Effect of the insecticide Lambda-cyhalothrin on rice spider populations in southern Brazil

The adverse effect that insecticides have on spider populations is of major concern since these predators are a component in the biological control of rice insect pest populations. This study was designed to determine the effect of the pyrethroid insecticide, Lambda-cyhalothrin, on spider populations in rice fields. Treatments consisted of plots treated once with the insecticide and untreated plots (control). Spiders were sampled four times per crop year over a two year period in each of three sites in the state of Rio Grande dol Sul, Brazil. Treatments were replicated four times. Sweep net sampling was conducted to sample the araneofauna. Overall 1,961 spiders were collected, with a predominance of juveniles over adults. Between treatments, there were significantly larger numbers of spiders and species richness in areas without insecticide. There were also significant differences among areas (sites) and among days after the insecticide application. Of the 12 recorded families, Araneidae and Tetragnathidae dominated, and were both strongly affected by insecticide application, decreasing more than 50% in those locations. Overall, 37 species were recorded (insecticide: 29; without: 26), eleven species were exclusive for rice without insecticide and eight for rice with insecticide. Alpaida veniliae (Keyserling, 1865) (n = 236) and Tetragnathaaff.jaculator (108) were the most abundant in both treatments, but with large decreases in abundance in rice with insecticide. Among guilds, the predominance was for orb weavers (67.3%) in all locations, but with much lower values for those spiders in rice with insecticide application. This study illustrates that the spider fauna in rice fields are adversely affected by the application of the insecticide Lambda-cyhalothrin.