Insect Ecology at Purdue - Ian Kaplan's Lab

Plant Volatiles

Pests are notoriously effective early-season colonists and receive a ‘head start’ on feeding and reproduction during the time lag before natural enemies arrive. As a result, herbivores are predicted to outbreak in ephemeral, early-succession habitats that provide enemy-free space. This presents a dilemma for using biocontrol in agriculture, especially annual cropping systems: enemies are always one-step behind the pest. When this asynchrony is corrected, however, pest populations are more stable and rarely erupt. Thus, the crux of the problem, and perhaps the Achilles’ heel of biocontrol, is that natural enemies occur at low densities and/or arrive too late to offset crop damage.

Herbivore-induced plant volatiles (HIPVs) are potent attractants for entomophagous arthropods and researchers have long speculated that HIPVs can be used to lure natural enemies into crops, reestablishing predator-prey relationships that become decoupled in disturbed agricultural habitats (Fig. 1). In other words, how can we make the scent of crops more attractive to foraging predators and parasitoids? This represents a radical departure from the rationale underpinning traditional efforts to enhance natural enemy impact on herbivorous pests, which emphasize the provisioning of habitat and supplemental food, such as cultivating floral borders.

biocontrol diagram

Figure 1. A phytocentric perspective on biocontrol. Plant traits play a central role in mediating the outcome of enemy-prey interactions at higher trophic levels.

Our lab is taking multiple approaches to test the various uses of HIPVs in synergizing the effects of beneficial arthropods in the field with a particular focus on the ubiquitous induced volatile – methyl salicylate (MeSA). Current work entails:

  1. Experimentally screening various HIPV compounds and blends to maximize the ‘pull’ exerted on natural enemies;
  2. Testing for the role of inter- and intra-plant priming to elucidate the mechanism(s) underlying attraction (Fig. 2);
  3. Evaluating the spatial consequences of attraction in the context of source-sink dynamics, i.e., does ‘pulling’ individuals to one area necessarily remove them from another area?;
  4. Quantifying the roles of associative learning and habituation in the maintenance of the attractive response to HIPVs by generalist predators; and
  5. Assessing the utility of HIPVs as arrestants to decrease emigration of mass-released predators in augmentation biocontrol (in contrast with their putative role as attractants to increase emigration in conservation biocontrol).

Figure 2. Potential mechanisms underlying natural enemy attraction to fields baited with synthetic HIPVs, in this case using MeSA as an example. In Scenario 1, parasitoids are directly attracted to MeSA being emitted from slow-release dispensers embedded within the crop. In Scenario 2, parasitoids are responding to plant-derived HIPVs that were induced via exposure to synthetic lures. In these first two scenarios, natural enemy foraging efficiency is predicted to decrease because the signal is not associated with the presence of herbivores and thus wasps waste time searching prey-free plants. In Scenario 3, lures prime neighboring plants, which then amplifies the pest-induced volatile response that occurs when the crop is damaged. In all cases, habituation is a prime concern from over-exposing entomophagous arthropods to large quantities of the focal compound.

This work is in being conducted in tomato and other vegetable crops in collaboration with Drs. Natalia Dudareva and Michael Gutensohn in the Department of Horticulture at Purdue.