Effects of plant density on the survival rate of cabbage pests

The population density of herbivores depends on the spatial scale as well as the temporal scale. In a small‐scale, short‐term experiment, the number of individuals entering from the surrounding area will be most influential in determining the herbivore density. In large‐scale, long‐term experiments, however, the density of herbivores will rather be influenced by the survival rate of individuals inside the field because most of the herbivorous population derives from the parents that developed inside the field. If we want to predict the large‐scale long‐term density of herbivores, therefore, emphasis should be placed on the estimation of survival rate. To elucidate the effects of plant density on the large‐scale long‐term abundance of cabbage pests, we examined the survival rates of three lepidopterous pests, the small white butterfly Pieris rapae crucivora Boisduval (Pieridae), the beet semi‐looper Autographa nigrisigna (Walker) (Noctuidae), and the diamondback moth Plutella xylostella (Linnaeus) (Yponomeutidae) under two levels of plant spacing (sparse plot, 2 m × 2 m interval; dense plot, 0.5 m × 0.5 m interval). The experiment with four blocks was repeated in two seasons. The number of eggs per plant was larger in the sparse plots than in the dense plots for all species. The survival rate of eggs and larvae, on the contrary, was lower in the sparse plots than in the dense plots. The lower survival rate of eggs in the sparse plots was mainly caused by the density dependency, while the lower survival rate of larvae in the sparse plots was mainly caused by the direct effects of plant density. It was thus suggested that the density of herbivores may become lower in the sparsely planted field in the long run because of the higher mortality of larvae.