Brassica patch geometry and adjacent matrix: how small-scale changes in landscape can affect Plutella xylostella populations

Herbivorous insects are predicted to have higher densities in larger patches, while their natural enemies will be more affected by the edge of these patches. In this paper, we investigate the effect of the relationship between brassica horticultural plot shape and the adjacent land use on the abundance of P. xylostella and its chances to be parasitised. Samples were collected from brassica plots during spring. Landscape metrics of the study area were estimated using Landsat images. We found a negative relationship between the pest abundance and the perimeter-to-area ratio of the brassica plot, and a positive relationship with land cover estimated from Landsat images. The proportion of parasitised immatures showed opposite relationship with the same variables. Implications of these findings are discussed.