Effects of pesticides on mite predators (Acari: Phytoseiidae) and colonization of apple trees by Tetranychus urticae.

A 2-year survey of mite populations and pesticide use was carried out in Nova Scotia, Canada, in apple orchards where the two-spotted spider mite ( Tetranychus urticae Koch) was the dominant phytophagous mite. Fungicides were the only class of pesticides that affected cumulative number of T. urticae-days per leaf in tree canopies and colonization success — the ratio of T. urticae-days to number of immigrating T. urticae caught in sticky bands on tree trunks. In 2000, increased numbers of T. urticae-days in the tree canopy were associated with more frequent applications of ethylene bis-dithiocarbamate (EBDC) fungicides and with higher fungicide toxicity scores, which indicate cumulative level of suppression of the phytoseiid predator Typhlodromus pyri Scheuten by all fungicide applications. Higher rates of colonization success were also associated with higher toxicity scores. EBDC’s applied in 2000 promoted T. urticae immigration as indicated by their counts on sticky bands. In 2000 and 2001, number of T. pyri-days in the tree canopies was decreased by more frequent EBDC applications and by higher fungicide toxicity scores. Promotion of T. urticae in tree canopies by EBDC’s was attributed to their toxicity to T. pyri. Both T. pyri and another phytoseiid, Amblyseius fallacis (Garman), were found in ground cover vegetation. Hence, increased immigration from the ground cover attributed to the toxicity of EBDC’s to T. pyri and, especially, to A. fallacis, which is a specialist predator of spider mites and an effective natural enemy of T. urticae. [ABSTRACT FROM AUTHOR]