THE POPULATION-DYNAMICS OF AN INTRODUCED TREE, SESBANIA-PUNICEA, IN SOUTH-AFRICA, IN RESPONSE TO LONG-TERM DAMAGE CAUSED BY DIFFERENT COMBINATIONS OF 3 SPECIES OF BIOLOGICAL-CONTROL AGENTS

This paper contributes to the relatively sparse literature on the effe cts of insect herbivory on the population dynamics of plants and is pr obably unique in that it reports the long-term effects of combinations of three insect herbivore species on the population densities of a mo derately long-lived tree species. The tree is Sesbania punicea, a legu minous perennial from South America that has been the target of a biol ogical control programme in South Africa for almost 20 years. Sixteen infestations of the weed have been monitored for periods of up to 10 y ears to determine changes in the density of the mature, reproductive p lants under the influence of different combinations of three biologica l control agents (i.e. with one, two or three of the agent species pre sent in the weed infestation). The three biological control agents, al l weevil species, include Trichapion lativentre, which primarily destr oys the flower-buds, Rhyssomatus marginatus, which destroys the develo ping seeds, and Neodiplogrammus quadrivittatus, whose larvae bore into the trunk and stems of the plants. While T. lativentre occurs through out the range of the weed in South Africa, the other two species are l ess mobile, more recent introductions and are largely confined to the vicinity of selected release sites. There has been a significant decli ne in the density of mature S. punicea in areas where two or more of t he agents are established. The decline of the weed has been most evide nt where N. quadrivittatus is active and particularly so where both of the other two weevil species are also present.