Temperature Effects on Development of Three Cereal Aphid Parasitoids (Hymenoptera: Aphidiidae)

Temperature is an important climatological variable that influences the biology and ecology of insects. Poor climatic adaptation can limit the effec­ tiveness of parasitic insects in biological control. Two exotic paras tes (Syrian Diaeretiella rapae (M'Intosh) and Argentinean Aphid us colemani Viereck) imported for biological control of the Russian wheat aphid, Diuraphis noxia (Mordvilko), and one native parasite (Diaeretiella rapae) were reared in growth chambers in three fluctuating temperature regimes with average daily temperatures of 12, 18, and 24°C. Estimates of temperature thresholds for immature development were 3.3, 5 and 2.8°C, for Oklahoman D. rapae, Syrian D. rapae, and A. colemani, respectively. Estimates of thermal require­ ments for development from egg to adult were 297, 278, and 301 degree-days for the three parasitoids. Dry weights of adults reared in different fluctuating temperature regimes did not differ significantly among sexes, but adults from regimes with low average temperatures of 12 and 18°C had significantly greater weights than those reared in a regime with an average temperature of 24°C. Results suggest that developmental response to temperature will not limit the effectiveness of the exotic parasites in biological control. Diaeretiella rapae M' Intosh is a cosmopolitan aphidiid that commonly parasitizes a wide range of hosts in agroecosystems (Mackauer and Stary 1967). Aphidius colemani Viereck is widely distributed in the southern hemi­ sphere and broadly oligophagous on Aphididae (Stary 1975). Both species par­ asitize several economically important aphid pests of small grain crops in­ cluding the greenbug, Schizaphis graminum (Rondani), and Russian wheat aphid, Diuraphis noxia (Mordvilko) (Mackauer and Stary 1967, Stary 1975). Both A. colemani and D. rapae were imported into the United States for clas­ sical biological control of the Russian wheat aphid in the Southern Great Plains and elsewhere in the western US (Gould and Prokrym 1994). Temperature is an important abiotic variable that influences insect popu­ lations in the field. Temperature can influence insect population growth through its effects on development rate, survival, fecundity, and dispersal (Ratte 1984, Rankin and Singer 1984). Temperature may partially determine the effectiveness of a parasitoid as a biological control agent in a particular re­ gion. For example, a parasitoid with a higher developmental threshold tem­ perature than its host may have limited effectiveness i regions where tem­ peratures below the parasitoids developmental threshold but above the hosts 1 USDA, ARS, SPA, Plant Science Research Laboratory, 1301 N. Western St., Still­ water, OK 74075. 1 Elliot et al.: Temperature Effects on Development of Three Cereal Aphid Parasito Published by ValpoScholar, 1995 200 THE GREAT LAKES ENTOMOLOGIST Yol. 28, No.3 & 4 threshold occur often, because development from birth to maturity, and hence population growth rate would lag behind that of its host (Campbell et al. 1974). Thus, in addition to factors such a earching ability, host preference, and host suitability, knowledge of developmental responses to temperature should be considered when deciding whether to release an exotic parasitoid for biological control (Bernal and Gonzalez 1993). Estimates of developmental thresholds for D. rapae populations from dif­ ferent regions range from 2.1 to 7.0'C (Campbell et al. 1974, Bernal and Gon­ zalez 1993). The estimates may result from adaptation to geographic differ­ ences in climate (Campbell et al. 1974). To the best of our knowledge, the developmental threshold of A. colemani has not been reported. The first ob­ jective of this study was to determine developmental thresholds and temper­ ature requirements for development of three cereal aphid p rasitoids. Two of the parasitoids were imported into the US; Diaeretiella rapae was imported from Syria, while A. colemani was imported from Argentina. A native Okla­ homa population of D. rapae was also examined. Because this parasitoid is en­ demic to the S uthern Great Plains, and presumably well adapt d to the cli­ mate there, estimation of temperature effects on its development may serve as a benchmark against which to assess the exotic parasitoids. The second ob­ jective was to determine the influence of temperature during immature stages on adult size. Body size is related to fecundity in aphidiids (Hofsvang 1991) and is therefore an important factor influencing parasitoid population dy­ namics. MATERIALS AND METHODS