Entomologia Experimentalis et Applicata 101: 9–17, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 9 Plant-mediated interactions between the rice water weevil and fall armyworm in rice Kelly V. Tindall & Michael J. Stout Department of Entomology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA (Phone: 225-578-1850; Fax: 225-578-1643) Accepted: July 31, 2001 Key words: plant-mediated interactions, competition, rice, root herbivores, foliage-feeding herbivores, Lissorhop- trus oryzophilus, Spodoptera frugiperda Abstract Greenhouse studies were conducted to investigate plant-mediated interactions between an above-ground and a below-ground herbivore when sharing a common host plant, rice (Oryza sativa L). Two common pests of rice were used: the rice water weevil (RWW), Lissorhoptrus oryzophilus Kuschel, as the root herbivore, and the fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) as the foliage-feeding herbivore. Rice water weevil larval performance was assessed by measuring larval density and average weight in response to different levels of defoliation by FAW larvae. The reciprocal experiment was done to evaluate FAW performance(growth rate) in response to RWW feeding. Severe defoliation by FAW decreased RWW densities by 32% and reduced larval weights by 48% compared to larvae on roots of non-defoliated plants. Effects in the converse experiments were not as strong. FAW growth rates were reduced 9–37% when feeding on rice leaves from plants damaged by RWW compared to larvae feed leaves from the no damage treatment. These reciprocal negative effects show that RWW and FAW are potential competitorswhen sharing a rice plant. Because RWW and FAW did not interact directly, competition was plant-mediated. Introduction When insect herbivores share a common host plant, several types of interactions may occur: competition (each insect has a detrimental effect on the other), mutualism (each insect has a positive effect on the other), amensalism (one detrimental effect, one neu- tral effect), contramensalism (one positive effect, one negative effect), or commensalism (one positive ef- fect, one neutral effect) (Arthur & Mitchell, 1989). In many cases, interactions have been shown to be plant-mediated; i.e., insects do not interact directly but indirectly via herbivore-induced changes in plant chemistry or physiology (Hatcher, 1995; Denno et al., 1995; Moran & Schultz, 1998; Rieske & Raffa, 1998; Inbar et al., 1999; Denno et al., 2000). Insect herbivory has been shown to cause a variety of responses in plants, including stimulation or inhibition of growth and photosynthesis, changes in nutrient content, and increases in allelochemical concentrations (Tallamay & Raupp, 1991; Hatcher, 1995; Karban & Baldwin, 1997; Agrawal et al., 1999). Changes in the plant can affect survival, reproduction, and/or feeding behavior of co-occurring herbivores. Interactions involving root herbivores have not re- ceived adequate attention, despite the prominence of root herbivores in many systems (Brown & Gange, 1990; Masters et al., 1993; Denno et al., 1995). In par- ticular, there have been few studies of plant-mediated interactions between above-ground and root herbi- vores (Gange & Brown, 1989; Moran & Whitham, 1990; Masters & Brown, 1992; Masters et al., 1993; Hatcher, 1995). There are many reasons, however, to think that such interactions are common. Root her- bivory can have a profound influence on plant growth and physiology and may modify plant allocation pat- terns, decrease nutrient absorption, or reduce root and shoot growth (Anderson, 1987; Hatcher, 1995; Mur-