Testing the Effects on Mimosa pigra of a Biological Control Agent Neurostrota gunniella (Lepidoptera: Gracillaridae), Plant Competition and Fungi Under Field Conditions

Abstract

The impact of herbivory, fungi and competition on Mimosa pigra was studied in the field in Australia using insecticide and fungicide exclusion experiments and monitoring. The insect Neurostrota gunniella , a stem-boring moth imported for the biological control of M. pigra , had no apparent effect on seedling survival, which was, however, strongly reduced by inter- and intra-specific competition. In mature plants, the insect reduced radial growth of the canopy by about 14% in a season, but the insecticide had a negative effect on seed production, perhaps because it disrupted native pollinating insects. Fungicide had no effect, implying that native fungi were not affecting plant performance. A 5-year monitoring study showed a strong negative correlation between the density of the insect and seed output, such that seed output was 60% lower than normal at the highest densities of the insect. Furthermore, even at the lowest insect densities found in the field, the mean seed output and leaf litterfall were less than 50% of the value found in a study carried out prior to the insect's release in the mid 1980s. The authors conclude tentatively that there has been an impact of the insect on seed output, though this would not be sufficient to control the weed. Although measuring the impact of biological control agents in the field is fraught with difficulties, the asymmetrical designs recently proposed for environmental impact assessment may represent a solution.