Peter C Ellsworth

Abstract:

We conducted an online survey to assess the potential effects of herbicide-tolerant (HT) and insect-resistant (IR) crops on integrated pest management (IPM) practices in the Western United States For HT crops, participants perceived a decrease in several IPM practices, including crop and herbicide rotations and the combined use of multiple weed control strategies. The most serious potential consequences were considered to be a shift in weed species composition and development of herbicide-resistant weeds. For IR crops, respondents perceived a beneficial reduction in application of both broad-spectrum and selective insecticides. The most significant issues for IR crops were believed to be potential development of target pest resistance and difficulties with management of insect refuges. The survey results support the need for continued emphasis on comprehensive strategies in IPM education programs to prolong the usefulness of HT and IR crops. © 2009 AgBioForum.

Evolution of resistance by pests can reduce the benefits of transgenic crops that produce toxins from Bacillus thuringiensis (Bt) for insect control. One of the world's most important cotton pests, pink bollworm (Pectinophora gossypiella), has been targeted for control by transgenic cotton producing Bt toxin Cry1Ac in several countries for more than a decade. In China, the frequency of resistance to Cry1Ac has increased, but control failures have not been reported. In western India, pink bollworm resistance to Cry1Ac has caused widespread control failures of Bt cotton. By contrast, in the state of Arizona in the southwestern United States, monitoring data from bioassays and DNA screening demonstrate sustained susceptibility to Cry1Ac for 16 y. From 1996-2005, the main factors that delayed resistance in Arizona appear to be abundant refuges of non-Bt cotton, recessive inheritance of resistance, fitness costs associated with resistance and incomplete resistance. From 2006-2011, refuge abundance was greatly reduced in Arizona, while mass releases of sterile pink bollworm moths were made to delay resistance as part of a multi-tactic eradication program. Sustained susceptibility of pink bollworm to Bt cotton in Arizona has provided a cornerstone for the pink bollworm eradication program and for integrated pest management in cotton. Reduced insecticide use against pink bollworm and other cotton pests has yielded economic benefits for growers, as well as broad environmental and health benefits. We encourage increased efforts to combine Bt crops with other tactics in integrated pest management programs.

We determined effects of aerial sprays of the insect growth regulator pyriproxyfen on sweetpotato whitefly, Bemisia tabaci (Gennadius) (B biotype), in Arizona cotton (Gossypium spp.) fields. We measured survival for males and females from a susceptible strain and a laboratory-selected resistant strain, as well as for hybrid female progeny from crosses between the strains. Insects were exposed directly to pyriproxyfen sprays in the field or indirectly in the laboratory by rearing them on sprayed leaves collected from the field. In all tests, survival was higher for the resistant strain than the susceptible strain, but did not differ between sexes in each strain. Survival to the adult stage did not differ between eggs and nymphs directly exposed to sprays. For susceptible and hybrid individuals, survival was lower on leaves collected the day of spraying than on leaves collected 2 wk after spraying. In contrast, survival of resistant individuals did not differ based on the timing of exposure. Dominance of resistance to pyriproxyfen depended on the type of exposure. Resistance was partially or completely dominant in direct exposure bioassays and on leaves collected 2 wk after spraying (h > 0.6). Resistance was partially recessive on leaves collected the day of spraying (mean h = 0.34). Rapid evolution of resistance to pyriproxyfen could occur if individuals in field populations with traits similar to those of the laboratory-selected strain examined here were treated intensively with this insecticide.

Abstract:

It has been less than 6 years since the devastation of the whitefly in Arizona and southern California. Numbers were so dense that windshields were clouded with the bodies of the adults, unprotected cotton fields were 'biologically' defoliated, and fields stood in 'permanent' wilt due to the excessive stress imposed by the immatures. Today our program has evolved from an effective, yet 2-dimensional system of chemical management to a multi-faceted, 3-dimensional and integrated management strategy (Ellsworth et al. 1996a; Ellsworth and Naranjo 1999). Early on the three 'keys' to whitefly management were identified by us and others as 1) Sampling and detection, 2) Effective chemical use, and 3) Avoidance of the problem. Now, this matrix of factors can be represented in the form of a pyramid, an inherently stable structure (Fig. 1). 'Avoidance' is the foundation block upon which 'Effective Chemical Use' and 'Sampling' rest. Confronted with a pest crisis, short term survival depends on the upper two levels of the pyramid. However, sustainable, long-term strategies ultimately must depend on the development of a solid foundation, 'avoidance'. At the same time, a pyramid-strategy developed for one pest must be compatible with like strategies in place for all pests of a system. The building blocks of a successful pest management program can be further subdivided into component parts. Sampling in cotton involves multi-stage and binomial methods of classifying whitefly populations (Ellsworth et al. 1995, 1996c; Diehl et al. 1997a, b, c) and sits at the apex of the pyramid. This represents its overarching importance in the implementation of all insect control tactics. Further, sampling plays a central role in the refinement and understanding of our management strategies. Without well-designed sampling tools, progress in all areas of whitefly management would be hampered. These tools have been adapted for new chemistry as it was developed. Effective chemical use consists principally of the use of action thresholds, availability and understanding of selective and effective chemistry, and a proactive resistance management plan. Action thresholds have been developed that are effective at preventing yield and quality losses (Ellsworth and Meade 1994; Naranjo et al. 1998). These, too, are insect stage-specific and have been optimized for proper deployment of insect growth regulators (IGRs) (Ellsworth et al. 1996c, 1997a,b, 1998a; Ellsworth 1998). The IGRs, Knack® and Applaud®, became available for the first time in this country in 1996 and have had a sensational impact on the selective management of this pest. [However, one cannot understate the importance of concomitant use of Admire® (imidacloprid) in melons and vegetables to the overall, area-wide lowering of pest dynamics.] All chemistry has been organized into a 3-stage program of deployment for resistance management (Ellsworth et al. 1996a). The proactive nature of this program has led to the restriction of use of the new IGRs such that their modes of action may be preserved for as long as possible while providing relief for resistance risk to all products.