Aphelinus albipodus (Hymenoptera: Aphelinidae) and Diaeretiella rapae (Hymenoptera: Braconidae) parasitism on Diuraphis noxia (Homoptera: Aphididae) infesting barley plants differing in plant resistance to aphids

Russian wheat aphid, Diuraphis noxia (Mordvilko), as a pest of small grains, ha's prompted research into biological control and host plant resistance. In the presence of Russian wheat aphid, leaves of a susceptible barley (Morex) are curled and chlorotic and sustain large densities of this aphid, while leaves of a resistant barley (STARS-9301B) remain hat and green and sustain fewer aphids. Might parasitism of Russian wheat aphid by Aphelinus albipodus Hayat & Fatima and Diaeretiella rapae McIntosh be affected differently by these plant types? When presented the plants separately and based on parasitism rate relative to aphid density, the larger D. rapae was more effective in parasitizing relatively high densities of aphids within curled leaves of Morex than relatively low densities of aphids on uncurled leaves of STARS-9301B. Parasitism by A. albipodus did not significantly differ among the plants. When given a choice of plants, approximately equal rates of parasitism occurred on the two plant lines for both parasitoid species, and parasitism by D. rapae was greater than A. albipodus. These data indicate that using parasitoid size as an indicator of success in a physically restricted environment may be misleading, when considered in a plant environment responsive in several manners to aphids (chlorosis, curling, and ability to sustain Russian wheat aphid). We expect that use of resistant barley will result in decreased parasitoid abundance as aphid densities decrease. However, parasitism rates are expected to be approximately equal on resistant and susceptible barley. In this system, plant resistance and biocontrol are compatible management strategies. (C) 1998 Academic Press.