Predation rates of spiders on a mirid bug in organic rice paddies : an approach using molecular gut-content analysis

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The predation rates of three groups of spiders (Lycosidae, Tetragnatha spp. and Pachygnatha clercki) on the mirid bug Stenotus rubrovittatus (Hemiptera : Miridae) in organic rice paddies were evaluated using DNA-based gut-content analysis. The bug predation rates per day per individual spider in the field can be estimated using the detection period of prey's DNA obtained from the laboratory feeding trials, and the rate of spiders that is positive for a bug's DNA at a given time in the field obtained in the DNA analysis of the field samples.In total, 1,199 field spiders were assayed among three groups that differ in preferred microhabitats as well as predation habits, revealing that predation rates significantly increased with the density of prey irrespective of group. Our estimates revealed that a maximum of 0.03-0.07 of spiders feed on the mirid bug, which corresponds to one adult per day. This low frequency of predation may indicate the difficulty in suppressing mirid bug density by spider predation alone in organic paddy fields. To enhance the role of spiders as biocontrol agents, some additional management strategies must be implemented, such as increasing the diversity of spiders in paddy ecosystems.