VOL. 34, NO. 4 SOUTHWESTERN ENTOMOLOGIST DEC. 2009 Tritrophic Interactions among Host Plants, Whiteflies, and Parasitoids Shoil M. Greenberg1, Walker A. Jones2, and Tong-Xian Liu3 Abstract. Effects of cotton, Gossypium hirsutum L.; green bean, Phaseolus vulgaris L.; and sweet potato, Ipomoea batatas (L.) Lam.; on mortality and development of sweetpotato whitefly, Bemisia tabaci (Gennadius) biotype B; bandedwinged whitefly, Trialeurodes abutilonea (Haldeman); and greenhouse whitefly, T. vaporariorum (Westwood); and on the key biological parameters of an exotic parasitoid species, Eretmocerus mundus Mercet, and an indigenous parasitoid, Encarsia pergandiella Howard, were compared in the laboratory. Cotton was most suitable for sweetpotato whitefly, and bean was most suitable for greenhouse whitefly. No significant differences were found between these two whitefly species on sweet potato. Preimaginal mortality of sweetpotato whitefly on cotton was 35.2% versus 77.3% of greenhouse whitefly. Developmental time of sweetpotato whitefly was significantly shorter (17.5 days) than that of greenhouse whitefly (23.2 days). The mortality and developmental time of bandedwinged whitefly did not differ on the different host plants. Parasitism by Er. mundus was greatest in sweetpotato whitefly and least in greenhouse whitefly when both whiteflies were reared on cotton. Parasitism of bandedwinged whitefly was intermediate. Parasitism by En. pergandiella was significantly greater than that by Er. mundus attacking the same whitefly species reared on bean or cotton, except parasitism of sweetpotato whitefly. Emergence of Er. mundus was greatest from sweetpotato whitefly on cotton, and least for bandedwinged whitefly on bean. Emergence of En. pergandiella was significantly greater than that of Er. mundus among host plants and whitefly species except sweetpotato whitefly. Introduction Whiteflies (Hemiptera: Aleyrodidae) are among the most widespread and economically important insect pests worldwide. The sweetpotato whitefly, Bemisia tabaci (Gennadius), may feed on 506 plants in 74 families (Greathead 1986). Sweetpotato whitefly biotype B ranks among the most noxious insects attacking agronomic and ornamental crops (Cock 1993). The bandedwinged whitefly, Trialeurodes abutilonea (Haldeman), is a polyphagous feeder on ≈140 species of plants, including many important species in the genus Hibiscus (Russell 1963, Liu and Stansly 2000). Similarly, the greenhouse whitefly, Trialeurodes vaporariorum (Westwood), is an important pest of vegetables and ornamental crops in greenhouses. ________________________ 1Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture, Weslaco, TX. 2European Biological Control Laboratory, ARS-USDA, Montpellier, France. 3Vegetable IPM Laboratory, Texas AgriLife Research, Texas A&M University System, Weslaco, TX. Correspondence: S. M. Greenberg, Research Entomologist, BIRU-SARC-ARS-USDA, Weslaco, TX; E-mail address: [email protected]. 431 Although the bioecology of sweetpotato whitefly and greenhouse whitefly is well understood, little information is available on the bioecology of bandedwinged whitefly under different geographical and environmental conditions. During 2008, 30,355 ha of cotton in the Lower Rio Grande Valley of Texas were infested by bandedwinged whitefly while only 4,047 ha were infested by sweetpotato whitefly (Williams 2009). This contrasted with the U.S. trend where sweetpotato whitefly was more damaging than bandedwinged whitefly in cotton during the same year (147,138 versus 109,853 ha infested by sweetpotato whitefly and bandedwinged whitefly, respectively; Williams 2009). Liu and Stansly (2000) speculated that bandedwinged whitefly, originally restricted to Malvaceae had adapted to other host species in areas where malvaceous hosts have become rare. If this is the case, the adaptation process probably will continue and the host range of bandedwinged whitefly may continue to expand. Information on the interactions among plant species, whiteflies, and their natural enemies is important in the development of locally appropriate integrated pest management strategies including biological control. At present, whitefly control programs in the southwestern U.S. are based mostly on the application of insecticide (Palumbo et al. 2001). However, frequent and intensive use of broad-spectrum insecticides increases the risk of resistance to insecticide, environmental contamination, and human exposure. Conservation biological control is one of the most economically feasible alternatives to conventional pest management practices in field crops. Among the most effective natural enemies of Bemisia whiteflies are parasitoids in the family Aphelinidae (Hymenoptera) including Eretmocerus mundus Mercet (exotic) and Encarsia pergandiella Howard (indigenous). Er. mundus is the most common parasitoid of sweetpotato whitefly in southern Europe and has been successfully established in many parts of the US. En. pergandiella is a heteronomous hyperparasitoid (autoparasitoid); females develop as primary parasitoids on immature whiteflies, but males develop as secondary parasitoids on females of their own or on related species (Hunter 1989). En. pergandiella can cause 94% parasitism of sweetpotato whitefly in South Texas (Goolsby et al. 1998). Encarsia formosa Gahan and Er. eremicus Rose and Zolnerowich are perhaps the most effective species for control of greenhouse whitefly (Hoddle et al. 1998, Gelman et al. 2005). However, no information is available on the use of these species to control bandedwinged whitefly (Liu and Stansly 2000) and little information is available on the use of Er. mundus and En. pergandiella to control greenhouse whitefly. This information is important in areas such as the southwestern US where both pests species co-exist in the field and greenhouse. Most investigations tend to focus on direct interactions between whiteflies and their host plants or the insects and their natural enemies. The full set of tritrophic interactions needs to be assessed to provide a basis for effective pest management strategies against whiteflies. Qualities of the host plant may have indirect effects on the fitness of the third trophic level. For example, high host/pest densities leading to abundant parasitoids or low parasitoid:host ratio may result in longer developmental time and smaller size of parasitoid progeny and a greater proportion of male progeny because of possible differences in nutritional quantity. The objectives of this study were to determine the effects of three host plants, bean, cotton, and sweet potato, on key fitness parameters of three whitefly species, sweetpotato whitefly, bandedwinged whitefly, and greenhouse whitefly, and two parasitoid species, Er. mundus and En. pergandiella, developing on these 432 pests and host plants. Parameters measured included survivorship, developmental time, sex ratio, preovipositional period, daily fecundity, and size. Knowledge of the interrelationship among host plants, whiteflies, and their parasitoids is critical to developing rearing techniques, making decisions in augmentative releases, developing predictive models, and understanding the mechanisms involved in competition by parasitoids. Materials and Methods Host Plants. Bean, cotton, and sweet potato were used as host plants in these studies. Leaves were excised and each leaf petiole was placed in a floral aquapic filled with a hydroponic solution (Aqua-Ponics International, Los Angeles, CA). Excised leaves readily rooted and did not deteriorate under fluorescent lights (20 watt, Vita-Life©, Duro-Test Lighting, Elk Grove, IL) in an incubator. Whitefly Cultures. A culture of sweetpotato whitefly originated from adults collected from cabbage, Brassica oleracea L., in Hidalgo County, TX, and maintained on sweet potato in a greenhouse of the Subtropical Agricultural Research Center, ARS-USDA, Weslaco, TX. A culture of greenhouse whitefly originated from individuals received from the Department of Entomology, University of Georgia, Griffin, GA, where they were reared on green bean. A culture of bandedwinged whitefly was started from individuals collected from cotton in Hidalgo County, TX, and maintained on cotton in a greenhouse. Before the experiment, the sweetpotato whitefly, bandedwinged whitefly, and greenhouse whitefly were cultured for three generations on the three host plants (cotton, bean, and sweet potato). We and others (Van Boxtel et al. 1978, Dorsman and van de Vire 1987, Liu and Stansly 2000) observed that when adult whiteflies were transferred from one plant species to another, the insects required a period of adjustment for at least three generations on the host plant. Parasitoid Cultures. Er. mundus was originally collected from sweetpotato whitefly on cotton near Murcia, Spain, and provided by USDA, APHIS Mission Plant Protection Center, Mission, TX (MPPC culture # M92014). En. pergandiella was collected from sweetpotato whitefly on cotton at Weslaco, TX. We maintained all the parasitoid cultures on sweetpotato whitefly reared on sweet potato. Host Plant Effects on Whiteflies. We determined mortality and developmental time separately by the whitefly instar. Whiteflies were confined within a 4.5-cm-diameter clip cage to the underside of each excised test leaf. Each rooted leaf with eggs was placed in a 120 x 25-mm dish covered with polyester organdy for ventilation. Hydroponics solution was added