Abstract

The biology of Mesostoa kerri Austin & Wharton, amember of the endemic Australian subfamily Mesostoinae, was investigated inthe laboratory and at the only known field site at Reedy Creek, SouthAustralia, where it causes stem galls onBanksia marginata Cav. Galls vary in shape fromspherical to elongate, with larger elongate galls appearing to inhibit distalfoliage growth. Their internal structure is characterised by a large number ofchambers occupied by wasp larvae, a melanised cambial layer, and partialdisruption and proliferation of xylem tissue. Larvae were observed to feeddirectly on the walls of their chambers. There is a direct relationshipbetween the size of galls and number of resident wasps, with the largest gallscontaining up to 300 individuals. Total wasps dissected from galls had a sexratio close to 1 : 1, although the ratio varied among galls from strongly maleto strongly female biased. Females are sluggish in behaviour and have not beenobserved to fly. They oviposit into preapical green stems and the pattern ofoviposition may determine the shape and size of the resultant gall. Male waspsemerge just prior to females and probably wait on galls for females to emergebefore mating. A number of factors, including the absence of early-stage gallsand substantial foliage growth distal to some galls inSeptember–October, indicates that M. kerri has anannual life cycle. Other gall residents occurred in 57% of sampledgalls, and included 11 species of Hymenoptera, 3 of Coleoptera and 1 ofLepidoptera. Data available for other Mesostoinae suggest thatBanksia is the exclusive host group and all species aregall formers.