John C. Palumbo

Insecticidal control and resistance management for Bemisia tabaci

Abstract Historically, Bemisia tabaci has been difficult to control with conventional insecticides in agronomic and horticultural production systems. In the past 10 years, new insecticide chemistries have been introduced that provide a diversity of novel modes of action and routes of activity to effectively control whiteflies. The chemistries that have had the most immediate impact on B. tabaci control include the nicotinoids and insect growth regulators (IGRs). The nicotinoids are systemic neurotoxins that target acetylcholine receptors in the insect nervous system. Imidacloprid, the first nicotinoid registered, has been largely responsible for the sustained management of B. tabaci in horticultural production systems worldwide. The non-neurotoxic IGRs, including buprofezin, a chitin synthesis inhibitor, and pyriproxyfen, a juvenile hormone analog, have also played important roles in controlling B. tabaci, particularly on cotton in North America and Israel. Collectively, the novel biochemical attributes and biological activities of these insecticides make them extremely effective. Consequently, intensive use of these products in some production systems has resulted in reduced susceptibility of B. tabaci. Insecticide resistance management strategies based on the structured and restricted use of these new modes of action, and coupled with the use of cultural and biological pest management tactics, presently provides the best model for combating insecticide resistance in B. tabaci.

Localized migration and dispersal by the sweet potato whitefly, Bemisia tabaci

Laboratory populations of the sweet potato whitefly, Bemisia tabaci, have been shown to consist of both migratory and trivial flying morphs. The behavior of these forms as part of the process of short-range migration was examined under field conditions. Insects were marked in a field of melons using fluorescent dust during two consecutive growing seasons. During the first growing season, passive traps used to collect living whiteflies, were placed along 16 equally spaced transects radiating from the field to a distance of up to 1.0 km. Wind out of the north-east consistently carried migrating whiteflies to traps placed along transects in the south-western quadrant because cold air drainages dictate wind direction during early morning hours in the desert South-west. For this reason, during the second season traps were laid out over fallow ground in a rectangular grid extending 2.7 km to the south-west of the marked field. If dispersal was entirely passive, patterns could be described using a diffusion model. Statistical examination of the data, howèver, demonstrated that the distribution on all days was patchy. Geostatistical techniques were used to describe the observed patchiness. Traps in the immediate vicinity of the marked field caught more whiteflies than the daily median. Large numbers were also collected from near the periphery of the grid. White-flies were far less prevalent in the grids center. As a result, the distribution of captured whiteflies can be described as bimodal. These patterns confirm behavior observed in the laboratory, i.e., a portion of the population are trivial fliers that do not engage in migration and are consequently captured in traps near the field, and a portion initially respond to cues associated with skylight, ignoring cues provided by the ground, and fly for a period of time before landing in distant traps. During both years movement out of the field had an exaggerated directional component on 13 of 14 days.

Large-scale, spatially-explicit test of the refuge strategy for delaying insecticide resistance

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.

Baseline susceptibilities of B- and Q-biotype Bemisia tabaci to anthranilic diamides in Arizona

BACKGROUND Development of pyriproxyfen and neonicotinoid resistance in the B-biotype whitefly and recent introduction of the Q biotype have the potential to threaten current whitefly management programs in Arizona. The possibility of integrating the novel anthranilic diamides chlorantraniliprole and cyantraniliprole into the current program to tackle these threats largely depends on whether these compounds have cross-resistance with pyriproxyfen and neonicotinoids in whiteflies. To address this question, the authors bioassayed a susceptible B-biotype strain, a pyriproxyfen-resistant B-biotype strain, four multiply resistant Q-biotype strains and 16 B-biotype field populations from Arizona with a systemic uptake bioassay developed in the present study. RESULTS The magnitude of variations in LC(50) and LC(99) among the B-biotype populations or the Q-biotype strains was less than fivefold and tenfold, respectively, for both chlorantraniliprole and cyantraniliprole. The Q-biotype strains were relatively more tolerant than the B-biotype populations. No correlations were observed between the LC(50) (or LC(99)) values of the two diamides against the B- and Q-biotype populations tested and their survival rates at a discriminating dose of pyriproxyfen or imidacloprid. CONCLUSION These results indicate the absence of cross-resistance between the two anthranilic diamides and the currently used neonicotinoids and pyriproxyfen. Future variation in susceptibility of field populations to chlorantraniliprole and cyantraniliprole could be documented according to the baseline susceptibility range of the populations tested in this study.

Newer insecticides for plant virus disease management.

Effective management of insect and mite vectors of plant pathogens is of crucial importance to minimize vector-borne diseases in crops. Pesticides play an important role in managing vector populations by reducing the number of individuals that can acquire and transmit a virus, thereby potentially lowering disease incidence. Certain insecticides exhibit properties other than lethal toxicity that affect feeding behaviours or otherwise interfere with virus transmission. To evaluate the potential of various treatments against the Bemisia tabaci-transmitted Cucurbit yellow stunting disorder virus (CYSDV), insecticide field trials were conducted in Yuma, AZ, USA, during spring and autumn growing seasons. Differences in vector-intensity each season led to mixed results, but at least five insecticide treatments showed promise in limiting virus spread during spring 2008. Increasing concern among growers in this region regarding recent epidemics of CYSDV is leading to more intensive use of insecticides that threatens to erupt into unmanageable resistance. Sustainability of insecticides is an important goal of pest management and more specifically resistance management, especially for some of the most notorious vector species such as B. tabaci and Myzus persiscae that are likely to develop resistance.

Bagrada hilaris (Hemiptera: Pentatomidae), An Invasive Stink Bug Attacking Cole Crops in the Southwestern United States

An invasive stink bug, Bagrada hilaris (Burmeister) (Hemiptera: Pentatomidae), was discovered in the Western Hemisphere in 2008 near Los Angeles, CA, presumably introduced on container shipments arriving at the Port of Long Beach. In the subsequent 4 years, it has spread throughout southern California, southern areas of Arizona, Nevada, and Utah, southern and west-central New Mexico, and extreme western Texas. We initiated studies on the seasonality and host range of this invasive insect as it adapts to this non-native habitat. We have learned that the bagrada bug has a single population peak between July and October in urban areas dominated by native and introduced weeds, and two population peaks (March-May and September-November) timed with the production of cole crops in agricultural areas. In greenhouse tests, we found 14 plants on which bagrada bug fed heavily, out of 38 agricultural crops and weed plants evaluated. The opportunistic use of host plants other than crucifers, the use of soil oviposition sites, and tolerance of warm climates may contribute to its invasive potential.

Ecological Determinants of Bemisia tabaci Resistance to Insecticides

The global importance of Bemisia tabaci (Gennadius) offers unique opportunities to examine patterns of infestation among diverse habitats and identify major factors that determine pest status. Its occurrence on field, vegetable and ornamental crops grown under open or protected conditions in temperate or tropical environments plays a critical role in the pest status of B. tabaci. Management practices also figure heavily into the arcane formula that ultimately determines the severity of infestation and degree of crop damage caused by B. tabaci. Decades of experience have taught valuable lessons regarding problems that arise when management practices are inadequate or inappropriate to meet the challenge of a B. tabaci onslaught. In some cases, inadequacy has taken the form of over-reactive management that responded to burgeoning B. tabaci infestations with brute-force application of insecticides.

Assessing Transmission of Crop Diseases by Insect Vectors in a Landscape Context

ABSTRACT Theory indicates that landscape composition affects transmission of vector-borne crop diseases, but few empirical studies have investigated how landscape composition affects plant disease epidemiology. Since 2006, Bemisia tabaci (Gennadius) has vectored the cucurbit yellow stunting disorder virus (CYSDV) to cantaloupe and honeydew melons (Cucumis melo L.) in the southwestern United States and northern Mexico, causing significant reductions in yield of fall melons and increased use of insecticides. Here, we show that a landscape-based approach allowing simultaneous assessment of impacts of local (i.e., planting date) and regional (i.e., landscape composition) factors provides valuable insights on how to reduce crop disease risks. Specifically, we found that planting fall melon fields early in the growing season, eliminating plants germinating from seeds produced by spring melons after harvest, and planting fall melon fields away from cotton and spring melon fields may significantly reduce the incidence of CYSDV infection in fall melons. Because the largest scale of significance of the positive association between abundance of cotton and spring melon fields and CYSDV incidence was 1,750 and 3,000 m, respectively, reducing areas of cotton and spring melon fields within these distances from fall melon fields may decrease CYSDV incidence. Our results indicate that landscape-based studies will be fruitful to alleviate limitations imposed on crop production by vector-borne diseases.

Biology, Ecology, and Management of an Invasive Stink Bug, Bagrada hilaris, in North America.

The painted bug, Bagrada hilaris, native to eastern and southern Africa and Asia, was detected in California in 2008, and it has spread rapidly throughout several southwestern US states. A polyphagous insect, it is particularly damaging to the billion dollar cole crop industry. B. hilaris frequently causes damage when it migrates to newly planted crops from weedy hosts. Feeding produces circular or star-shaped chlorotic lesions that become necrotic, and infested plants may be distorted. Currently, no reliable sampling methods for B. hilaris exist, nor are there effective natural enemies in the United States. Therefore, management has relied on multiple applications of insecticides and cultural practices such as removal of weedy hosts, destruction of crop residues, timing of planting, and use of transplants. Several pyrethroid and neonicotinoid insecticides are most effective for controlling the insect. Reliable sampling methods and further development of integrated pest management strategies to manage this invasive pest are urgently needed as its range continues to expand.

DEPOSITION, OFF-TARGET MOVEMENT, AND EFFICACY OF CAPTURE™ AND THIODAN™ APPLIED TO CANTALOUPES USING FIVE SPRAYERS

Five sprayers were tested to assess deposition efficiency, whitefly control, cantaloupe yield, and off-target movement. These were: an Electrostatic Spraying Systems sprayer (with the charging circuit on and off), a Micromax CDA without air assist, a DeGanya (FMC) with air assist, and a conventional twin nozzle system. The ESS-on and FMC had the highest deposition efficiency on the leaf undersides, and the conventional the lowest. Deposition on the leaf undersides decreased as the canopy closed, with the FMC showing lesser decreases than the ESS systems. For the first two application dates, the ESS-on and ESS-off were associated with greater insect control than the CDA or conventional systems. Later in the season no differences were detected. Overall, cantaloupe yields were not significantly different than for the untreated control. The ESS systems, however, were associated with significantly greater yield of no. 12 cantaloupes than the untreated control. Off-target movement was significantly greater for the FMC than any other system, except the conventional.