1. The harlequin bug, a herbivore on bladderpod, is attacked by two special
ist egg parasitoids Trissolcus murgantiae and Ooencyrtus johnsonii. Ooencyr
tus can out-compete Trissolcus in the laboratory, but coexistence is the no
rm in field populations. Despite the heavy mortality inflicted by the two p
arasitoids, the host-parasitoid interaction is persistent in all sites that
have been studied in southern California.
2. I manipulated inter-patch distances in a field experiment to determine w
hether spatial processes drive parasitoid coexistence and/or host-parasitoi
d dynamics. I first tested the hypothesis that the parasitoids coexist via
a dispersal-competition trade-off. Both parasitoid species took significant
ly longer to colonize isolated patches than well-connected patches, suggest
ing that they have comparable dispersal abilities. Ooencyrtus did not exclu
de Trissolcus even when inter-patch distances were reduced to 25-30% of tho
se observed in natural populations. These data suggest that parasitoid coex
istence can occur in the absence of a dispersal advantage to the inferior c
ompetitor.
3. Since the treatments with isolated vs. well-connected patches did not di
ffer in parasitoid composition, I next asked whether isolation would destab
ilize, or drive extinct, the host-multiparasitoid interaction. No local ext
inctions of bugs or parasitoids were observed during the 18-month study per
iod. Bug populations in the isolated patches were no more variable than tho
se in the well-connected patches. In fact, temporal variability in the expe
rimentally isolated patches was comparable to that observed in highly isola
ted natural populations.
4. These data argue against a strong effect of spatial processes on host-pa
rasitoid dynamics. Local processes may mediate both parasitoid coexistence
as well as the host-parasitoid interaction.