Potential for Interspecific Competition Between Congeneric Longhorned Beetle Species (Coleoptera: Cerambycidae) in an Adventive Environment.

The cerambycid beetle, Phoracantha semipunctata F., was introduced into California in the mid-1980s and killed large numbers of Eucalyptus host trees. The populations of the borer declined to very low levels in the mid-1990s following the establishment of the congener, Phoracantha recurva Newman, and the intentional introduction of the egg parasitoid, Avetianella longoi Siscaro. The distributions of the beetles overlap in the Australian native range, but one species has replaced the other in the adventive range in California. One possible explanation is differential susceptibility to natural enemies introduced for biological control. An alternative explanation for the reduced abundance of P. semipunctata is asymmetric interspecific competition between the two species. To test this hypothesis, equal larval densities of each species were introduced into host logs. In all cases, more P. recurva adults emerged than P. semipunctata adults, but the presence of congeners did not have a different effect than the presence of an equal density of conspecific individuals. Neither the temporal order of introduction or bark thickness altered the outcome of potential competitive interactions. Consequently, it appears that the ecological replacement of one borer with another in the adventive environment in southern California may not be a result of bottom-up intraguild competitive interactions. The top-down effects of natural enemies on P. semipunctata have most likely led to its decline.

Influence of oleoresin constituents fromPinus ponderosa andPinus jeffreyi on growth of mycangial fungi fromDendroctonus ponderosae andDendroctonus jeffreyi.

Dendroctonus jeffreyi andD. ponderosae are morphologically similar sympatric species of pine bark beetles over portions of their geographic ranges; however,D. jeffreyi is monophagous onP. jeffreyi whileD. ponderosae is highly polyphagous. Both species carry a species of mycangial fungi that are also very similar in appearance. Growth of the two mycangial fungi and of the fungusLeptographium terebrantis (associated with the polyphagous and non-tree-killingDendroctonus valens) in the presence of oleoresin constituents of host and nonhost conifers was tested by placing individual chemicals on agar growth medium and by growing the cultures in saturated atmospheres of the chemicals. The fungus associated withD. jeffreyi showed greater tolerance for chemical constituents placed on the growth medium than the other two fungi, and growth after three days was enhanced by heptane, the dominant constituent ofP. jeffreyi oleoresin. Growth of all three species of fungi was reduced by the resin constituents when the chemicals were presented as saturated atmospheres. The results suggest that the influence of the tree on growth of the symbiotic fungi of the bark beetles during the initial attack process may be different than after colonization by the beetles is complete. The difference in the responses of the apparently related species of mycangial fungi may provide some new insight into the evolutionary history of these beetle/mycangial fungus/host tree systems.

Response ofDendroctonus brevicomis andIps paraconfusus (Coleoptera: Scolytidae) to combinations of synthetic pheromone attractants and inhibitors verbenone and ipsdienol.

DendroctonusBrevicomis andIps paraconfusus are sympatric bark beetle species colonizingPinus ponderosa in western North America. Interspecific and intraspecific competition for resources is, in part, mediated through semiochemicals. The response ofD. brevicomis to its attractant pheromone was significantly reduced by simultaneous release of either verbenone or racemic ipsdienol. Trap catch was significantly further reduced by including both inhibitors with the attractant pheromones. However, although the response ofIps paraconfusus was significantly inhibited with the addition of either verbenone or racemic ipsdienol, both compounds together did not result in a significant further reduction in trap catch. There was a trend for greater reduction in response ofD. brevicomis to attractant pheromones with increased release rates of either 69% (+)-/31% (-)-verbenone or 84% (-)-/16% (+)-verbenone. Response of associates to attractants and inhibitor combinations was also determined.