Pesticide Effect on Faunal Composition, Abundance, and Body Length of Spiders (Araneae) in Apple Orchards

Abstract

We studied differences in family and guild compositions, seasonal trends of abundance, and mean body length of spiders in Massachusetts commercial apple, Malus pumila (P. Mill), orchards under different pesticide regimes. The 4 guilds were active nocturnal hunters (Anyphaenidae), visual hunters (Oxyopidae and Salticidae), ambushers and runners (Thomisidae and Philodromidae), and web builders (mostly Araneidae and Theridiidae). In 11 orchard blocks in 1991 and 1992, we compared spider populations in 1st-level integrated pest management (IPM) plots (where insecticides were applied as needed through August) with those in 2nd-level IPM plots (where insecticides were applied only until early June). In a single orchard block in 1995, we compared spider populations in three 2nd-level IPM plots with those in plots that were not treated with pesticides. In all treatments, spider numbers increased over time. However, by the end of the growing season, spiders were 3 times more abundant in 1991 and 2 times more abundant in 1992 in 2nd-level than in 1st-level IPM plots. After treatment, the mean number of spiders collected in 2nd-level IPM plots in 1995 dropped to approximately 0, whereas it remained approximately the same in the unsprayed plots. Later in the season, spider numbers increased more rapidly in the unsprayed plots. We found no significant differences in family and guild compositions of spiders between 1st-level and 2nd-level IPM plots in 1992. In 1995, however, we found proportionately more philodromids and less theridiids in 2nd-level IPM plots than in the unsprayed plots. The mean body length of spiders did not differ between 1st-level and 2nd-level IPM plots in 1992. Following treatment, in 1995, mean body lengths of visual hunters and web builders were numerically smaller in 2nd-level IPM then in no pesticide plots. These differences were most probably caused by insecticide-caused mortality of resident adults in May in 2nd-level IPM plots. Because body lengths of spiders are related to the sizes and therefore the types of prey they consume, the negative effect of pesticides on mean body lengths of visual hunters and web builders may affect their role as predators of different types of arthropod pests. We conclude that insecticides, even if applied only early in the season, have a marked season-long negative effect on spider populations found on apple trees in commercial orchards in Massachusetts. This effect may differ depending on the spider species.