Sublethal Effects of Pyrethroids on Insect Parasitoids: What We Need to Further Know

In the past decade pyrethroids have emerged as a major class of highly active insecticides due to their high bio-efficacy and relatively low toxicity in comparison to organochlorine and organophosphate pesticides, which are more acutely toxic to birds and mammals than the pyrethroids. These chemicals have been largely used for the control agricultural pests, with Lepidoptera representing the highest percentage (40%) of pyrethroid targeted insects, followed by sucking insects and Coleoptera (Wirtz et al., 2009). On the other hand, insect parasitoids are of high importance in natural and agricultural ecosystems where they regulate or influence the population density of their hosts, being therefore worldwide used in the control of several agricultural pests, particularly Lepidoptera and Hemiptera. Parasitoids can be released in the field either by inundative (for an immediate and non sustaining reduction of the target host population) or inoculative (where the objective is to release the natural enemies early in the season and so is the progeny who will have a later effect on the target host population) approaches. In the first approach, parasitoids are positioned as a fast-acting replacement of insecticides, while in the second parasitoids are considered as one aspect of integrated pest management (Smith, 1996). According to the review by Collier and Steenwyk (2004), biological control by augmentative releases of natural enemies is not likely to replace insecticides in the near future, in part due to the apparent lack of efficacy compared to conventional insecticide applications. Thus the integration of biological and chemical controls is an essential alternative to the conventional use of insecticides that requires knowledge of the lethal and sublethal effects that chemicals may have on the natural enemies. Actually most of the studies regarding the effects of insecticides (inclusive for pyrethroids) on parasitoids and other natural enemies have relied on the evaluation of acute toxicity by determination of a median lethal dose (LD50) or concentration (LC50) (Desneux et al., 2007). Though estimation of LD50 or LC50 is a simple and a fast approach to compare and evaluate the acute toxicity of insecticides to parasitoids, it overlooks sublethal effects that will interfere with the physiology and behaviour of the natural enemies. Insecticides may directly interfere with the efficacy of biological control