Natural impact of the flea-beetle, Longitarsus sp., on Heliotropium amplexicaule in Argentina and its potential for use as a biological control agent in Australia.

Heliotropium amplexicaule (Boraginaceae) was introduced into Australia from South America over 100 years ago and has since become a serious weed in summer pastures. In 1998, CSIRO started a classical biological control project against H. amplexicaule to search for and evaluate natural enemies in its native range in Argentina. This study quantifies the impact of one such natural enemy, the fleabeetle Longitarsus sp. (Coleoptera: Chrysomelidae), on H. amplexicaule, to determine its potential for control of the weed in Australia. Longitarsus sp. adults chewed small feeding holes in the leaves of H. amplexicaule plants, leading to necrosis and death of the remaining leaf tissue. Feeding started from the basal leaves, with new leaves attacked as they developed on the growing plants. Unattacked control plants achieved maximum size by early March. The mean growth of attacked plants at the two attacked sites was only 54 and 48%, respectively, of that shown by these control plants. Moreover, these plants showed greatly reduced vigour and half of them had died by mid-March at one site and mid-May at the other. There was no plant mortality at the control site. Finally, control plants produced significantly larger quantities of flowers and seed over a longer period than did attacked plants. The overall impact of Longitarsus sp. is due to a combination of adult feeding on leaf tissue and larval feeding on the root system. The study also demonstrates a synergism between the flea-beetle and pathogenic micro-organisms causing subsequent leaf necrosis. Such impact on plant survival and reproductive potential suggests that Longitarsus sp. could be a good biological control agent for H. amplexicaule in Australia, provided it demonstrates sufficient host specificity.