EFFECT OF HABITAT SUBDIVISION ON THE POPULATION-DYNAMICS OF HERBIVOROUS AND PREDATORY INSECTS IN CENTRAL CHILE

Habitat subdivision may facilitate the persistence at low densities of both predator and prey populations. But habitat subdivision may also allow herbivores to escape from predators and therefore reach higher l ocal densities. The outcome will depend on the dispersal behaviour of organisms. The effect of habitat subdivision on populations of aphidop hagous coccinellids Eriopis connexa and Hippodamia variegata and their prey Brevicoryne brassicae was evaluated in crops of Brassica olerace ae. In particular, the following responses were analysed: a) density/p lant of herbivorous and predatory insects, b) colonisation of herbivor ous and predatory insects, c) movement between plant patches and resid ence time of predators, d) in situ recruitment of predators, and e) fi nal weight of plants. Nine continuous and subdivided plots of B. olera cea were set up following a 3 x 3 completely randomised block design. The three treatments were: a) continuous, b) subdivided plus weeds, an d c) subdivided plus cloth barriers. Density per plant of herbivorous and predatory insects was lower in subdivided plus barriers habitats. Colonisation by herbivores and predators was more rapid in continuous or subdivided plus weeds habitats. Residence time and in situ recruitm ent of predators was greater in continuous habitats. Plant weight was greater in subdivided than continuous habitats. Therefore, habitat sub division, particularly when patches were more isolated, negatively aff ected the population densities of both herbivorous and predatory insec ts. This is likely due to lesser colonisation and in situ recruitment, and greater emigration of insects in those subdivided habitats.