The Compatibility of Insecticides With Conservation Biological Control in Arizona Cotton

Abstract

Selective tools, including selective insecticides and transgenic cotton, have played a crucial role in reducing insecticide usage and conserving arthropod predator populations within the Integrated Pest Management (IPM) plan for Arizona cotton. In this system, it is essential to comprehensively assess the efficacy of insecticides against cotton’s primary pests, Bemisia argentifolii and Lygus hesperus, as well as their impacts on non-target arthropods, including natural enemies, to maintain the stability and advancement of our IPM plan, which relies heavily on biological control. Insecticides can disrupt the intricate predator-prey relationships in cotton ecosystems, potentially affecting the entire food web and its biological control functions. The Arizona cotton system hosts over 27 taxa, including generalist predators that feed on key pests and each other, allowing for functional redundancy and resilience within the food web. In Arizona, six key generalist predators manage whitefly populations, and the abundance of just one of these predators at adequate levels can effectively maintain biological control. As predator populations fluctuate throughout the season, the impact of insecticides can vary depending on the species or life stages affected at the time of application. Ecological interactions of arthropods with cotton plants, such as variations in presence and behavior based on plant conditions, can affect non-target arthropod distribution and exposure, challenging the assessment of insecticide impacts. Given these complex ecological interactions, impacts of candidate pest control technologies on non-target arthropods are best assessed within the system of interest through field trials, where products will be used. Thus, an important detail of testing insecticide selectivity is the spatial scale necessary to detect and understand changes in arthropod dynamics without inter-plot interference. Study plots should be carefully chosen to be large enough to exclude external influences such as pesticide drift and biological migration, avoid interferences that could mask the effectiveness of beneficial arthropods, and sufficiently represent and predict larger commercial-scale outcomes. This dissertation addressed this critical factor in assessing insecticide selectivity focusing on plot size in Appendix A. A two-year replicated field experiment was conducted to determine the optimal plot size for studying non-target arthropods, evaluating three square plot sizes (144 m2, 324 m2, and 576 m2) across three insecticide treatments, including an untreated check, a positive control (an insecticide with known negative effects on non-target arthropods), and a selective insecticide. Results indicated that the smallest plot size (144 m2) was sufficient for accurately measuring insecticidal effects on non-target arthropods in cotton. This result on plot size marks a significant advancement in non-target testing, forming the foundation of our recommendations on product safety for growers. By demonstrating that smaller plots are sufficient, it helps avoid the impracticalities and higher costs associated with larger plots, such as increased sampling efforts, land rental, water usage, labor, and equipment. This is especially relevant in cases where seed availability is limited, such as with regulated trials of genetically engineered pest control traits. These findings are crucial for regulatory bodies to accurately assess data submitted for insecticide or trait registration, particularly as the evaluation of invertebrate effects is becoming increasingly important in the registration of new pesticides. The impacts of four globally significant insecticides on non-target arthropods were evaluated through years of field testing using adequate plot sizes: isocycloseram and afidopyropen in Appendix B, and indoxacarb and spiromesifen in Appendix C. The results indicated that indoxacarb, afidopyropen and spiromesifen are fully selective insecticides, while isocycloseram is a partially selective insecticide. This work helped to develop a model-approach, based on sets of analytical information, for comprehensive non-target assessment of candidate pest control technologies that could be broadly applicable to many agricultural systems. This robust, model-approach to assess non- target arthropod impact includes: 1) Arthropod community analyses over 27 different taxa across various orders and families, many of which are also important in other crop systems;2) Abundance of proven individual key predators in the Arizona cotton system: Collops spp., Orius tristicolor, Geocoris spp., Misumenops celer, Drapetis nr. divergens, and Chrysoperla carnea s.l; 3) Comparative assessment of key predator to prey ratios as indicators of functioning biological control; 4) Direct measurement of biological control and other sources of in-field mortality through a sentinel prey method. Our findings have been widely disseminated through Extension meetings, where thousands of stakeholders have been educated on insecticide selectivity and biological control (Appendix D). The results of this dissertation provide valuable insights for regulatory agencies, manufacturers and researchers of other agricultural systems. This dissertation helped to develop a model system for comprehensive non-target assessment of candidate pest control technologies applicable to many agricultural systems. This dissertation also highlights the uniqueness of the Arizona IPM system, which is currently the only place in the world conducting such detailed and comprehensive research on non-target arthropod impacts of insecticides in field trials.