Citrus call for backup to fight root-destroying pests

Citrus fruits are delicious. Their delicate balance of sweetness and tartness is a biochemical masterpiece. It's no wonder that they, of all nature's tasty options, are the highest value fruit crop in terms of international trade, with over 105 million tons produced annually. But these tempting produce face a persistant villain that seeks to destroy their roots; a menace known, cleverly, as the citrus root weevil.

The weevil's grubby larvae feed like maggots on the vital roots of citrus plants. Native to the Caribbean, this crop-destroying pest wreaks havoc on citrus farms in Florida and California, causing approximately $70 million in damage annually in the US. Farmers have tried a variety of control methods, including ants, parasitic wasps, and even a virus, but they simply don't keep up with the birth rate of the pesky weevils. A female weevil can lay up to 5,000 eggs at a time, each of which burrows into the roots of its host plant, depriving it of water and nutrients and making it vulnerable to infection by deadly fungi.

But the citrus plants don't just sit idly and allow the weevil larvae to eat their tender roots: they call for backup, emitting chemical signals that attract weevil larvae-killing nematodes.

A Diaprepes abbreviatus larvae feeding c/o USDA

To date, the best method of control are what are called entomopathogenic nematodes. These little worm-like creatures are parasites of the weevil larvae, and can be highly effective at controlling the pests. However, they aren't always as efficient as farmers would like, and scientists have been searching for ways to make the nematodes attack the weevils with more vigor. While researching the dynamic between the citrus roots, weevil larvae, and parasitic nematodes, they discovered something extraordinary: certain citrus plants actually release chemicals to attract the nematodes when they're being eaten by weevils.

The researchers took roots from the common citrus rootstock Swingle citrumelo and watched how nematodes reacted when the roots were eaten by weevils, mechanically damaged, or left alone. They were surprised to find that more nematodes were attracted to the roots when the larvae were feeding as compared to any other kind of damage or no damage at all. This meant that the plants specifically attracted nematodes when they might be useful in controlling a pest. The real question, though, was how?


Using the magic of modern science, the researchers isolated and analyzed the chemicals present and emitted by damaged roots. They found that when damaged by weevil larvae, the roots emitted a unique cocktail of compounds that attract parasitic nematodes.


Scientists are now working to isolate and test the specific compounds present in the extracts from these roots to determine which molecules are responsible for this effect. Ultimately, this research may lead to a more effective control of citrus root weevils. Instead of just adding nematodes, farmers might be able to put a chemical near the roots of their trees to attract the nematodes to the infested roots, thus increasing their effectiveness at fighting the devastating pests.


But perhaps the more interesting evolutionary question is how do the plants know what is harming their roots? How do they tailor their response and only emit these chemicals when being eaten instead of being cut, scraped, or otherwise damaged? The more we study plants, the more we realize how complex and extraordinary they are. And to think, they do this without a brain!



Citation:Ali, J., Alborn, H., & Stelinski, L. (2010). Subterranean Herbivore-induced Volatiles Released by Citrus Roots upon Feeding by Diaprepes abbreviatus Recruit Entomopathogenic Nematodes Journal of Chemical Ecology, 36 (4), 361-368 DOI: 10.1007/s10886-010-9773-7