The spatial population dynamics of insects exploiting a patchy food resource

The population dynamics of ten species of phytophagous insects and seven parasitoids inhabiting the flowerheads of two herbaceous plants, Centaurea nigra and Arctium minus, were studied, and three main aspects of their ecology were examined, namely, rates of population extinction, density dependence in population changes from one generation to the next, and movements between populations. The study was based on monitoring the insect populations on more than 50 patches of each plant, scattered over 5 km² of arable farmland and the results were used to test the relative importance of immigration and population regulation to the persistence of these populations. This paper describes the study of movements between patches of food plant. Experimentally planted new patches of plants were rapidly colonised by all species and this appeared to be unaffected by distance from a source population, up to the maximum distance of 800 m considered in the experiment. Large patches tended to be colonised more readily than small ones. Movements between plant patches were studied with the use of chemical markers (Rb, Sr, Dy and Cs) which were applied as chloride salts to individual patches, and which were translocated to the flowerheads and so to insects feeding on the seed, and to their parasitoids. Initial experiments in the laboratory showed that these elements could be readily detected by ICP (Inductively Coupled Plasma) mass spectrometry in the bodies of all species reared on potted plants sprayed with solutions containing them. Background levels of strontium were patchily high on the study area, but the other elements were naturally either absent or in very low concentrations. Four patches of each plant were marked with a different element in 1991. In 1992, samples of four species of tephritid fly and two parasitoids were collected from all patches, and analysed for the four elements. These analyses showed that individual of all species moved considerable distances, with movements of up to 2 km being commonly recorded. Estimates of rates of immigration to patches showed that movement plays an important role in the population dynamics of these insects. There was some evidence that immigration was density-dependent: it was highest when the resident populations (numbers per flowerhead) were low.