Factors influencing the diversity of gall-forming insect species on Australian plants

Factors with the potential to influence the species richness of insects which induce galls on plants include host plant taxon, plant adaptations to low soil fertility, and size and dryness of the host plant geographic range. These factors were investigated for gall-forming insects on Australian plants using a data base compiled from existing records and new information collected from field surveys. The data base indicated that galling was concentrated in certain insect and plant lineages. Seven broad groups of insects were reported to cause galls. These were coccoid bugs, chalcidoid wasps, thrips, flies, psyllids, beetles and moths. The most gall-prone plant genus was Eucalyptus. Evidence from differences in galling on eucalypt species suggested that host plant species with many gall species belong to large subgenera. Gall species numbers seem to be limited on host plants with small geographic ranges. Soil fertility and environmental dryness did not appear to affect gall species richness. The factors which might affect gall species richness were tested more rigorously by field surveys designed specifically for that purpose. To test whether the pattern of high gall species richness associated with plants adapted to low soil fertility, found for other biogeographical regions, applies in the Australian region, a field comparison of galling on vegetation at infertile and fertile sites was carried out. I recorded the number of gall-forming insect species (morphospecies), complex and simple gall species, host plant species, total plant species, and total soil phosphorus (a measure of soil fertility) at eight sites in the Sydney region of NSW, Australia. Total soil phosphorus levels ranged from 65-961 mg P/kg. Gall-forming insect species richness was greater at less fertile sites than at more fertile sites, as was the proportion of gall-forming insect species with complex gall morphologies. At all sites where they occurred, myrtaceous tree species supported numerous gall insect species, while most other plant species were associated with only a few, or no gall insect species. The greater gall-forming insect diversity and higher proportion of gall species with complex gall morphology at lower fertility soil sites could be accounted for largely by the greater number of myrtaceous tree species (mainly of the genus Eucalyptus) at these sites. Gall-proneness was concentrated in only a few plant lineages and was not an intrinsic characteristic of plants adapted to infertile soils. Identification of the relevant characteristics of these gall susceptible plant groups is needed to help advance understanding of patterns in gall diversity and complexity. Accordingly, the emphasis of the thesis shifted to consideration of factors which might influence gall species richness within gall-prone Australian plant genera. A field survey was designed to test whether host plant geographic range size, usually found to be positively correlated with insect species richness in other parts of the world, affected the number of gall species on Australian Eucalyptus species. I assessed the local and regional species richness of gall-forming insects on five pairs of closely related eucalypt species. One pair belonged to the subgenus Corymbia, one to Monocalyptus, and three to different sections of Symphyomyrtus. Each eucalypt pair comprised a large and a small geographic range species. Species pairs were from coastal or inland regions of eastern Australia. The total number of gall species on large geographic range eucalypt species was greater than on small range eucalypt species but only after the strong effect of eucalypt taxonomic grouping was taken into account. There was no relationship between eucalypt species geographic range size and the size of local gall species assemblages, but the variation in insect species composition between local sites was higher on large range eucalypt species than on small range eucalypt species. Thus the effect of host plant geographic range size on regional insect species richness was due to greater between-site differentiation among the more widespread local sites of host plant species with large ranges, rather than to greater numbers of gall species at each local site. This study confirmed the role of host plant geographic range size in the determination of insect species richness and provided evidence of the importance of host plant taxon. Indications that dry environments favour galling, as suggested by gall surveys in North America and Brazil, were looked for but none were found. To test whether patterns of galling found for Australian eucalypts generalized to a different radiation of gall insect species another field survey was undertaken, this time with Acacia as the host plant genus. This field survey was designed to determine the effect of Abstract 6 acacia taxonomic group, geographic range size, and rainfall zone on gall-forming insect species numbers. Local and regional gall insect species richness was estimated on eight pairs of closely related acacia species, from four Acacia sections, with ranges in low or high rainfall areas of NSW, Australia. Each acacia pair comprised a large and a small geographic range species. Unlike eucalypts, where host plant geographic range size was positively related to regional gall species richness, there was no relationship between acacia range size and galling. It seems likely that the low absolute numbers of gall insect species on phyllodinous acacias may have masked the species-area effect. There was also no detectable effect of host plant range rainfall zone on gall species numbers on acacias. The lack of reliable gall sites on acacias in low rainfall areas, caused by the unreliability of rainfall, and the dominating effect of low soil fertility in wet as well as dry areas, may be stronger than the proposed positive effects of fewer enemies and diseases in drier areas. There was no relationship between acacia taxon size and gall species richness but galling on acacias in the bipinnate acacia taxon group (which has true leaves) was significantly higher than on most phyllodinous acacia taxa (which have modified petioles instead of leaves). Thus the effect of taxon in acacias is via structural differences between taxon groups, rather than differences in taxon size, as it was for eucalypts. The results of the work undertaken for this thesis suggest that patterns of insect galling in Australia generally conform to global patterns in terms of the importance of host plant taxon and geographic range size. Sclerophylly, in response to widespread low soil fertility, seems to exert a strong effect but needs further investigation. Unlike other parts of the world, dry environments in Australia do not appear to be more favourable for gall insects than wet environments. The influence of meristem availability, host plant architecture and latitude on gall species richness, factors so far not fully explored, are promising areas for future research.