Foraging mode: A factor affecting the susceptibility of spiders (Araneae) to insecticide applications

Abstract

Field experiments have revealed that some species of spiders are more sensitive to insecticides than others. Among many factors influencing their susceptibility, foraging mode seems to play an important role. Aspects of foraging mode that appear to be relevant are whether the spider is diurnal or nocturnal, a hunter or a web-maker. Six spider species, Araniella opisthographa, Clubiona neglecta, Dictyna uncinata, Pardosa agrestis, Philodromus cespitum and Theridion impressum were used in the study. P agrestis and P cespitum are diurnal hunters that may come into direct contact with insecticide. C neglecta is nocturnal and so is exposed to residues only. The remaining three species are web-makers building webs that vary in the extent to which they can protect the spider from direct spray. The effect of sprays was tested under laboratory conditions (Potter tower) with three commercial insecticides, an insect growth regulator (hexaflumuron), a selective organophosphorus (phosalone) and a non-selective pyrethroid insecticide (permethrin) using a four-day exposure period. Data were analysed using bootstrap method and randomization tests. The results obtained showed that hunting spiders were more susceptible to the insecticides tested than web-makers (in their webs). Diurnal hunting spiders (Philodromus and Pardosa) were severely affected only by permethrin. A high mortality was observed for the nocturnal hunter, Clubiona, after application of phosalone and permethrin. This species appears to be very sensitive to residues of both insecticides. Comparing the effect on web-making spiders, with and without webs, it was observed that the sparse orb-web of Araniella did not protect its owner at all, but the dense cribellate and frame-webs of Dictyna and Theridion, respectively, reduced the mortality caused by permethrin significantly in comparison with specimens without webs. Of other factors studied, posture (normal and upside-down position) did not influence the susceptibility. Mortality increased slightly with body size after permethrin application. © 1999 Society of Chemical Industry