HOST-PLANT ALTERS THE SHAPE OF THE FUNCTIONAL-RESPONSE OF AN APHID PREDATOR (COLEOPTERA, COCCINELLIDAE)

The response of the lady beetle Propyka quatuordecimpunctata (L.) to t he density of the Russian wheat aphid, Diuraphis noxia (Mordvilko), wa s measured in petri dish arenas as well as on plants with divergent le af architectures. Logistic regression was used to distinguish the shap e of the functional response (type II or III), and nonlinear least-squ ares regression was used to estimate attack coefficients (a) and handl ing times (TI,). In dishes, the behavior of both beetle larvae and adu lts closely conformed to a type II response. Estimates of a and T-h on whole plants were considerably different from those obtained from dis hes, and they also depended on plant species. Beetle adults consumed m ore aphids on the slender-leaved Indian ricegrass, Oryzopsis hymenoide s (Roemer & Schultes) Ricker, than on the broad-leaved crested wheatgr ass, Agropyron desertorum (Fisher ex Link) Schultes, at each aphid den sity. Moreover, logistic regression suggested a type II response on In dian ricegrass versus a type III (sigmoidal) response on crested wheat grass. The complex response on crested wheatgrass may have been caused by density-dependent changes in the proportion of aphids in refuges ( such as rolled leaves). By modifying the shapes and parameters of func tional responses, plant traits can influence the stability of predator -prey dynamics and the success of biological control.