Differential responses in spider oviposition on crop-edge gradients in agroecosystems with different management

Abstract

Semi-natural habitats in agricultural landscapes offer reproduction and feeding sites for natural predators of pests. Spiders are among the most abundant and diverse generalist predators and can efficiently suppress pest populations in agroecosystems. However, despite the importance of these habitats, there is a significant gap of knowledge on oviposition sites of spiders in agroecosystems. Our main goal was to investigate if semi-natural habitats play an important role in spider oviposition in two crops with different agricultural managements. Our specific objectives were to estimate where spider oviposition is highest (in semi-natural habitats or in crops); if there are different responses between crops (alfalfa and soybeans); and to measure trap-nests efficiency as shelter and breeding places for vegetation-dwelling spiders. We used trap-nest technique, a novel method to collect spiders and their eggsacs. We carried out the study in 15 sites in soybean and 10 sites in alfalfa crops in order to sample spiders’ eggsacs and shelters. Each sampled site consisted of a semi-natural habitat (enclosures or edges) and its adjacent crop. We found that the number of eggsacs decreased from semi-natural habitats towards the interior of the soybean crop and the opposite pattern in alfalfa, where the number of eggsacs significantly increased within the crop. Our results provide evidence that spider reproduction takes place outside the crops, albeit only in soybean crops. Our findings were different in alfalfa with minimum use of agrochemicals and with a lesser seasonal disturbance, with a higher spider oviposition compared to semi-natural habitats. In addition, our results have several important implications for the management of agroecosystems. We consider that the use of these trap-nests is a suitable and a reliable tool to study vegetation-dwelling spiders in different ecosystems.