Utilization of Hybrid Oak Hosts by a Monophagous Gall Wasp: How Little Host Character is Sufficient?

We examined the utilization by the cynipid gall former, Andricus californicus, of oak trees ranging across a hybrid continuum from pure Quercus dumosa to pure Q. engelmannii. Two components of utilization were considered, the proportion of trees parasitized and the density of galls on parasitized trees. Although the wasp normally only parasitizes Q. dumosa, galls were found on trees with up to 85% of the genetic character of the non-host, as determined by a hybrid index. The proportion of hybrids utilized did not differ statistically from the proportion of pure Q. dumosa utilized. We consider competing hypotheses about the genetic mechanism behind host choice in the light of our results. The model that is most consistent with the observed incidence pattern is based on a "cluster concept": A number of independent genetic factors in the host determine its acceptability, however, these factors are substitutable, such that no single factor is critical, but some threshold number of factors must be exceeded.