Malacorhinus irregularis for biological control of Mimosa pigra: host-specificity, life cycle, and establishment in Australia

Malacorhinus irregularis Jacoby (Coleoptera: Chrysomelidae: Galerucinae: Galerucini), from Mexico is identified as a potential biological control agent for Mimosa pigra L. (Mimosaceae), a serious weed of northern Australia and Asia. The adults feed on leaves of the host, and the larvae develop on seedlings, roots, and perhaps other plant parts. The damage to the target plant is substantial, indicating that this insect could be an effective control agent. Host-specificity tests examined the suitability of seedlings and leaves for larval development, and suitability of leaves for adult feeding. In no-choice tests, no larval development occurred on any of the 81 test plant species other than M. pigra. The extent of adult feeding on the test plants was negligible in the tests using a choice-minus-control design, being less than 1% of that which occurred on M. pigra. We conclude that M. irregularis is a specialist on its host and the risk associated with its release in Australia is low. It was released in infestations of M. pigra in the Northern Territory of Australia in 2000. Establishment and abundance was monitored at one site where the number and distribution of adult beetles fluctuated widely as soil moisture conditions varied through the seasons. Adults were found for two years after release and local defoliation of plants was attributed to this species. Although only limited observations were made, adult feeding was not recorded from Neptunia major (Benth). Windler plants growing in close proximity to M. pigra, indicating specificity under field conditions.