Steven J. Castle

Selection for imidacloprid resistance in silverleaf whiteflies from the Imperial Valley and development of a hydroponic bioassay for resistance monitoring

A field-collected population of the silverleaf whitefly, Bemisia argentifolii, was selected with the nicotinyl compound, imidacloprid, over 32 generations to determine if resistance would develop when maintained under continuous selection pressure in a greenhouse. Resistance was slow to increase at first with low to moderate levels of resistance (RR from 6- to 17-fold) in the first 15 generations of selection. Further selection steadily led to higher levels of resistance, with the greatest resistance ratio at 82-fold, the gradual rise suggesting the involvement of a polygenic system. At the end of the selection, slopes of probit regressions were substantially steeper than earlier, indicating increased homogeneity of imidacloprid resistance in this strain. A hydroponic bioassay featuring systemic uptake of imidacloprid through roots was developed to monitor the changes in resistance to imidacloprid in the selected whitefly strain and in seven field-collected strains from Imperial Valley, California. Six out of seven field-collected strains exhibited low LC50 values (0·002 to 0·512 mg ml-1) compared to the selected resistant strain, with one exception where the LC50 was 0·926 mg ml-1 (RR=15·0). Variation in responses to imidacloprid in the field strains suggest that this technique is sufficiently sensitive to detect differences in susceptibilities of whitefly populations. The imidacloprid-resistant strain showed no cross-resistance to endosulfan, chlorpyrifos or methomyl (RR ranging from 0·4- to 1·5-fold). A low level of cross-resistance was observed to bifenthrin in the IM-R strain at 7-fold. The success of selection for resistance to imidacloprid has serious implications for whitefly control programs that rely heavily on imidacloprid. ©1997 SCI

Compatibility of Two Systemic Neonicotinoids, Imidacloprid and Thiamethoxam, with Various Natural Enemies of Agricultural Pests

ABSTRACT Two systemic neonicotinoids, imidacloprid and thiamethoxam, are widely used for residual control of several insect pests in cotton (Gossypium spp.), vegetables, and citrus (Citrus spp.). We evaluated their impact on six species of beneficial arthropods, including four parasitoid species—Aphytis melinus Debach, Gonatocerus ashmeadi Girault, Eretmocerus eremicus Rose & Zolnerowich, and Encarsia formosa Gahan—and two generalist predators—Geocoris punctipes (Say) and Orius insidiosus (Say)—in the laboratory by using a systemic uptake bioassay. Exposure to systemically treated leaves of both neonicotinoids had negative effects on adult survival in all four parasitoids, with higher potency against A. melinus as indicated by a low LC50. Mortality was also high for G. ashmeadi, E. eremicus, and E. formosa after exposure to both compounds but only after 48 h posttreatment. The two predators G. punctipes and O. insidiosus were variably susceptible to imidacloprid and thiamethoxam after 96-h exposure. However, toxicity to these predators may be related to their feeding on foliage and not just contact with surface residues. Our laboratory results contradict suggestions of little impact of these systemic neonicotinoids on parasitoids or predators but field studies will be needed to better quantify the levels of such impacts under natural conditions.

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.

Compatibility of the insect pathogenic fungus Beauveria bassiana with neem against sweetpotato whitefly, Bemisia tabaci, on eggplant

A study on the compatibility of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin (Ascomycota: Hypocreales) with neem was conducted against sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), on eggplant. Initially, three concentrations of B. bassiana (106, 107, and 108 conidia ml−1) and three concentrations of neem (0.25, 0.5, and 1.0%) were used as individual treatments against B. tabaci. The highest concentration of B. bassiana yielded the highest B. tabaci egg (25.2%) and nymph mortalities (73.0%), but this was not significantly different from the mortalities caused by the 107 conidia ml−1 suspension. Similarly, the highest concentration of neem yielded the highest egg (27.3%) and nymph mortalities (75.5%), which was also not significantly different from the 0.5% suspension. Therefore, 0.5% neem was used along with 107B. bassiana conidia ml−1 suspension as an integrated pest management program against B. tabaci. The combination of B. bassiana and neem yielded the highest B. tabaci egg (29.5%) and nymph mortalities (97.2%), and the lowest LT50 (2.08 day) value. Moreover, an integrated combination of B. bassiana with neem caused 27.6 and 20.5% more nymphal mortality than individual treatments of B. bassiana and neem, respectively, 7 days post‐application. Thus, a combined application of an entomopathogenic fungus and a botanical insecticide may benefit from both, and it has proven effective for the control of B. tabaci on eggplant.

Sampling plans, selective insecticides and sustainability: the case for IPM as ‘informed pest management’

Integrated Pest Management (IPM) is considered the central paradigm of insect pest management and is often characterized as a comprehensive use of multiple control tactics to reduce pest status while minimizing economic and environmental costs. As the principal precursor of IPM, the integrated control concept formulated the economic theory behind pest management decisions and specified an applied methodology for carrying out pest control. Sampling, economic thresholds and selective insecticides were three of the critical elements of that methodology and are now considered indispensable to the goals of IPM. We examine each of these elements in the context of contemporaneous information as well as accumulated experience and knowledge required for their skillful implementation in an IPM program. We conclude that while IPM is principally about integrating control tactics into an effective and sustainable approach to pest control, this overarching goal can only be achieved through well-trained practitioners, knowledgeable of the tenets conceived in the integrated control concept that ultimately yield informed pest management.

Inter-Regional Differences in Baseline Toxicity of Bemisia argentifolii (Homoptera: Aleyrodidae) to the Two Insect Growth Regulators, Buprofezin and Pyriproxyfen

Abstract A survey of 53 Bemisia argentifolii Bellows & Perring populations from different agricultural regions in California and Arizona was conducted from 1997 to 1999 to establish baseline toxicological responses to buprofezin and pyriproxyfen. Although both compounds proved to be highly toxic even in minute quantities to specific stages, geographical and temporal differences in responses were detected using a leaf spray bioassay technique. Monitoring for three years revealed that six to seven populations had higher LC50 values but not greater survival when exposed to these two insecticides. A significant difference in relative susceptibility to buprofezin was first observed in late season 1997 in San Joaquin Valley populations with LC50s ranging from 16 to 22 mg (AI)/liter−1 compared with LC50s of 1 to 3 mg (AI)/liter−1 in Imperial, Palo Verde Valley and Yuma populations. Whiteflies collected in subsequent years from these and other locations showed an increase in susceptibility to buprofezin. Regional differences in susceptibilities to pyriproxyfen were minimal within the same years. Three years of sampling revealed consistently higher LC50s to pyriproxyfen in populations from Palo Verde Valley, CA, compared with whiteflies from Imperial, San Joaquin Valley or Yuma. As was the case with buprofezin, a decline in LC50s to pyriproxyfen was observed in whiteflies from all locations sampled in 1999. However, no correlation was observed between buprofezin and pyriproxyfen toxicity in any of the strains. The variable toxicities observed to both compounds over a period of 3 yr may be due principally to inherent differences among geographical populations or due to past chemical use which may confer positive or negative cross-resistance to buprofezin or pyriproxyfen.

Establishment of Baseline Susceptibility Data to Various Insecticides for Homalodisca coagulata (Homoptera: Cicadellidae) by Comparative Bioassay Techniques

Abstract Homalodisca coagulata Say , adults from three locations in California were subjected to insecticide bioassays to establish baseline toxicity. Initially, two bioassay techniques, petri dish and leaf dip, were compared to determine the most useful method to establish baseline susceptibility data under laboratory and greenhouse conditions. Comparative dose-response data were determined by both techniques to endosulfan, dimethoate, cyfluthrin, and acetamiprid. Toxic values were similar to some insecticides with both techniques but not for all insecticides, revealing susceptibility differences among the three populations of H. coagulata. In subsequent tests, the petri dish technique was selected to establish baseline susceptibility data to various contact insecticides. A systemic uptake bioassay was adapted to estimate dose-mortality responses to a systemic insecticide, imidacloprid. A 2-yr comparison of toxicological responses showed all three populations of H. coagulata to be highly susceptible to 10 insecticides, including chlorpyrifos, dimethoate, endosulfan, bifenthrin, cyfluthrin, esfenvalerate, fenpropathrin, acetamiprid, imidacloprid, and thiamethoxam. In general, two pyrethroids, bifenthrin and esfenvalerate, were the most toxic compounds, followed by two neonicotinoids, acetamiprid and imidacloprid. The LC50 values for all insecticides tested were lower than concentrations used as recommended field rates. Baseline data varied for the three geographically distinct H. coagulata populations with the petri dish technique. Adult H. coagulata collected from San Bernardino County were significantly more susceptible to select pyrethroids compared with adults from Riverside or Kern counties. Adults from San Bernardino County also were more sensitive to two neonicotinoids, acetamiprid and imidacloprid. The highest LC50 values were to endosulfan, which nonetheless proved highly toxic to H. coagulata from all three regions. In the majority of the tests, mortality increased over time resulting in increased susceptibility at 48 h compared with 24 h. These results indicate a wide selection of highly effective insecticides that could aid in managing H. coagulata populations in California.

Population Dynamics, Demography, Dispersal and Spread of Bemisia tabaci

Understanding the population-level processes of any pest insect is central to predicting temporal and spatial changes in abundance and occurrence, as well as in developing effective pest management strategies, whether on single crops on individual farms or multiple crops within agricultural landscapes. Four components drive population dynamics in the time-space continuum: birth rates, death rates, immigration rates, and emigration rates. This deceptively simple fact becomes immensely complex, however, when one considers all the interacting biotic and abiotic factors that influence each of these key population rates. The challenge becomes even greater for a broadly polyphagous and non-diapausing pest like Bemisia tabaci that can thrive and reproduce on multiple host plants over the entire year in areas it inhabits (Mound and Halsey 1978; Watson et al. 1992; Naranjo et al. 2004).

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.

Areawide Models Comparing Synchronous Versus Asynchronous Treatments for Control of Dispersing Insect Pests

Integrated pest management (IPM) has the goal of combining several control methods that reduce populations of pest insects and their damage to tolerable levels and thereby reduce the use of costly pesticides that may harm the environment. Insect populations can be monitored during the season to determine when the densities exceed an economic threshold that requires treatment, often as an insecticide application. We developed a simulation model where insect populations varied in exponential growth in fields and dispersed to adjacent fields each day of a season. The first model monitored populations of individual fields in a grid of fields and treated any field with insecticide if it exceeded a threshold population (asynchronous model) as done in traditional IPM. The second model treated the entire grid of fields with insecticide when the average population of all fields exceeded the threshold (synchronous model). We found that the synchronous model at all growth and dispersal rates tested had average field populations during a season that were significantly lower and required fewer treatments than the asynchronous method. Parameters such as percentage of fallow fields, number of fields, and treatment threshold had little affect on relative differences between the two models. The simulations indicate that cooperation among growers in areawide monitoring of fields to obtain an average population estimate for use in treatment thresholds would result in significantly less insect damage and fewer insecticide treatments. The synchronous method is more efficient because population refugia are precluded from which dispersal could reintroduce insects.