PARENT SESSION
Oral Session # 43: A. Insect Ecology I; B. Traditional Ecological Knowledge.
Presiding: E Borer and SP Yanoviak
Wednesday, August 6. 8:00 AM to 11:30 AM, SITCC Meeting Room 105.

Factors determining spatial distribution of an herbivorous insect: A model of patch density-dependent rates of parasitism vs. dispersal mortality.

Yoo, Ho Jung^*,1, ¹ Purdue University, West Lafayette, IN, USA

ABSTRACT- The western tussock moth, Orgyia vetusta, is a poor disperser in patchy coastal habitats. Only first instar larvae are capable of significant between-patch movement, which is accomplished by passive wind dispersal. At a site in central California, however, moth abundance is greater on isolated patches of its host plant, Lupinus chamissonis, despite the likelihood that dispersal mortality is higher in areas of low patch density. Empirical studies have previously shown that 1) mortality in crawlers (instars II to VI) is density-dependent, and 2) fecundity, pupal predation, and pupal parasitism by a wasp are significantly correlated with local patch density. Only pupal parasitism, however, varies in a manner consistent with the observed abundance pattern. To determine whether variation in pupal parasitism is sufficient to explain the distribution of moths, a spatially-explicit agent-based simulation model was developed. Within-patch dynamics included all four significant relationships in demographic processes mentioned in 1) and 2) above. Dispersal between patches was localized. A simulation experiment was run in a 2 x 2 factorial design to qualitatively assess the independent and joint effects of local (vs. global) dispersal and the spatially varying (vs. fixed) birth and death rates. The results indicated that while local dispersal alone causes highest abundances in areas of high patch density, the effect is essentially reversed when variability in pupal parasitism by the wasp is included; areas of high patch density attain the lowest moth abundances. This result was robust to a realistic range of parameter values for dispersal distances, emigration rates, and larval death rates. Thus, attack patterns by a parasitoid appear to have a greater influence than does dispersal ability on moth distribution.

Key words: spatially-explicit simulation, Orgyia vetusta, limited dispersal