Interactions between a gall–inducing wasp Trichilogaster acaciaelongifoliae (Hymenoptera: Pteromalidae) and its host plant Acacia longifolia (Fabaceae)

Abstract

Sallow Wattle (Acacia longifolia subsp. longifolia) is a native Australian shrub which is an invasive weed in some parts of Australia, and internationally. A gall-forming wasp (Trichilogaster acaciaelongifoliae), also native to Australia, causes abnormal growth of tissues (galls) in Sallow Wattle. This wasp is used outside of Australia to control invasive populations of this plant species. However, in Australia, the wasp is not effective in managing the spread of Sallow Wattle. This study investigates various aspects of the relationship between the wasp and its host plant in Australian ecosystems to better understand the physiological and ecological processes involved. The study shows that this wasp is host-specific on Sallow Wattle. The feeding action of the larval wasps increase secondary plant compounds in gall tissue, which may assist the plant to defend itself chemically against other insects and microorganisms. The growth of the galls redirects resources which are otherwise used by the plant for growth and reproduction. A second insect species was found within the galls and was identified as Megastigmus sp. This second species is likely to be a parasitoid, killing the larvae of the gall-former and occupying the gall. The presence of Megastigmus sp. in Australian ecosystems may be a key factor affecting the ability of T. acaciaelongifoliae to control Sallow Wattle in its native range. The structure of galls formed by each type of gall-inducing insects is unique and the process of gall induction also varies across species. The present study has specifically examined the initiation and development of galls formed by T. acaciaelongifoliae on A. l. longifolia. Unlike other hymenopteran groups, which induce galls during oviposition, T. acaciaelongifoliae appears to form galls on A. l. longifolia via the larval feeding process. Three major stages of gall development were identified and described: induction of gall, growth and maturation of gall, and shrinking and desiccation of gall. These findings have significantly extended our current knowledge of gall induction and development by the hymenopteran group of insects. Total antioxidant capacity (TAC), total phenol (TP), and total anthocyanin (TA) were measured in galls formed by T. acaciaelongifoliae at different growth stages of galls and in other plant tissue samples of A. l. longifolia to understand the effect of gall formation on plant phytochemistry. The results indicated differences in the amounts of phytochemicals in tissue samples from galls of different growth stages of galls and between gall tissue samples and other plant samples of A. l. longifolia. The highest amount of total antioxidant capacity, total phenols and total anthocyanin were recorded in samples of early stages galls, whereas the minimum amounts of phytochemicals were in stems of A. l. longifolia. Amounts of antioxidant capacity, phenols and anthocyanin gradually declined as galls developed and larvae became less active in their feeding activity prior to pupation. It is assumed that the active feeding action of the larvae results in increased amounts of these chemicals in the early growth stages of the galls. The effect of galls formed by the wasp, T. acaciaelongifoliae on the growth and reproduction of A. l. longifolia was investigated in the native home range of both species, where the plant is invasive. Differences in the average number of phyllodes per sub-branch were found between galled and ungalled plants. Galls were also shown to affect the growth rate of branches. The number of galls correlated positively with twig mortality; and negatively with the number of seedpods per sub-branch. While galls formed by T. acaciaelongifoliae have impacts on the growth and reproduction of A. l. longifolia plants, the plant continues to invade Australian ecosystems. An experiment was conducted to investigate the host plant preference of T. acaciaelongifoliae. Ten different native host plant species (co-occurring with A. l. longifolia in the study locations) were tested in two set of experiments; a ‘free choice test’ and ‘no choice test’. The results showed that T. acaciaelongifoliae is highly host-specific on A. l. longifolia plants. Thus, it was concluded that the presence of other plant species does not explain the continued invasiveness of A. l. longifolia in Australia. A second insect species was found in the galls developed by T. acaciaelongifoliae on A. l. longifolia. The insect species has been identified as another hymenopteran from the genus Megastigmus. Since no T. acaciaelongifoliae emerged from the galls occupied by Megastigmus sp, it is proposed that Megastigmus sp. may feed upon T. acaciaelongifoliae larvae and kill them inside the galls. This might be a key factor affecting the performance of the wasp, T. acaciaelongifoliae in controlling A. l. longifolia in its native distribution. Parasitism rates of Megastigmus sp. should be investigated in future experiments.Doctor of Philosoph