Prickly acacia biocontrol phase II: host specificity testing of agents from India

Summary

​Prickly acacia (Vachellia nilotica ssp. indica), a multipurpose tree native to the Indian subcontinent, is a Weed of National Significance and is widespread throughout the grazing areas of northern Australia. Biological control of prickly acacia has been in progress since the early 1980s, but with limited success to date. Based on genetic and climate matching studies, native surveys for potential biological control agents were conducted in India, resulting in the prioritisation of several species.

The brown leaf-webber, Phycita sp. A, was imported into quarantine in January 2011 and host specificity testing commenced in June 2011. In no-choice larval development trials the leaf-webber completed development on 16 out of 27 non-target plant species tested. Despite occurring only on prickly acacia in the field, testing of the brown leaf-webber was terminated in December 2012 due to unacceptable non-target feeding.

The second leaf-webber, Phycita sp. B, was imported into quarantine in October and December 2013 and host-specificity testing commenced in May 2014 after colony establishment, though maintaining the colony has proved difficult due to issues with stimulating reproduction. Host-specificity testing of a scale insect, Anomalococcus indicus, commenced in July 2011. The scale insect completed development on 17 of the 83 non-target plant species tested during no-choice trials. However, when provided with a choice, prickly acacia was the preferred host. In view of the field host specificity of the scale insect in India, choice trials under field conditions in India involving non-target test plants on which the scale insect completed development in no-choice tests in quarantine are in progress.

Key findings

Results to date suggest that Neptunia major and Vachellia sutherlandii are likely to be susceptible to the scale insect attack under field conditions. Tests for the susceptibility of other non-target plants for the scale insect under field conditions in India are in progress. The green leaf-webber larvae completed development on 10 out of 19 test plant species under no-choice conditions but in no-choice oviposition trials egg have been laid only on prickly acacia and N. major. However, in paired choice oviposition trials, eggs were laid only on prickly acacia and not on N. major. No further progress on screening the remaining test plants for the green leaf-webber was made due to difficulties in maintaining a culture of the insect in quarantine.

A colony of the leaf-weevil Dereodus denticollis could not be established in the quarantine due to difficulties with its oviposition and larval feeding. Three promising gall-inducing biocontrol insects (a thrips gall, a mite gall and a midge gall) have been identified in Ethiopia. Based on damage potential, field host range and geographic range, the gall thrips, Acaciothrips ebneri (Karny) (Thysanoptera: Phlaeothripidae), inducing rosette galls in shoot tips and sprouting axillary buds resulting in shoot tip dieback, was imported into high-security quarantine in Brisbane, Australia and host specificity tests are in progress. No-choice tests on 14 non-target test plant species so far suggest that the gall thrips is highly host specific with no galls on any of the non-target plant species.