NATURAL ENEMIES AND HOST-PLANT ASYNCHRONY CONTRIBUTING TO THE FAILURE OF THE INTRODUCED MOTH, COLEOPHORA-PARTHENICA MEYRICK (LEPIDOPTERA, COLEOPHORIDAE), TO CONTROL RUSSIAN THISTLE

Host-plant synchronization, predators, and parasitoids limited the population growth of Coleophora parthenica Meyrick (Lepidoptera: Coleophoridae), a moth introduced into California for the biological control of Russian thistle, Salsola australis R. Brown (Chenopodiaceae), that was studied in the Coachella Valley of southern California during 1979/80 and 1985/86. The oviposition period of the overwintered F3 generation largely coincided with the highest mortality of young plants. Field observations indicated that this plant mortality caused the death of the C. parthenica larvae living on these plants, and was mainly responsible for the drastic population decrease of C. parthenica in early spring. During summer generations, rodent predation and parasitism of the larvae were minimal; spider predation accounted for 28.7 and 30.4% of the F1 and F2 moths, respectively. The C. parthenica population slightly increased during summer 1979 and was highest in the desiccated plants during winter 1979/80. During winter 1979/80, intrinsic larval mortality was 8.5%; rodents removed 25% of the larvae, and larval parasitism reached 42.1%, as compared with only 13.5 and 11.0% larval parasitism in 1984/85 and 1985/86, respectively. Rodent predation was limited by the diameter of the branches, reaching highest rates in the second order branches. Parasitoids did build up high densities, especially during mild winters, as in 1979/80 and predominantly attacked larvae in the thinner third order branches, which the rodents could not reach. Thus, poor host-plant synchronization, predators, and parasitoids considerably limited the population growth of C. parthenica. The extent of these mortality factors, together with the limited impact of the larval mining, helps to explain the failure of this moth to control Russian thistle. © 1990.