Life stage specific impact of dimethoate on the predatory mite Hypoaspis aculeifer canestrini (Gamasida: Laelapidae).

Toxicants may affect juveniles more than adults because of physiological and behavioral aspects. When developing toxicity tests, this issue is often not addressed, and the most sensitive end point may be ignored. The topic was raised by a European working group aiming to develop a standard toxicity test with the predatory mite, Hypoaspis (Geolaelaps) aculeifer Canestrini, as this species was included in the EU Guidance documentfortesting of pesticides. To assess whether the juvenile life stages are the most susceptible, we examined the acute toxicity of dimethoate on larvae, protonymphs, deutonymphs, males, and females of H. aculeifer. The mites were exposed to 0, 2, 4, and 6 mg dimethoate kg(-1) for 7 days in an OECD artificial soil (5% organic matter). Total juvenile biomass, reproduction, mortality, and population growth rate (pgr) lambda were assessed at the end of the test. A comparison of mortality ranked the sensitivity of the life stages: Larvae (LC50 = 3.8 mg kg(-1)) > protonymphs (LC50 = 5.3 mg kg(-1)) > males (LC50 = 5.6 mg kg(-1)) > deutonymphs (LC50 = 7.1 mg kg(-1)) > females (LC50 = 7.6 mg kg(-1)). Effects on reproduction and pgr were significant at 2 mg dimethoate kg(-1), with population decline starting at this concentration. Thus, a test system with H. aculeifer including reproduction as end point is a rational approach, as reproduction will encompass juvenile mortality, at least with respect to dimethoate. Moreover, we suggest that pgr should be included in chronic standard tests because of high ecological relevance and the feasibility of applying it.