BIOLOGY OF MESOSTOA-KERRI (INSECTA, HYMENOPTERA, BRACONIDAE, MESOSTOINAE), AN ENDEMIC AUSTRALIAN WASP THAT CAUSES STEM GALLS ON BANKSIA-MARGINATA

The biology of Mesostoa kerri Austin & Wharton, a member of the endemi c Australian subfamily Mesostoinae, was investigated in the laboratory and at the only known field site at Reedy Creek, South Australia, whe re it causes stem galls on Baizksia marginata Cav. Galls vary in shape from spherical to elongate, with larger elongate galls appearing to i nhibit distal foliage growth. Their internal structure is characterise d by a large number of chambers occupied by wasp larvae, a melanised c ambial layer, and partial disruption and proliferation of xylem tissue . Larvae were observed to feed directly on the walls of their chambers . There is a direct relationship between the size of galls and number of resident wasps, with the largest galls containing up to 300 individ uals. Total wasps dissected from galls had a sex ratio close to 1:1, a lthough the ratio varied among galls from strongly male to strongly fe male biased. Females are sluggish in behaviour and have not been obser ved to fly. They oviposit into preapical green stems and the pattern o f oviposition may determine the shape and size of the resultant gall. Male wasps emerge just prior to females and probably wait on galls for females to emerge before mating. A number of factors, including the a bsence of early-stage galls and substantial foliage growth distal to s ome galls in September-October, indicates that M. kerri has an annual life cycle. Other gall residents occurred in 57% of sampled galls, and included 11 species of Hymenoptera, 3 of Coleoptera and 1 of Lepidopt era. Data available for other Mesostoinae suggest that Banksia is the exclusive host group and all species are gall formers.