Peter C Ellsworth

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.

The refuge strategy is used worldwide to delay the evolution of pest resistance to insecticides that are either sprayed or produced by transgenic Bacillus thuringiensis (Bt) crops. This strategy is based on the idea that refuges of host plants where pests are not exposed to an insecticide promote survival of susceptible pests. Despite widespread adoption of this approach, large-scale tests of the refuge strategy have been problematic. Here we tested the refuge strategy with 8 y of data on refuges and resistance to the insecticide pyriproxyfen in 84 populations of the sweetpotato whitefly (Bemisia tabaci) from cotton fields in central Arizona. We found that spatial variation in resistance to pyriproxyfen within each year was not affected by refuges of melons or alfalfa near cotton fields. However, resistance was negatively associated with the area of cotton refuges and positively associated with the area of cotton treated with pyriproxyfen. A statistical model based on the first 4 y of data, incorporating the spatial distribution of cotton treated and not treated with pyriproxyfen, adequately predicted the spatial variation in resistance observed in the last 4 y of the study, confirming that cotton refuges delayed resistance and treated cotton fields accelerated resistance. By providing a systematic assessment of the effectiveness of refuges and the scale of their effects, the spatially explicit approach applied here could be useful for testing and improving the refuge strategy in other crop-pest systems.

Abstract:

Maize plants were grown under four moisture regimes (wet to extreme deficit) and three constant temperatures (20°, 25° & 30°C) in a phytotron. Each plant was infested with one E-race European corn borer [Ostrinia nubilalis (Hubn.)] (ECB) egg mass at pollen shed. ECB development, location, and establishment were recorded over the course of 12 destructive sample dates (4/temperature). ECB developmental rates were not significantly affected by soil moisture treatments, but were significantly affected by temperature. In spite of successful establishment of four distinctly different soil moisture regimes, the maize stalk tissue water levels were not significantly different among soil water treatments. Instead, the maize plants exhibited accelerated leaf senescence in response to the water deficit conditions. Among the soil water treatments, differences were found in larval establishment, vertical distribution and dispersion, and feeding site selection; however, those effects were slight and could not explain the similarity in ECB developmental rates observed in these treatments. In maize, the larval environment within the stalk was effectively insulated from changes in the external environment by the plant's ability to maintain a relatively high and stable stalk tissue water content. Thus, large changes to the soil environment had essentially no effect on ECB development, though drastic consequences for the plant. This study indicates that ECB rates of development are relatively insensitive to changes in the soil water environment as well as the associated changes in the maize plant that accompany severe drought stress. The significance of these findings to insect modelling, crop physiology, and insect-crop interactions is discussed. © 1989 Kluwer Academic Publishers.

Abstract:

Understanding the effect of cropping patterns on population dynamics, dispersal, and habitat selection of insect pests has been an unresolved challenge. Here, we studied the western tarnished plant bug, Lygus hesperus (Knight) (Heteroptera: Miridae), in cotton during early summer in central Arizona. We used a general approach based on global positioning system (GPS) and geographic information system (GIS) technologies combined with spatial statistics to assess the maximum distance at which forage and seed alfalfa, fallow fields with weeds, and cotton affect L. hesperus population density. Using a set of 50 cotton fields as focal fields, we found that forage and seed alfalfa as well as weeds acted as L. hesperus sources for these cotton fields. The source effect did not extend beyond 375, 500, and 1500 m for forage alfalfa, weeds, and seed alfalfa, respectively. Conversely, cotton fields acted as L. hesperus sinks, but this effect did not extend further than 750 m from the focal cotton fields. These findings suggest that specific spatial arrangements of these field types could reduce L. hesperus damage to cotton. The spatially explicit approach used here provides a direct evaluation of the effects of agroecosystem heterogeneity on pest population dynamics, dispersal, and habitat selection, which is a significant asset for the development and improvement of areawide pest management. © 2006 The Authors.