TEMPERATURE REQUIREMENTS OF 4 PARASITES OF THE RUSSIAN WHEAT APHID DIURAPHIS-NOXIA

We studied the effects of six constant temperatures (7.2, 10.0, 15.5, 21.1, 26.6, and 29.4-degrees-C) on developmental rates and developmental times in four parasites of Diuraphis noxia (Mordwilko) (Homoptera:Aphididae): Aphelinus asychis Walker from Chile, A. asychis W. from France, Aphidius matricariae Haliday from Iraq, and Diaeretiella rapae (M'Intosh) from Pakistan. Differences were not detected in developmental time between sexes in D. rapae at any of the temperatures tested. Males developed faster than females in A. matricariae at 10 and 21.1-degrees-C, A. asychis Chile at 15.5 and 29.4-degrees-C, and A. asychis France at 10, 15.5, and 21.1-degrees-C (P < 0.05). Developmental thresholds (t), and times-to-adult in degree-days (K) of the four parasites are: 7.1-degrees-C and 248.8 for A. asychis Chile, 6.4-degrees-C and 246.7 for A. asychis France, 1.4-degrees-C and 311.4 for A. matricariae, and 2.1-degrees-C and 293.4 for D. rapae, respectively. Based on the predicted accumulation of degree-days and number of generations per year for the four parasites at three climatically distinct localities in California, we concluded that the parasite with the lowest developmental threshold (A. matricariae) may complete ca. 4.5, 3.6, and 2.1 more generations than the parasite with the highest developmental threshold (A. asychis from Chile) at the coldest, intermediate, and warmest localities, respectively. It was suggested that parasites with lower developmental thresholds, despite their generally greater time-to-adult, will have greater population densities following the winter season than parasites with higher developmental thresholds, they may appear earlier and thus have greater potential as effective biological control agents especially in annual crops such as small grains. When attempting to introduce parasites for biological control of a target host the developmental thresholds of both the parasite and the host should be considered in conjunction with other attributes such as searching efficiency, fecundity, host preference, and others.