Control of Folivorous Larvae Damage by Forest Bird Predation

The contribution of biodiversity to ecosystem stability and productivity has been reported in experiments and surveys on grasslands, forest vegetation, and coastal areas. However, to date, no long-term studies have been conducted from this perspective on the role of birds as predators of forest folivorous insects. Therefore, changes in breeding bird density and in frass production of insect larvae were surveyed for 10 – 20 years in three different forests, including zones with nest boxes and without nest boxes. As the first report of this study, we describe the effect bird predation in controlling the larval damage by birds. The production of folivorous larval frass (g/m2) during May – July was estimated to decrease by an average of 70% due to bird predation. The coefficient of variation (CV) of observed larval frass amount (OFA) in the larch Larix kaempferi forest was significantly smaller in the nest box set forest than in the no-nest box forest. The CV of OFA decreased in forests with higher predatory potential (TPP) of bird communities per OFA. Predation-induced reduction in frass per TPP obtained from the regression model showed a density-dependent functional response. In conclusion, forest insectivorous birds are thought to always play a major role in the frass reduction and stabilization of folivorous larvae damage. To increase the control effect of insectivorous birds, it is necessary to increase the density and diversity of the bird community by introducing broad-leaved trees or holed trees inside and outside the plantations. The effect of predation of larvae by birds on the growth of larch forest will be reported separately.