WHEN ENTOMOLOGISTS hip to chemical ecology speak of HIPPOs, they mean HERBIVORE INDUCED PLANT PROTECTION ODORS, not the massive hippopotamus native to riverine Africa. At Washington State University (WSU), David James and his entomological colleagues have spent the last several years testing fragrant wintergreen oil, a common HIPPO produced by distressed plants, as a means to boost biological control of crop pests by natural enemies.
The same fragrant wintergreen oil (high in methyl salicylate) used in mints and mouthwashes can be formulated into slow-release dispensers to attract beneficial insects into crops in greater numbers earlier in the season than would otherwise be the case. The dispensers minimize environmental and worker health impacts and crop damage (phytotoxicity). Also, the dispensers can last 3-4 months, providing extended periods of natural enemy attraction. Sprays evaporate more quickly, potentially meaning more applications. Nevertheless, spray technologies are still very popular and 2% wintergreen oil in canola oil is being test sprayed worldwide in crops ranging from hops and cotton to soybeans, strawberries and sweet corn.
In their earliest hop yard and grape vineyard field tests, James and his coworkers found that wintergreen oil attracted significantly higher numbers of pest natural enemies like predatory green lacewings, minute pirate bugs (Orius spp.), spider mite-eating lady beetles (Stethorus spp.), and aphid-eating syrphid flies. By getting higher numbers of natural enemies into the fields earlier than might otherwise have been the case, pests like spider mites were kept under biological control all season long. Leafhopper numbers were also lower, leading James to suspect that parasitic wasp species attacking this pest were also boosted by wintergreen oil.
This is the basis for commercial products such as PredaLure, sold by Rincon-Vitova Insectaries and others to maximize natural enemies providing biological pest control. Plants are virtual chatterboxes of chemical communication, and wintergreen oil and methyl salicylate are just the tip of the iceberg in terms of natural compounds awaiting field testing. Farnesene, caryophyllene and the male-produced lacewing aggregation pheromone iridodial are among the other green lacewing attractants attracting research attention.
When attacked by insects, spider mites, and pathogens (e.g. viruses, bacteria, fungi) or under environmental stress (e.g. chilling, drought, salinity), plants can ooze chemical exudates from their roots into the soil and waft a variety of communication chemicals into the wind. Thereby neutralizing pathogenic soil microbes, luring in pest-eating natural enemies and tipping off downwind neighbors in the plant community to ramp up their immune response in preparation for impending pest attacks.
We have only scratched the surface of what exists and what is possible to develop for biological insect control.