Biological control of a tropical weed: A population model and experiment for Sida acuta

Published source details Lonsdale W. M., Farrell G. & Wilson C. G. (1995) Biological control of a tropical weed: A population model and experiment for Sida acuta. Journal of Applied Ecology, 32, 391-399.
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Summary

Sida acuta (Malvaceae) native to central America now has a pan-tropical distribution and in some areas, such as parts infests pastures and cropland The effects of defoliation of S.acuta by an introduced biological control agent, the chrysomelid beetle Calligrapha pantherina, were investigated in northern Australia with an insecticidal exclusion experiment.

Biologocal control insect: The chrysomelid beetle Calligrapha pantherina (native to Mexico) was first released in northern Australia in 1989 where it was observed to almost completely defoliate the invasive S.acuta. Both adult and beetle larvae feed on leaves, flowers and fruits of this, and also two other, Sida spp.

Effects on seed production: A heavily infested pasture near Noonamah (12˚38'S, 131˚5'E) 50 km from the city of Darwin in the Northern Territories was selected for C.pantherina releases. On 24 November 1989 123 adult beetles were released, with a further release of 181 on 8 January 1990 in one corner of the field. As the population expanded through the pasture, 20, 1 m² quadrats were laid out on 2 April 1990 amongst healthy Sida plants ahead of the advancing colonization front. Ten quadrats were randomly selected for treatment with the systemic granular insecticide carbofuran (Crop King Furadan™) applied to the soil at 1 g a.i./m². The beetles subsequently expande across the entire field, and plants in quadrats were allowed to flower and fruit until flowering had ceased on 25 May.

Vegetation in quadrats was then cut and sorted into S.acuta and other vegetation. The number of S.acuta plants per quadrat, the number of pods/plant (in untreated quadrats) or a sample of five plants (in insecticide treated quadrats) were counted. Cuttings were dried and weighed.

On 13 June 1991 weed densities were again monitored in 35, 1 m² quadrats placed at random in the pasture (but excluding the area sampled previously). A further 20 biomass samples were taken.

Calligrapha pantherina was found to reduce annual seed production of S.acuta from around 8,001 seeds/m² to 731/m². However, there was no measurable effect on individual plant survival, mass per seed, or total biomass of the weed in the year of defoliation. Whilst the insecticide treatment dramatically reduced defoliation, had no significant effect on overall biomass (due to predominance of woody material) mass per seed or density of the plants or other weed species.

A model for annual plants was adapted and used to predict the density at flowering of S.acuta in the following year that would result from the measured reduction in seed output. This predicted a fall in density from 76 plants/m² to 42/m² in the year following defoliation. The actual density in the field was 50/m², not significantly different from the predicted value.

Conclusions: If the level of reduction in S.acuta seed output recorded in this study were maintained, its density would continue to fall to a level that would depend on the searching efficiency of the beetle. However, the authors highlight that there are still considerable uncertainties in the estimation of some of the model's plant parameters, and the grazing and population dynamics of the beetle remain to be determined.

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