Impacts of the biocontrol agent Malacorhinus irregularis (Coleoptera, Chrysomelidae) on Mimosa pigra seedlings and the importance of root nodules

This study investigated the impacts of the biocontrol agent Malacorhinus irregularis Jacoby (Coleoptera, Chrysomelidae) on the weed Mimosa pigra L. (Mimosaceae). We used controlled experiments to determine whether larvae of different developmental stages can destroy mimosa seedlings, whether larvae can survive and develop when feeding on root nodules, whether larvae prefer root nodules or seedlings, and the importance of N₂ fixation to mimosa. One third instar larva destroyed a mean of 1.6 seedlings overall, although this varied with larval density. First instar larvae spent more time on seedlings than on nodules, but final instar larvae spent more time on nodules. Larvae survived and developed on root nodules and on seedlings. Mimosa plants growing in pots only produced high numbers of root nodules when growing in low N conditions, indicating that mimosa responds to soil low N status by increasing symbiotic N₂ fixation. The higher N content in mimosa leaves than leaves of native plants from north Australian wetlands, and the ability to vigorously nodulate in conditions with a low N supply suggest that mimosa relies on N₂ fixation during times of low soil N availability and at sites with low N status. We propose that Malacorhinus below ground herbivory on root nodules and seedlings complements the above ground herbivory of other established biocontrol agents against mimosa.