Thrips generations occurring each year as a function of weather are represented as a probability distribution. A model of cotton seedling growth was also developed to predict susceptibility as a function of weather. Combining these two models resulted in a model of seedling injury, which was validated and developed for implementation as a software tool.

CONCLUSIONS

Experimental validation of the implemented model demonstrated the utility of its output in predicting infestation risk. Successful implementation and use of the software tool derived from this model was enabled by close cooperation with university extension personnel, agricultural consultants, and growers, underscoring the importance of stakeholder and expert input to the success of applied analytical research. © 2020 Society of Chemical Industry.

Understanding the potential impact of continued seed treatment use for resistance management in Cry51Aa2.834_16 Bt cotton against Frankliniella fusca

Transgenic cotton expressing Cry51Aa2.834_16 Bt toxin (hereafter referred to as MON 88702) has the potential to be an important tool for pest management due to its unique activity against tobacco thrips, Frankliniella fusca. Unlike other Bt toxins targeting lepidopteran cotton pests, MON 88702 does not cause direct mortality but has an antixenotic effect that suppresses F. fusca oviposition. Previous work has shown neonicotinoid seed treated (NST) crops have similar behavioral effects on thrips. This study used non-choice and common garden experiments to examine how the presence of MON 88702 cotton and soybean (another F. fusca host) with and without NSTs might alter F. fusca infestation distributions. In a no-choice environment, significant larval establishment differences were observed, with untreated soybean plants becoming most heavily infested. In choice experiments, plants expressing MON 88702 or were neonicotinoid treated had significantly lower larval establishment. Larval density decreased as dispersal distance increased, suggesting reproductive decisions were negatively related to distance from the release point. Understanding how F. fusca responds to MON 88702 in an environment where adults can choose among multiple host plants will provide valuable context for projections regarding design of MON 88702 resistance refuges. Reduced larval establishment on NST cotton and soybean suggests that area-wide use of NSTs could reduce the number of susceptible F. fusca generated in unstructured crop refuges for MON 88702. These results also suggest that although the presence of NST MON 88702 could suppress reproduction and resistance selection, over time this benefit could erode resulting in increased larval establishment on NST cotton and soybean due to increased frequency of neonicotinoid resistant F. fusca populations.

Novel mechanism of thrips suppression by Cry51Aa2.834_16 Bt toxin expressed in cotton

BACKGROUND

Genetically engineered (GE) crops that express insecticidal traits have improved the sustainability of insect pest management worldwide, but many important pest orders are not controlled by commercially available toxins. Development of the first transgenic thysanopteran- and hemipteran-active Bacillus thuringiensis (Bt) Cry51Aa2.834_16 toxin expressed in MON 88702 cotton will significantly expand the diversity of pests controlled in the crop. Here, we examined MON 88702 cotton activity against two thrips species within the same genera, Frankliniella fusca and Frankliniella occidentalis. We used a multi-component cotton tissue assay approach to understand effects on adult longevity, fecundity, and larval development.

RESULTS

We found that in no-choice assays, cotton plants expressing MON 88702 suppress oviposition, when compared to a non-Bt cotton. MON 88702 did not kill a large proportion of F. fusca larvae or adults but killed most F. occidentalis larvae. Time series experiments with F. occidentalis larvae documented significant developmental lags for MON 88702 exposed individuals. We also found that female thrips preferred to oviposit on non-Bt cotton when provided a choice.

CONCLUSION

Together these results describe the activity of MON 88702 against thrips. They document clear differences in toxin performance between different thrips species and throughout the insects' life cycle. Most importantly, we show that MON 88702 was associated with reduced oviposition via behavioral avoidance to the toxin. This is a novel mechanism of action for pest control for a Bt crop plant. Together, these results provide a basis to describe the mechanism of population control for MON 88702 cotton. © 2019 Society of Chemical Industry.

Stability of neonicotinoid sensitivity in Frankliniella fusca populations found in agroecosystems of the southeastern USA

BACKGROUND

Insecticide resistance arises at a given location in response to selection acting on novel genotypes or standing variation, or allelic migration. Fitness costs of resistance may slow resistance evolution or result in reversion to susceptibility, but consistent and geographically widespread use of insecticides may provide sufficient selection to offset the fitness costs of resistance. Understanding this relationship is important to the success of insecticide resistance management. We report the existence of fitness costs of neonicotinoid resistance in field-collected populations of the tobacco thrips (Frankliniella fusca), which increasingly challenge upland cotton production in the southeastern USA.

RESULTS

Populations (14 of 15 in 2015; 4 of 5 in 2016) investigated showed a loss of resistance to imidacloprid after multiple generations without exposure to the insecticide. Populations studied in 2016 were each split into two colonies, and one of each pair was repeatedly exposed to imidacloprid. In three of the four populations that lost resistance, imidacloprid-exposed colonies lost resistance significantly more slowly than did corresponding unexposed colonies.

CONCLUSION

For imidacloprid resistance to be broadly increasing in the landscapes of the southeastern USA despite fitness costs of resistance, selection for resistance must be sufficient to overcome the costs. Findings encourage investigation into why costs are overcome in this system, potentially including geographic extent of neonicotinoid use or prevalence of low-dose exposure. © 2019 Society of Chemical Industry.

Neonicotinoid Insecticide Resistance in Tobacco Thrips (Thysanoptera: Thripidae) of Mississippi

Insecticidal efficacy of neonicotinoid insecticides used against tobacco thrips, Frankliniella fusca (Hinds) (Thysanoptera: Thripidae), in cotton, Gossypium hirisutum L. (Malvales: Malvaceae), was evaluated for field populations collected in Mississippi during 2014-2016. Resistance was documented in 16 and 57% of populations to imidacloprid and thiamethoxam, respectively. Resistance levels did not vary by host plant for any neonicotinoid, but resistance levels varied between the two main agricultural areas (Delta and Hills) of Mississippi and among years for some neonicotinoids. In spite of documented resistance, neonicotinoid seed treatments are still used on cotton in the midsouthern United States due to the lack of reliable alternative management strategies. The development of alternative thrips management strategies is critical to the sustainability of cotton production in the midsouthern United States.