Insect pest density per leaf area as a measure of pest load

The abundance of insect pests is expressed as the density per plant in most cases. This measure, however, is not always an appropriate measure of dens ity, since the size of a plant varies greatly with its growth stage. To eva luate the importance of selecting an appropriate measure of density, the dy namics of the density of cabbage pests per leaf area was compared with that per plant. The leaf area was continuously estimated in the field in a noni nvasive manner, using the allometric relationship between leaf area and lea f length. Density per leaf area and density per plant showed widely differe nt dynamics in some herbivores. Aphid density per plant increased gradually with the growth of cabbages, while aphid density per leaf area decreased w ith cabbage growth, suggesting that injury by aphids was more severe in the early stages of plant growth. The larval density of the diamondback moth, Plutella xylostella (Linnaeus), per plant increased with cabbage growth, wh ile the density per leaf area showed a peak level at 15 days after transpla nting. Such differences suggest that population dynamics measured per plant may sometimes be a misleading description of actual insect-plant interacti ons.