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 density, since the size of a plant varies greatly with its growth stage. To evaluate the importance of selecting an appropriate measure of density, the dynamics 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 noninvasive manner, using the allometric relationship between leaf area and leaf length. Density per leaf area and density per plant showed widely different dynamics in some herbivores. Aphid density per plant increased gradually with the growth of cabbages, while aphid density per leaf area decreased with 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, while the density per leaf area showed a peak level at 15 days after transplanting. Such differences suggest that population dynamics measured per plant may sometimes be a misleading description of actual insect-plant interactions. (Copyright by the Japanese Society of Applied Entomology and Zoology)

Figure 1.   Comparison of the dynamic of aphids (Myzus persicae, Brevicoryne brassicae, and Lipaphis erysimi ) per plant and those per leaf area. Green, blue, magenta, and red symbols correspond to four densities of cabbages; 0.25, 1.0, 4.0, and 8.0 plants/m₂, respectively. Population per plant is rather constant during the cabbage growth except for the latter period. However, population per unit leaf area consistently decreased during the cabbage growth. Thus, the result becomes much different depending on the measure of population density. (Copyright by the Japanese Society of Applied Entomology and Zoology)