INSECT HERBIVORES AND PLANT POPULATION DYNAMICS

It is one thing to show that herbivorous insects affect plant performance. It is an entirely different matter to demonstrate that insect herbivory affects plant population dynamics. There is a vast literature on insect pests of crop plants that shows how attack by defoliating, sucking, stem-mining, and gall-forming species can delay seed ripening, reduce seed production and individual seed weights, reduce the rates of shoot and root growth, increase the susceptibility of plants to disease, and reduce the competitive ability of plants relative to their un attacked neighbors. This literature tells us virtually nothing, however, about the importance of insects in natural communities, chiefly because we have so little information on the regulation of plant populations in the wild. For example, we do not know whether plant recruitment is seed limited , so we cannot predict whether there would be an increase in plant numbers if the simple experiment of sowing extra seeds were carried out. Information on the nature of population regulation is vital because if plant recruitment is not seed limited, then insects that reduce seed production will not have an important effect on plant population dynamics. This review concentrates on material published from 1 984 to 1987. Earlier material has been covered in other reviews (36, 64, 70, 94, 1 07, 12 1 , 1 82, 206) . The first part examines the impact of insect feeding on different aspects of plant performance. The second part examines two of the best sources of evidence on the impact of insect feeding on plant population dynamics: (a) the release of specialist insect herbivores against target weed species in classical