Effects of combination between web density and size of spider mite on predation by a generalist and a specialist phytoseiid mite.

Complicated three-dimensional webs of silk threads produced by Tetranychus spider mites provide protection from predation by many generalist phytoseiid mite species. Accessibility to prey may be the most significant criterion for successful predation in complicated webs. To assess the protective effects of combination between web size and density, we compared predation on eggs of the two-spotted spider mite, Tetranychus urticae, by a generalist (Euseius sojaensis) and a specialist (Neoseiulus womersleyi) phytoseiid mite in the laboratory under manipulated web size and density. Web sizes negatively affected to the predation. Significant interactions were found between phytoseiid mite species and prey distribution; egg consumption by E. sojaensis was more in uniform distribution, but that by N. womersleyi, in contrast, was larger in contagious distribution. However, the egg consumption by E. sojaensis and N. womersleyi were both mitigated in larger webs. This area size depending mitigation was more effective to the predation by E. sojaensis. Although the mechanism of web size depending mitigation is unknown, web sizes might affect prey searching efficiency of this phytoseiid mite. Consequently, combination between web density and size are likely to affect not only a generalist E. sojaensis but also a specialist N. wormersleyi.