Assessment of Knack Field Performance Through Precision Field and Laboratory Bioassays in Cotton

Abstract

When a product performs better or worse than expectations, there are many biological, ecological, and operational factors that must be examined. Genetic resistance to the pesticide itself is often a concern. The control interval depends on the ecological impact of biotic (e.g., presence and function of natural enemies) and abiotic (e.g., frequency and severity of storms) factors. Timing, rates, and application methods used are also key factors affecting product performance. A four-year study to evaluate pyriproxyfen (Knack®) field performance in Arizona was initiated in 2004, after levels of whitefly susceptibility in statewide surveys were observed to be decreasing. Grower sites in Maricopa, Buckeye, Wellton, and Marana were used. We controlled for major operational factors by using a common timing, rate, and aerial application for each Knack spray. Resistance bio-assays were conducted on progeny of field-collected adults, pre- and post-spray. New eggs were marked in-field prior to spraying and examined in the field and lab in order to isolate Knack-associated mortality caused by direct toxicity as well as by ecological factors (e.g., bioresidual). Nymphal bioassays were used to evaluate metamorphosis inhibition. Population trends were estimated using standard sampling methods. Appropriate check plots were compared to the Knack treatment. Study results suggest Knack field performance and pyriproxyfen resistance has not changed significantly among the years or locations examined to date. In 2005, many struggled to gain control over whitefly populations. This work indicated that Knack performance and resistance parameters were within the range expected for the last several years. However, operational and ecological barriers to the performance of Knack and other chemistry were in play. Late planted conditions, lush winter vegetation capable of hosting whiteflies, poor growing conditions, and an extended period of extreme immigration pressure were all factors that diminished the impact of Knack and other products in 2005. In contrast, the winter preceding 2006 was among the driest on record followed by a very active monsoon season in central Arizona. High winds and dust movement, and a very active natural enemy community helped to continually lower whitefly populations. The result was a whitefly season characterized as light, with overall foliar insecticide usage setting a 28-yr record low for Arizona cotton. Barring all other operational and ecological factors at work, control intervals should have been similar each year. Yet, observed intervals have been different (e.g., 2005 vs. 2006) and point to the importance of these external factors in assessing product performance. Work will continue in 2007 to identify factors that contribute to whitefly outbreak conditions. These data will be key to understanding any performance changes, either due to operational or ecological factors mentioned above or due to innate changes in whitefly susceptibility. This will be important in advising growers about the risk factors associated with whitefly outbreaks and should lead to recommendations for minimizing these risks.