Spalangia endius Walker is a parasitoid wasp that oviposits in the pupal stage of certain fly species, killing the fly in the process. Its fly hosts include economic pests in livestock-rearing operations, and it is sold commercially as a biological control agent. Male and female S. endius may encounter the commonly used pesticide imidacloprid while walking or resting on treated surfaces, and females may encounter imidacloprid while searching for hosts in contaminated manure. Contact with imidacloprid has been shown to affect survival and subsequent ability to parasitize hosts in S. endius. The present study examined the sublethal effect of imidacloprid on mating behavior. How pesticides affect mating in parasitoid wasps of pests is of economic importance. If pesticides suppress mating, wasp populations will be more male-biased. A more male-biased sex ratio will slow population growth of parasitoid wasps, reducing numbers available to parasitize hosts. In addition, only females parasitize hosts, so a female-deficient population will result in reduced parasitization rates, and ultimately curtail control of pest hosts. Pesticide treatment of female and male S. endius was by exposure to a surface concentration that induces low mortality. First, the effect of pesticide treatment on aspects of mating behavior, offspring sex ratios, and mate choice was examined. In a male mate choice experiment, untreated males were presented with a pesticide-treated female and a control female. A female's treatment had no significant effect on whether she was the first female to be contacted or mounted, but significantly more males copulated with control females first. Among females that were mounted, receptivity (opening of the female's genital orifice, a behavior necessary for copulation) was observed in 1 of 21 treated females and 10 of 11 control females. Males do not appear to contribute to copulation being more likely with control females than with treated females; when the experiment was repeated but with dead females, there was no difference between treated and untreated females in which was first contacted, mounted and copulation attempted with. Female S. endius were subject to a mating choice assay in the same way as males, but with the choice being between a live pesticide-treated male and a live control male. Almost all first contacts, first mounts and first copulations involved the control male. Only one of the 28 pesticide-treated males mounted; he then copulated with the female. Thus, a male being pesticide-treated reduced his mating, when the female had the choice of an untreated male. In contrast, when pesticide-treated males mated, their ability to produce offspring (daughters; sons lack fathers), as measured by their mate's sex ratio, was unaffected. In addition, whether a male was pesticide-treated had no detectable effect on whether he contacted, mounted or copulated first with a mated female or with a virgin female. Both treated and untreated males were more likely to copulate first with the virgin. Secondly, effects of male and female parasitoids being allowed to burrow through used fly rearing media were examined. Three treatments were tested: for 48 h, a wasp was exposed to pesticide or not and then was exposed to media for 24 h, or a wasp was exposed to pesticide for 48 and then for 24 h to no media. Results suggest that duration until mounting and until copulation, but not until contact, were increased for male wasps that had been exposed to pesticide and then to media relative to wasps that were exposed to just pesticide or just media. Media exposure had no effect on female duration to contact, mounting, and copulation, but pesticide exposure did affect her time to copulation, suggesting that some deleterious pesticide exposure effects persist at least 24h after exposure. These experiments with S. endius demonstrate that neonicotinoids can suppress mating. This suppression is likely to result in S. endius populations that are more male-biased if females do not have access to untreated males. Thus, livestock-rearing operations may be inefficiently spending money if they use imidacloprid in combination with release of S. endius for pest management.