Interactions between Biological Control Efforts and Insecticide Applications in Tropical Rice Agroecosystems: The Potential Role of Intraguild Predation

Integrated pest management attempts to combine cultural, chemical, and biological approaches to bring about pest reductions and improve crop yields. In Asian wet rice agriculture, as in many crop systems, there is a real question as to how compatible chemical control methods are with other pest control approaches. Working in an irrigated rice paddy on Java, Indonesia, we crossed a natural enemy treatment (addition of wolf spiders,Lycosa pseudoannulataBoesenberg et Strand) with an insecticide treatment (monocrotophos) in a balanced, replicated, two-way factorial design to explicitly examine the potential interactions between chemical and biological control methods. Although adding either wolf spiders or insecticide to field plots significantly reduced abundance of pests (sucking homopterans), combining the two treatments together generated a significant, season-long interaction effect such that pest densities did not decrease. In other words, pest densities in plots receiving both spiders and insecticide were statistically comparable to those in plots receiving neither pest control method. Furthermore, we found additive effects of wolf spiders and insecticide on other generalist predators, and from those data we hypothesize that intraguild predation and ensuing indirect effects may be responsible for the interaction effect on pest density. Our results indicate that, far from being complementary and compatible approaches to pest reduction, combining treatments of natural enemy addition and insecticide application may be quite counterproductive.