Eucalypt hybrid susceptibility to Gonipterus scutellatus (Coleoptera: Curculionidae)

Damage caused by the eucalypt snout weevil Gonipterus scutellatus Gyllenhal (Coleoptera: Curculionidae) (= G. gibberus Boisduval) was found on a greater proportion of F1 Eucalyptus amygdalina Labill. X E. risdonii J. D. Hook. hybrid trees than either of the pure species, in a replicated field trial in south-east Tasmania. A greater proportion of E. risdonii trees was also damaged than E. amygdalina trees. A study of the pattern of oviposition within the trial revealed no difference in oviposition by G. scutellatus between E. risdonii and E. amygdalina. Oviposition by G. scutellatus was significantly higher on the F1 and F2 interspecific hybrids between these two eucalypt species compared with pure species crosses. There were no apparent differences in damage or oviposition levels between the F1 hybrids and the single large F2 progeny included in the trial. This finding provides little evidence that hybrid susceptibility is due to hybrid breakdown after the first generation. Rather, the evidence is consistent with the hypothesis that there may be different mechanisms of host defence operating in each species that are somehow diluted below a threshold level in the hybrids.