Abstract
Wakayama Prefecture is one of the main production areas of persimmon in Japan. However, western flower thrips, Flankliniella occidentalis (Pergande) has been causing serious damage to persimmon fruit, especially variety Tonewase, when the fruit color turned yellow in early September since 1997. F. occidentalis overwintered on the perennial weed such as chickweed, Stellaria neglecta, and had two peaks of population density on undergrowth of persimmon orchards: one in June when perennial weeds were growing and the other in July-August when summer weeds were growing. Thus F. occidentalis maintained its population through various undergrowth. Since the weeding time was different between persimmon and mandarin orange orchards, the adult F. occidentalis dispersed from an orchard where they existed at a higher density to the adjacent orchards. Therefore, the population density of F. occidentalis was higher in a persimmon-mandarin orange cultivation area than in a persimmon mono-cultivation area. The thrips density on undergrowth of foxtail ears, Setalia viridis, was higher in the persimmon orchard than in the grassland nearby. This is provably because pesticides applied to persimmon trees suppressed the occurrence of natural enemies such as the minute bug, Orius sauteri, on weeds by drafting, causing a resurgence of F. occidentalis in persimmon orchards.
