Spatial Density Dependence in Parasitism of Heliothis virescens (Lepidoptera: Noctuidae) by Microplitis croceipes (Hymenoptera: Braconidae) in the Field

Abstract

A field experiment addressed four questions concerning the interaction between Microplitis croceipes (Cresson) and Heliothis spp.: (1) what is the relation between the proportion of hosts parasitized and host density? (2) does this relation depend on the size of the areas (spatial scale) at which it is measured? (3) what is the relation between parasitoid search rate and host density? and (4) what is the relation between parasitoid distribution and host distribution? To answer these questions a cotton field was artificially infested with various densities of Heliothis virescens (F.). M. croceipes was released uniformly across the field. At the scale of 30 by 60 m plots, the proportion of hosts parasitized by M. croceipes (adjusted for competition with other parasitoid species) increased from 0.41 ± 0.08 ( x ± SE) to 0.60 ± 0.06 as host density increased from 6,178 to 16,896 larvae/ha. The calculated search rate (area searched per parasitoid per unit time) of M. croceipes was independent of host density, and the density dependence in parasitism by M. croceipes could be explained completely by density dependence in adult female distribution. Natural populations of Cardiochiles nigriceps Viereck and Cotesia marginiventris (Cresson) parasitized 9 and 7%, respectively, of the hosts collected. At the scale of plots, parasitism by C. nigriceps increased with host density, but parasitism by C. marginiventris was independent of host density. At the scale of 1 by 2 m quadrats, parasitism by all species was independent of host density. We discuss the implications of these results for biological control of Heliothis virescens and Helicoverpa zea and for theories of parasitoid foraging and population regulation.