Effect of field-weathered residues of pyriproxyfen on the predatory coccinellids Chilocorus circumdatus Gyllenhal and Cryptolaemus montrouzieri Mulsant

Abstract

The residual toxicity of field-weathered residues of the juvenile hormone analogue insecticide, pyriproxyfen was evaluated against two coccinellids, Cryptolaemus montrouzieri Mulsant and Chilocorus circumdatus Gyllenhal, key predators of mealybugs and scales in citrus in southeast Queensland.
Pyriproxyfen was applied as a high volume spray at 0.1, 1, 10, 25, 50 and 100 mg/L to late Valencia orange trees, and larvae and adults of C. montrouzieri and C. circumdatus were exposed to leaves picked at intervals varying from 0 to 112 days after spraying. The effect on fourth and fifth instar larvae and pupation and subsequent adult emergence, and on the viability of eggs from treated adults, was measured. Regression models were developed to relate larval mortality and egg hatch to pyriproxyfen concentration and weathering time.

Pyriproxyfen killed larvae or prevented pupation of 50% of C. montrouzieri for 64 days at 10 mg/L and for a predicted 167 days at 100 mg/L; the effect on C. circumdatus was even greater, lasting a predicted 210 days at 10 mg/L.

Egg-hatch (from adults exposed to treated leaves) was suppressed at 10 mg/L for 28 days in C. montrouzieri and for 50 days in C. circumdatus; at 100 mg/L, suppression extended to 50 days and a predicted 478 days, respectively.

The combined effects on larvae and eggs of rates between 10 and 100 mg/L would be extremely disruptive to both species. Disruption was much less at 2 mg/L (lasting up to 3 weeks) and at 5 mg/L (lasting up to 7 weeks). Because of its potency against scale insects, pyriproxyfen may yet have a role in Integrated Pest Management (IPM) providing it is used very sparingly at dosage rates no greater than 2 mg/L.