Testing the effects on Mimosa pigra of a biological control agent Neurostrota gunniella (Lepidoptera : Gracillaridae), plant competition and fungi under field conditions

The impact of herbivory, fungi and competition on Mimosa pigra was studied in the field in Australia using insecticide and fungicide exclusion experim ents and monitoring. The insect Neurostrota gunniella, a stem-boring moth i mported for the biological control of M. pigra, had no apparent effect on s eedling survival, which was however strongly reduced by inter- and intra-sp ecific competition. In mature plants, the insect reduced radial growth of t he canopy by about 14% in a season, but the insecticide had a negative effe ct on seed production, perhaps because it disrupted native pollinating inse cts. Fungicide had no effect, implying that native fungi were not affecting plant performance. A 5-year monitoring study showed a strong negative corr elation between the density of the insect and seed output, such that seed o utput was 60% lower than normal at the highest densities of the insect. Fur thermore, 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 autho rs conclude tentatively that theta has been an impact of the insect on seed output, though this would not be sufficient to control the weed Although m easuring the impact of biological control agents in the field is fraught wi th difficulties, the asymmetrical designs recently proposed for environment al impact assessment may represent a solution.