Effects of Detrital Subsidy on Arthropod Communities in Louisiana Rice Fields and Predation on Rice Water Weevil (Lissorhoptrus Oryzophilus)

Abstract

The rice water weevil (RWW), Lissorhoptrus oryzophilus (Kuschel), is the most important insect pest of rice in the United States. Integrated pest management strategies for RWW in Louisiana consist of cultural controls, resistant cultivars and chemical insecticides. The fourth component of IPM, biological control, is largely absent from the literature for RWW, making exploration of biological control a logical next step in developing a full set of IPM strategies. The three main types of biological control are augmentation, classical and conservation. With little known about RWW predators, conservation biological control makes the most sense as local natural enemy abundance is increased. Detrital subsidies have been shown to cause trophic cascades in agricultural system that can ultimately reduce herbivore populations via increased predator abundance. During the summer of 2013 and 2014, field experiments were carried out to determine if compost-manure additions to rice fields would cause an increase in invertebrate diversity and translates to a reduction in RWW numbers. Surveying of treatment (compost-manure additions) and control plots (no additions) for differences in invertebrates used four different sampling methods: root/soil corer, Gee crayfish trap, aquatic netting and floating pitfall traps. Based on sampling from this experiment, Notonecta sp., immature Pantanla sp. and T. lateralis were chosen to be used in aquaria experiments to test for predator effects on RWW. Detrital subsidies in both years failed to increase diversity of invertebrates or reduce RWW numbers. Plotting of feeding guilds over the course of both years showed predator populations paralleling prey populations. RWW oviposition and larval emergence was unaltered whether Notonecta sp. or Pantala sp. were present or not. Tropisternus lateralis, a herbivore/scavenger, also failed to alter RWW fecundity, suggesting that RWW may either not alter their behavior when other organisms are present or they may be able to differentiate between dangerous and non-dangerous arthropods. These experiments failed to cause a trophic cascade or identify predators of RWW. They did however demonstrate that a large prey population is present and that is utilized by a diverse predator assemblage still with the potential to be increased by detrital subsidies.