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

研究了三类蜘蛛(Lycosidae、Tetragnatha spp.)的捕食率。和Pachygnatha clercki)对有机稻田中的红腹小卷蛾(半翅目：盲小蜂科)进行了肠道内容物DNA分析。从实验室饲养试验中获得的猎物DNA的检测周期和野外样本DNA分析中获得的在给定时间的蜘蛛阳性率可以估计每只蜘蛛在野外每天的捕食率。总共分析了3组不同的微生境和捕食习性的1199只野外蜘蛛，结果表明，捕食率随着猎物密度的增加而显著增加，而不同组的猎物密度也不同。我们的估计显示，最多有0.03-0.07只蜘蛛以这种蜘蛛为食，相当于每天有一只成年蜘蛛。这种低频率的捕食可能表明，在有机稻田中，仅靠蜘蛛捕食来抑制寄生虫密度是困难的。为了加强蜘蛛作为生防剂的作用，必须实施一些额外的管理策略，例如增加稻田生态系统中蜘蛛的多样性。

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.