Toxicity of imidacloprid to Galendromus occidentalis, Neoseiulus fallacis and Amblyseius andersoni (Acari: Phytoseiidae) from hops in Washington State, USA.

The toxicity of systemic and spray formulations of imidacloprid to Galendromus occidentalis Nesbitt, Neoseiulus fallacis Garman and Amblyseius andersoni (Chant) from hop yards in Washington State was evaluated in laboratory bioassays. The field rate of imidacloprid for hop aphids (0.13 g a.i. l) was highly toxic (100% mortality) to G. occidentalis and N. fallacis but less so (35.6% mortality) to A. andersoni. Half and quarter rates were also highly toxic to G. occidentalis and N. fallacis (79.5-100% mortality) but again had lower toxicity to A. andersoni (8.2-31.3% mortality). Systemic toxicity (via consumption of spider mite motiles feeding on leaf discs cut from imidacloprid-treated (0.13 g a.i. l) dwarf bean plants) was also high for G. occidentalis (98.3% mortality), as was toxicity from dried residues (93-98% mortality). Residual toxicity to N. fallacis was also high (89% mortality). The significance of these results for biological control of spider mites in hops and other crops is discussed.