The Effects of Host‐Finding and Colonization Rates on Abundances of Parasitoids of a Gall Midge

Five wasp species parasitize Rhopalomyia californica, a midge that forms galls on Baccharis pilularis. The relations between wasp abundances and colonization and host—finding rates were addressed with two types of experiments. In the first experiment, galls were stripped from a large area of host plants in the field, and recolonization by the midge and wasps was monitored. Colonization rate, which includes dispersal, search, and successful parasitization, correlated with field abundance of the wasp species in undisturbed habitat, with one exception. Torymus koebelei was the slowest colonizer but was the most abundant wasp. In a second set of experiments, the host—finding rates of the wasp species were measured in the greenhouse, by exposing hosts at field density to single female wasps. The wasp species differed in the number of hosts parasitized per day. Furthermore, this measure of host—finding rate correlated with abundance in the field, again with one exception. T. koebelei was third in host—finding ability, although first in abundance. Colonization rate measured in the field correlates strongly with host—finding rate measured in the laboratory. This suggests that most of the differences among the wasps in colonization rate resulted from differences in ability to find hosts on a small spatial scale (e.g., on a single host plant) and not from differences in dispersal over longer distances. Host—finding rate and fecundity were found not to correlate. These experiments and others on the same system suggest that host—finding rate and larval competitive ability determine the abundances of these wasps.