Entomology
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Item Potential for entomopathogenic nematodes in biological control: a meta-analytical synthesis and insights from trophic cascade theory(2008) Denno, Robert; Gruner, Daniel; Kaplan, IanEntomopathogenic nematodes (EPN) are ubiquitous and generalized consumers of insects in soil food webs, occurring widely in and agricultural ecosystems on all continents. Augmentative releases of EPN have been used to enhance biological control of pests in agroecosystems. Pest managers strive to achieve a trophic cascade whereby natural-enemy effects permeate down through the food web to suppress host herbivores and increase crop production. Although trophic cascades have been studied in diverse aboveground arthropod-based systems, they are infrequently investigated in soil systems. Moreover, no overall quantitative assessment of the effectiveness of EPN in suppressing hosts with cascading benefits to plants has been made. Toward synthesizing the available but limited information on EPN and their ability to suppress prey and affect plant yield, we surveyed the literature and performed a meta-analysis of 35 published studies. Our analysis found that effect sizes for arthropod hosts as a result of EPN addition were consistently negative and indirect effects on plants were consistently positive. Results held across several different host metrics (abundance, fecundity and survival) and across several measures of plant performance (biomass, growth, yield and survival). Moreover, the relationship between plant and host effect size was strikingly and significantly negative. That is, the positive impact on plant responses generally increased as the negative effect of EPN on hosts intensified, providing strong support for the mechanism of trophic cascades. We also review the ways in which EPN might interact antagonistically with each other and other predators and pathogens to adversely affect host suppression and dampen trophic cascades. We conclude that the food web implications of multiple-enemy interactions involving EPN are poorly studied, but, as management techniques that promote the long-term persistence of EPN are improved, antagonistic interactions are more likely to arise. We hope that the likely occurrence of antagonistic interactions in soil food webs should stimulate researchers to conduct field experiments explicitly designed to examine multiple-enemy interactions involving EPN and their cascading effects to hosts and plants.Item Inducible plant responses linking above- and below-ground herbivory: ecological significance and underlying mechanisms(2007-12-05) Kaplan, Ian; Denno, Robert F; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Above- and below-ground organisms can indirectly affect one another via several recently-described mechanisms, one such mechanism being herbivore-induced plant responses. Because plants leaves occur above-ground and roots below-ground, systemic plant responses to foliar- and root-feeding consumers can result in reciprocal interactions between above- and below-ground herbivory. To first address the broader theoretical context underlying plant-mediated herbivore interactions I conducted a meta-analytical review of interspecific interactions in phytophagous insects (Chapter 1). Using a data-set of 333 observations of interspecific herbivore interactions compiled from 145 independently published studies, I quantitatively assessed: (a) the overall importance of competition in the ecology of insect herbivores, and (b) whether plant-feeding insects conform to traditional competition paradigms. Despite finding frequent evidence for competition, I found very little evidence that phytophagous insects conform to theoretical predictions for interspecific competition. Notably, the strength of interactions between herbivores was largely unaffected by feeding guild, and occurred among distantly-related species that were spatially- and temporally-separated from one another. Moreover, in most cases plants mediated these indirect interactions. Next, I used the tobacco (Nicotiana tabacum) system to explore plant responses to foliar-feeding insects and root-feeding nematodes. I found that aboveground insect herbivores had limited impact on the secondary chemistry of roots, but belowground nematode herbivores strongly affected leaf chemistry (Chapter 2). However, the magnitude of leaf-root induction was also affected by vascular connectivity, with stronger induced responses among plant tissues that were more closely aligned (Chapter 3). Last, I assessed the impact of induced responses on the performance and abundance of foliar and root herbivores using manipulative greenhouse (Chapter 4) and field studies (Chapter 5). Overall, I documented that root-feeding nematodes positively affect leaf-chewing insects by interfering with aboveground nicotine dynamics, whereas aboveground insects benefit root-feeding nematodes via alteration of source-sink dynamics.