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United States Department of Agriculture

Agricultural Research Service

Biological Control
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Conservation of Natural Enemies Through use of Selective Insecticides
Steve Naranjo, Peter Ellsworth (UA), James Hagler, David Akey

Insect Growth Regulators

A three year field study contrasted the effects of two insect growth regulators (IGRs) and conventional insecticides on natural enemy conservation in cotton within the context of alternative management strategies for Bemisia tabaci (Gennadius). Compared with an untreated control, insecticide regimes based on the initial use of the IGR buprofezin or pyriproxyfen reduced densities of eight predator taxa out of 20 examined in at least one year, including common species such as Geocoris punctipes (Say), Nabis alternatus Parshley, Chrysoperla carnea s.l., and the empidid fly Drapetis nr. divergens. Patterns of predator and pest population change relative to IGR application dates suggest that factors other than direct toxic effects, such as reduction in prey availability, were likely involved. In comparison, the use of conventional insecticides reduced populations of nearly all the predatory taxa examined in most years, including those affected by IGRs, with the impact being greater and more immediate in all cases. Predator:prey ratios were significantly increased by the use of IGRs compared with both the untreated control and a conventional insecticide regime in most instances. The application of conventional insecticides for suppression of Lygus hesperus Knight, another key pest in the system, in a split-plot design reduced densities of most predator taxa and diminished the selective advantage of the IGRs. Rates of parasitism by aphelinid parasitoids (Eretmocerus spp. and Encarsia spp.) were generally low and did not vary consistently due to B. tabaci or L. hesperus insecticide regimes over the three years. Our 3-year study demonstrates the more selective action of buprofezin and pyriproxyfen in an effective integrated control system for B. tabaci. The use of these IGRs could further facilitate biologically-based management in cotton production systems. (PDF reprint)

Acetamiprid (Neonicotinoid)

The integrated control concept emphasizes the importance of both chemical and biological control to pest suppression in agricultural systems. A two-year field study was conducted to evaluate the selectivity of acetamiprid for control of Bemisia tabaci (Gennadius) in cotton compared with a proven selective regime based on the insect growth regulators (IGRs) pyriproxyfen and buprofezin. Acetamiprid was highly effective in controlling all stages of B. tabaci compared with an untreated control and generally produced lower pest densities than the IGR regime. Univariate analyses indicated that nine of 17 taxa of arthropod predators were significantly depressed with the use of acetamiprid compared with an untreated control, including common species such as Geocoris punctipes (Say), Orius tristicolor (White), Chrysoperla carnea Stephens sensu lato, Collops vittatus (Say), Hippodamia convergens Guérin-Méneville, and Drapetis nr. divergens.  Compared with results from independent, concurrent studies using mixtures of broad-spectrum insecticides at the same research site, acetamiprid depressed populations of fewer predator taxa, but for eight predator taxa significantly affected by both regimes, the average population reduction was roughly equal.  In contrast, only four taxa were significantly reduced in the IGR regime compared with the untreated control and three of these were omnivores that function primarily as plant pests.  Principal response curves analyses (a time-dependent, multivariate ordination method) confirmed these patterns of population change for the entire predator community.  Predator:prey ratios generally increased with the use of both IGRs and acetamiprid compared with untreated control but ratios were consistently higher with IGRs.  Parasitism by aphelinid parasitoids was unaffected or depressed slightly in all insecticide regimes compared with the control.  Because of its high efficacy, acetamiprid may play an important role in later stages of B. tabaci control were less emphasis is placed on selectivity.  However, our results suggest that acetamiprid would be a poor substitute for the currently used IGRs in the initial stage of control where insecticide selectivity is crucial to a functional integrated control program for B. tabaci in cotton. (PDF reprint)

Life Table Analysis of Natural Enemy and Other Mortality Factors Affecting Sweetpotato Whitefly
Steve Naranjo, Peter Ellsworth (UA)

Natural mortality is an important determinant of the population dynamics of a species and an understanding of mortality forces should aid development of better management strategies for insect pests. An in situ, observational method was used to construct cohort-based, life tables for Bemisia tabaci (Gennadius) Biotype B (Homoptera: Aleyrodidae) over 14 generations on cotton in central Arizona, USA from 1997-1999. In descending order, median marginal rates of mortality were highest for predation, dislodgment, unknown causes, egg inviability, and parasitism. The highest mortality occurred during the 4th nymphal stadium, and the median rate of immature survival over 14 generations was 6.6%. Predation during the 4th nymphal stadium was the primary key factor. Irreplaceable mortality was highest for predation and dislodgment with the absence of these mortality factors leading to the greatest increases in estimated net reproduction. There was little evidence of direct or delayed density-dependence for any mortality factor. Wind, rainfall, and predator densities were associated with dislodgment, and rates of predation were related to densities of Geocoris spp., Orius tristicolor (White), Chrysoperla carnea s.l. Stephens and Lygus hesperus Knight. Simulations suggest immigration and emigration play important roles in site-specific dynamics by explaining departures from observed population trajectories based solely on endogenous reproduction and mortality. By direct measurement of these mortality factors and indirect evidence of adult movement, we conclude that efficient pest management may be best accomplished by fostering greater mortality during the 4th stadium, largely through conservation of predators and by managing immigrating adult populations at their sources. (PDF reprint

Intraguild Predation on Sweetpotato Whitefly Parasitoids
Steve Naranjo

Intraguild predation is pervasive in many managed and unmanaged ecosystems and may have negative, neutral or positive effects on the biological control of pest insects.  Both generalist predators and aphelinid parasitoids attack Bemisia tabaci (Gennadius) Biotype B (=B. argentifolii Perring & Bellows) on cotton in the southwestern USA.  Free-choice and no-choice laboratory assays were conducted to quantify prey consumption patterns and preference by three representative generalist predators, Geocoris punctipes (Say), Orius insidiosus (Say), and Hippodamia convergens Guérin-Méneville, offered 4th instar B. tabaci nymphs and nymphs parasitized by Eretmocerus sp. nr. emiratus.  All three predators showed a significant preference for larval and pupal stage parasitoids over early 4th instar nymphs, but G. punctipes and O. insidiosus were non-discriminating when offered a choice of larval parasitoids and late 4th instar nymphs.  The potential implications of these observed patterns for the field were examined through sensitivity analyses of existing field life table data.  First, preference for parasitized hosts alters the methods required for calculating marginal rates of parasitism.  Incorporating a preference variable in the estimation procedure had a very small positive effect (0.02 - 1.13 % change) on total generational mortalities observed in previous life table studies. However, further hypothetical analyses suggested that under circumstances of lower generational mortality and higher levels of either apparent parasitism or predation, high levels of predator preference for parasitized prey could alter estimates of total mortality as much as 14%.  Second, although intraguild predation was demonstrated, the implications for biological control are unclear.  Based on field life table data the rate of IGP ranged from 0.019 – 0.126 depending on predator species and prey comparison, but accounting for these levels of intraguild predation had only small negative effects on total generational mortalities (0.193 – 1.278 % change). (PDF reprint)

Establishment and Impact of Exotic Parasitoids for Biological Control of Sweetpotato Whitefly

Steve Naranjo

 

A large multi-institutional, interagency classical biological control program was initiated in the early 1990s to combat the invasion of the B biotype of Bemisia tabaci into the USA.  This large program was successful in the discovery, importation, rearing and release of more than 30 species/strains of aphelinid parasitoids (primarily Eretmocerus and Encarsia) from around the world into multiple states.  Establishment of several species in each targeted state has been documented. In Arizona two exotic species have become established (Eretmocerus nr. emiratus and Encarsia sophia) and have largely displaced native aphelinid species attacking B. tabaci. However, the impact of these establishments and the overall biological control program in Arizona and elsewhere has been poorly documented.  From 1996 through the present in situ life tables have been constructed for B. tabaci on cotton in central Arizona.  Analyses of these life tables demonstrate that parasitism varied across years at low to moderate levels but that there is no trend for increasing levels of parasitism since the exotics became established.  Additional analyses showed that the irreplaceable mortality supplied by parasitism has not consistently increased since establishment and that parasitism has no explanatory value in predicting total generational mortality.  Predation by sucking predators has consistently been the largest source of mortality, has consistently contributed the largest amount of irreplaceable mortality and represents the key-factor explaining variations in total mortality both before and after the establishment of exotic aphelinids.

 

Contribution of Conservation Biological Control to Integrated Control of Bemisia tabaci

Steve Naranjo, Peter Ellsworth

 

Integrated control systems are based on the complimentary contribution of chemical and biological control fostered by conservation of natural enemies. Yet, in the 50 years since the integrated control concept [ICC] (Stern et al., 1959) was introduced there are few operational programs and even fewer attempts to analyze the mechanisms that allow chemical and biological control to act in concert. The dearth of demonstrable evidence for the ICC has eroded the credibility of biological control and its usage in operational IPM plans. We used in situ life tables within an experimental design to measure and compare the contribution and interaction of biological control and insecticides as tactical components within three pest management systems for Bemisia tabaci (Gennadius) in cotton. Insecticides were the key factor immediately following applications of broad-spectrum materials or one of two selective insect growth regulators (IGRs), and this mortality replaced that provided by natural enemies. Two-6 wks later, however, mortality from natural enemies, primarily predation, in the IGR regimes rebounded to the high levels observed in untreated controls and became the key factor. Mortality from natural enemies remained depressed in the broad-spectrum insecticide regime. Single IGR applications were sufficient to suppress B. tabaci populations throughout the season, while up to five broad-spectrum applications were needed to achieve comparable control. The chemical residual of IGRs was limited to several weeks, demonstrating a key role for mortality from conserved natural enemies that extended the control interval. This “bioresidual” allows for long-term, commercially acceptable pest suppression following the use of selective insecticides. We provide a rare experimental illustration of integrated control, where chemical and biological controls “augment one another”. Our approach and methodology could be applied to demonstrate and validate integrated control in many other systems, addressing a critical need for implementation of biological control in practicing IPM systems. (PDF)

 


Last Modified: 5/14/2012
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