Ecological interactions between two natural enemies of the Light Brown Apple Moth

Abstract

The light brown apple moth (LBAM), Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), is the most damaging insect pest of grapevines in Australia, causing upwards of $70 million AUD worth of damage to the Australian wine grape industry annually. The gregarious ectoparasitoid Goniozus jacintae Farrugia (Hymenoptera: Bethylidae) and the solitary endoparasitoid Dolichogenidea tasmanica (Cameron) (Hymenoptera: Braconidae) are the two most common, native, natural enemies that parasitise LBAM. Dolichogenidea tasmanica parasitises the early larval instars of LBAM, whilst G. jacintae parasitises the later instars. Both parasitise the 3rd instar of LBAM, suggesting that direct interspecific competition may occur. Biological control methods that suppress pest populations to below economically damaging thresholds are sought after, but require a sound understanding of the interactions between parasitoids and hosts prior to their application. There must also first be a foundation of knowledge for the biology of each species in the interaction. Dolichogenidea tasmanica has been well-studied, but little is known about the behavioural ecology of G. jacintae. Hence, this project began by studying the biology of G. jacintae before moving on to the evaluation of interactions between the two parasitoids. Specifically, this project aimed to investigate: Part One: (1) Foraging behaviour of G. jacintae towards different larval instars of LBAM; (2) Oviposition behaviour of G. jacintae towards different larval sizes and instars of LBAM; and Part Two: (3) Interspecific competitive interactions between G. jacintae and D. tasmanica when attacking the same LBAM host. The key results were that: Part One: (1) Goniozus jacintae exhibits host-stage dependent foraging behaviour towards LBAM: different behaviours were shown at the pre- and post-flight stages and varied according to host instar, flight duration was shortest around 5th instar LBAM, slow walking behaviour was only seen in close proximity to potential hosts and was more common around larger hosts; (2) Goniozus jacintae females produced bigger broods on larger hosts, brood sex ratios were female biased with extremely low variance, and body size of offspring was positively correlated to the amount of host resource available; and (3) Goniozus jacintae has some ability to discriminate between unparasitized and previously parasitised hosts. The probability of oviposition on the second host encountered was influenced by parasitism status of both the current and previous host, clutch size laid on the second host was influenced by parasitism status, but G. jacintae laid more eggs on larger hosts and laid more eggs on the first host encountered. Collectively, these findings contribute towards determining the efficacy of G. jacintae as a potential biocontrol agent of LBAM and could ultimately lead to the improvement of LBAM biological control practices in the field. In addition, these findings contribute to the understanding of bethylid behavioural ecology and can be applied across a range of agro-ecosystems, promoting the long-term stability of managing lepidopteran pest species in the field.