Indirect non-target effects of insecticides on Tasmanian brown lacewing (Micromus tasmaniae) from feeding on lettuce aphid (Nasonovia ribisnigri)

Tasmanian brown lacewing, Micromus tasmaniae (Walker) (Neuroptera: Hemerobiidae), is an indigenous aphid predator in New Zealand and Australia. Lacewing mortality can result directly from contact with insecticides and/or indirectly from the consumption of insecticide-intoxicated prey. Laboratory bioassays were set up to examine the indirect non-target effects of insecticides on M. tasmaniae populations feeding on lettuce aphid, Nasonovia ribisnigri (Mosley). Larvae of M. tasmaniae were initially tested against three commonly used insecticides at two dose rates (approximate recommended field rate and half field rate). 1st to 4th instar aphids were fed on insecticide-treated lettuce for 24 h and then fed to 1st instar M. tasmaniae larvae. Pirimicarb (Pirimor® 50) caused 30–40% mortality of M. tasmaniae larvae exposed at 1st instar for 3 days. Pymetrozine (Chess® WG) showed less than 20% mortality, while imidacloprid (Confidor®) caused over 96% mortality of M. tasmaniae larvae over this period. A dose–response bioassay was then set-up to determine the impacts of lower doses of imidacloprid through until pupal development of M. tasmaniae larvae. There was no increase in mortality due to low doses (one sixth field rate or less) but from days three to eight, M. tasmaniae larvae showed significant evidence of delayed developmental rate from 3rd instar larvae into pupae. These results demonstrate that the use of imidacloprid, pirimicarb and pymetrozine for control of lettuce aphid has indirect non-target effects on M. tasmaniae. These effects need to be considered in developing Integrated Pest Management (IPM) systems for the New Zealand lettuce industry.