BIOLOGICAL-CONTROL OF A TROPICAL WEED - A POPULATION-MODEL AND EXPERIMENT FOR SIDA-ACUTA

1. The effects of defoliation of the malvaceous tropical weed Sida acu ta by an introduced biological control agent, the chrysomelid Calligra pha pantherina, were investigated in northern Australia with an insect icidal exclusion experiment. 2. Calligrapha pantherina was found to re duce annual seed production by an order of magnitude, from 8001 seeds m(-2) to 731 m(-2). However, there was no measurable effect on individ ual survival, mass per seed, or total biomass of the weed in the year of defoliation. 3. A model for annual plants was adapted and used to p redict the density at flowering of the weed in the following year that would result from the measured reduction in seed output. The model pr edicted a fall in density from 76.0 plants m(-2) to 42.0 m(-2) in the year following defoliation. The actual density was 50.2 m(-2), not sig nificantly different from the predicted value. 4. If this level of red uction in seed output were maintained, the density of the weed would c ontinue to fall to a level that would depend on the searching efficien cy of the beetle. However, there are still considerable uncertainties in the estimation of some of the model's plant parameters, particularl y for seed losses. Moreover, the grazing and population dynamics of th e beetle remain to be determined. 5. The model is used to show that th e presence of seed carry-over (or generational dormancy) usually lower s plant density but, where herbivory is patchy, it reduces the rate of extinction of populations.