Facilitation of ground-dwelling spider predation on mirid bugs by horizontal webs built by Tetragnatha spiders in organic paddy fields

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The mirid bug Stenotus rubrovittatus, a major insect pest in rice production, usually remains in the upper layer of paddies to feed on rice ears. However, the mirids are frequently trapped by horizontal webs of Tetragnatha spp. spiders and fall to the bottom layers of paddies where they are preyed upon by ground-dwelling predators. It is hypothesized that Tetragnatha spp. spiders facilitate bug predation by wolf spiders through trait-mediated effects, in which their horizontal webs force the bugs onto or near the ground and thereby into the hunting zones of wolf spiders. Molecular gut-content analysis of spiders coupled with field measurements revealed that the number of wolf spiders that tested positive for mirid bug predation increased significantly with the density of Tetragnatha spp. spiders in the paddies. We also observed a positive relationship between Tetragnatha spp. abundance and total cover by their webs in paddies. These results partially support our hypothesis.