DIFFERENTIAL IMPACT OF EGG PREDATION BY ZETZELLIA MALL (ACARI, STIGMAEIDAE) ON METASEIULUS-OCCIDENTALIS AND TYPHLODROMUS-PYRI (ACARI, PHYTOSEIIDAE)

The differential impact of Zetzellia mall on the phytoseiids Metaseiul us occidentalis and Typhlodromus pyri was studied in the laboratory an d by analysis of population data from orchard plots that contained eit her phytoseiid, similar numbers of prey mites and high or low densitie s of Z. mall. Five hypotheses were evaluated to explain why Z. mall ha d more impact on M. occidentalis in the field than on T. pyri. Given e qual opportunity, Z. mall adult females did not consume more M. occide ntalis eggs than T. pyri eggs nor did adult females of either phytosei id inflict greater mortality on Z. mall eggs or larvae through attack or consumption. There was no difference in the within-tree association of Z. mall adult females with eggs of either phytoseiid species nor w ere there differences in the way prey mites (all life stages) were spa tially partitioned between adult female Z. mall as compared with adult s and deutonymphs (combined) of either phytoseiid. The foraging area o f adult female Z. mali and the oviposition locations of the two phytos eiids from both field and laboratory data were compared using spatial statistics. Metaseiulus occidentalis laid significantly more eggs in t he primary foraging area of adult female Z. mall than T. pyri. This wa s the only factor identified which may explain the greater impact of Z . mall on M. occidentalis. The impact of these interspecific effects o n the persistence of predatory mite guilds and biological control are discussed.