The effects of different rates of the herbicide glyphosate on spiders in arable field margins.

Abstract

Field margins are susceptible to agro-chemical spray drift, and the effects of herbicide on spiders in semi-natural habitats have been little studied. In this experiment, an arable field margin was sprayed with three rates of glyphosate (90 g active ingredient/hectare (a.i/ha), 180 g a.i./ha & 360 g a.i./ha) and control plots left unsprayed. Spiders were sampled monthly (June-October) using a converted garden-vac and adult spiders were identified to species. A total of 23,393 spiders was sampled with the web-spinners representing more than 90% of the individuals. The effects of glyphosate application on the abundance of wandering and web-spinning prey-capture guilds, and the two most abundant species (Gonatium rubens and Lepthyphantes tenuis) were analyzed using ANOVA F tests. The highest rate of glyphosate consistently reduced the total number of spiders, the numbers of web-spinners, G. rubens and L. tenuis, but not numbers of wandering spiders. Changes in vegetation structure and microclimate caused by the glyphosate are implicated in the reduction of numbers of spiders in plots receiving the highest rate of glyphosate. We conclude that glyphosate drift at rates of more than 360 g a.i./ha (active ingredients per hectare) into arable field margins could result in significant losses of important arthropod predators in farmland and a reduction in spider biodiversity in agroecosystems.