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 calif ornicus, of oak trees ranging across a hybrid continuum from pure Quer cus dumosa to pure Q. engelmannii. Two components of utilization were considered, the proportion of trees parasitized and the density of gal ls on parasitized trees. Although the wasp normally only parasitizes Q . dumosa, galls were found on trees with up to 85% of the genetic char acter 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 factor s in the host determine its acceptability, however, these factors are substitutable, such that no single factor is critical, but some thresh old number of factors must be exceeded.