THE secret of one of North America鈥檚 most destructive plant pests has been revealed: it wreaks havoc by tuning in to its victims鈥� defences. The discovery spells bad news for next-generation insecticides designed to kill pests by stimulating plants into making pest-killing toxins.
As well as eating most varieties of corn, the corn earworm (Helicoverpa zea) eats tomatoes, green vegetables and cotton, making it the most widespread pest in North America. Close relatives of the pest also decimate crops in China, Australia and Europe. Since its introduction to the US mainland from Hawaii in the 1930s, the earworm has grown largely resistant to insecticides.
Now, May Berenbaum and her team at the University of Illinois at Urbana-Champaign have discovered the secret of the earworm鈥檚 success.
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When the pest starts munching on a plant, it immediately activates the plant鈥檚 natural defences. First, signalling chemicals called jasmonate and salicylate are released inside the plant. These chemicals then stimulate the production of toxins within the plant that should kill the pest when it eats them.
But Berenbaum and her team have now discovered that the earworm can stay one step ahead of the plant鈥檚 defences. Jasmonate and salicylate, the chemicals that are vital to triggering the plant鈥檚 toxic defence, also switch on a set of genes inside the pest, they found. These genes lead to the production of enzymes in the caterpillar鈥檚 gut that break down the plant鈥檚 toxins, and many synthetic pesticides, into harmless by-products (see Diagram). The enzymes render the plant鈥檚 toxins useless before the plant has even had time to make them. 鈥淚t has a jump start on the plant,鈥� says Mary Schuler, Berenbaum鈥檚 colleague.
The discovery of the earworm鈥檚 trick is the first evidence that insects can 鈥渆avesdrop鈥� on a plant鈥檚 defences, says Jack Schultz, an entomologist at Penn State University in University Park, Pennsylvania. It could be bad news for commercial insecticide manufacturers who are keen to develop next-generation insecticides that work by mimicking salicylate and jasmonate. Although these would stimulate a plant鈥檚 defences, they would also be likely to trigger the earworm鈥檚 own defences against the plant.
The ability to tap into a plant鈥檚 defences is unlikely to be unique to corn earworms. 鈥淚 would expect any insect that can be hurt by plant defences to benefit from this trick,鈥� says Schultz. A more promising way forward may be to develop pesticides that can鈥檛 be broken down by the earworm鈥檚 enzymes, says Schuler, whose work with Berenbaum is reported in this week鈥檚 issue of Nature (vol 419, p 712).