Biological Control of Bellyache Bush: host-specificity testing of the Jatropha leaf-miner

Summary

Introduced into northern Australia in the late 1800s as a hardy ornamental, bellyache bush (Jatropha gossypiifolia L.) has become a serious weed of rangelands and riparian zones of northern Australia. It forms dense thickets, reducing the usefulness of land for pastoral and grazing purposes. Dense infestations along the Burdekin River in Queensland have reduced the carrying capacity to zero. All parts of the plants are toxic to stock.

Biological control is an important component of the long-term management strategy for bellyache bush in Australia. Bellyache bush has been a target for biological control since 1996 but there are currently no biological control agents established in Australia. As part of a renewed biological control effort, recent exploration was undertaken south of the equator in central South America with the aim of identifying new potential biocontrol agents. A yet to be described leaf-mining moth Stomphastis sp. (Lepidoptera: Gracillariidae) from Peru, was prioritised for further studies. The aim of this project was to import Stomphastis sp. into quarantine in Australia, establish a colony and complete host specificity testing of the agent. If suitably specific, an application to release the agent will be compiled and submitted to the necessary Australian Government regulatory bodies.

Over 200 pupae and over 200 larvae of the Stomphastis sp. were collected from 12 sites in northern Peru and were imported into the quarantine facility at the Ecosciences Precinct in Brisbane Australia in November 2014.

Adult Stomphastis sp. are small (less than 1 cm long) and live for an average of 10 days in the quarantine glasshouse. The majority of eggs are laid on the underside of leaves, usually next to a leaf vein. Newly emerged larvae mine directly into the leaf from the egg and remain in the leaf as they develop until pupation. Mature larvae exit the leaves and locate a suitable place to pupate. Most larvae pupate on the leaves but they may also pupate on other substrates. A generation from adult to adult takes 22 days under quarantine conditions.

No-choice host specificity testing of Stomphastis sp. has been completed for 40 test plant species, with at least five replications for each species. The agent laid eggs on numerous non-target species, however development of the agent only occurred on bellyache bush and its congener Jatropha curcas. When female Stomphastis sp. were provided with both bellyache bush and J. curcas, they oviposited equally on both species. Approximately 80% of eggs develop into adults on each of these species. Jatropha curcas is native to tropical America. It is a declared weed in Western Australia and the Northern Territory. It is also an approved target for biological control. As such a non-target risk assessment is not required.

Test results provide strong evidence that Stomphastis sp. is highly host specific and is suitable for release in Australia. It is expected that like other Gracillariidae, Stomphastis sp. will be an adept disperser, a desirable characteristic given the expansive areas across which bellyache bush occurs. A relatively short generation time and high fecundity also bodes well for its future as a biological control agent.

A release application, which will be submitted to the relevant regulatory bodies, has been drafted, as has a proposed release strategy.