Influence of surrounding trees and bushes on the phytoseiid mite fauna on apple orchard trees in Finland

Abstract

Predaceous phytoseiid mites were collected from 14 apple orchards and from nearby deciduous trees and bushes in southern Finland. Phytoseiids were rare in insecticide-treated orchards bordered by just a few suitable host plants, whereas they were more abundant in other sprayed orchards that were surrounded by many suitable host plants. Phytoseius macropilis was the dominant phytoseiid species on unsprayed apple trees, but Euseius finalndicus was more common in sprayed orchards. In a 2 ha orchard surrounded by deciduous trees and bushes, the mean phytoseiid mite density was high only 15 days after a dimethoate treatment. Phytoseiid mites, primarily E. finlandicus, were encountered in the vicinity on the following trees: bird cherry (0.3 mites per leaf), hazel (9.8 per leaf), honeysuckle (2.7 per leaf), lime (2.4 per leaf), oak (1.2 per leaf) and great sallow (0.8 per leaf). Phytoseiid densities on adjacent vegetation had a noticeable effect on phytoseiid densities and species composition on apple orchard trees. Tall trees, such as oak and lime, are probably more important than low bushes as natural sources of phytoseiids for aerial dispersal into orchards. The migration of E. finlandicus into orchards from surrounding vegetation is concluded to be faster than that of Phytoseius macropilis.