Use of herbivore-inducedplant volatiles for manipulating natural enemies in agro-ecosystems

Abstract

Herbivore-induced plant volatiles (HIPVs) play an important role in the attraction of natural enemies of herbivores in agro-ecosystems. One such HIPV emitted by several plant species and often induced after herbivore attack is methyl salicylate (MeSA). This compound has been shown to attract biological control agents, including members of the families Coccinellidae, Chrysopidae, Syrphidae, Geocoridae, and Anthocoridae, in many agricultural crops. In addition, MeSA combined with companion plants in an attract-and-reward approach can enhance natural enemy ecosystem function. However, little is known whether or not manipulation of natural enemies by MeSA results in a reduction of pest populations, and ultimately results in higher crop yield. For this reason, in this work, I conducted studies to address the following two main hypotheses: (1) Hippodamia convergens Guérin-Méneville and Chrysoperla rufilabris Burmeister, two agriculturally-important predatory insects, respond physiologically and behaviorally to MeSA, which may lead to increased predation and oviposition, and (2) attraction of natural enemies to MeSA, alone or in combination with coriander (Coriandrum sativum L.) as a companion plant reduces pest populations and increases crop productivity in common bean (Phaseolus vulgaris L.). To test for my first hypothesis, I conducted laboratory studies using electro-antennography (EAG), behavioral studies in the greenhouse using cages and wind tunnels, and mark-release-recapture studies to investigate H. convergens and C. rufilabris attraction to MeSA over various distances in cranberry fields. I found that female H. convergens and C. rufilabris antennae can detect MeSA. Also, H. convergens females were attracted to, and arrested by, MeSA; this attraction increased predation of Ostrinia nubilalisHübner eggs. Attraction of C. rufilabris females to MeSA also increased oviposition. In field studies, higher numbers of marked H. convergens and C. rufilabris were caught on yellow sticky cards placed close (0-30 m) to the predator release site than at further distances, regardless of whether the cards were baited with MeSA or not. For my second hypothesis, I used visual sampling to count the number of arthropods (natural enemies and herbivores) on (a) bean plants alone, (b) bean plants baited with MeSA, (c) bean plants intercropped with coriander, and (d) bean plants baited with MeSA and intercropped with coriander throughout two growing seasons (2015-2016). Sentinel aphids were also used as a measure of ecosystem function (i.e. predation). Plant damage and biomass as well as the number and weight of pods and seeds were measured as a proxy for ecosystem services. At the natural enemy level, I found that predators from six different insect families were attracted to MeSA and coriander, when alone or in combination. At the herbivore level, spider mite and thrips populations were reduced with MeSA with or without coriander; however, MeSA increased, while coriander reduced, leaf beetle (Chrysomelidae) abundance. Coriander increased aphid predation rate in both years 2015 and 2016. At the plant level, MeSA and coriander reduced damage by spider mites. However, the effects of MeSA and/or coriander on crop biomass and yield were mostly non-significant. Altogether, my results have important implications for conservation and augmentative biological control. They demonstrate that predator responses to HIPVs can be complex in real agricultural settings because laboratory/greenhouse results do not necessarily translate to the field, and that manipulation of natural enemies may not enhance their ecosystem services as increases in natural enemy abundance did not cascade down to increase crop productivity.