Written by Sarah Kellett
Cross-species communication between citrus plants, bacteria, jumping plant lice and wasps begins with a fresh, minty smell.
Jumping plant lice, Diaphorina citri, are small insects that eat the sap of citrus trees like oranges and lemons. As they suck on tree blood, the plants produce the minty smell of wintergreen oil.
Wintergreen oil, or methyl salicylate, is also used in some toothpastes and mouthwash. But the smell on a tree seems to tell passing lice that a feast is on. Wintergreen oil can attract crowds of jumping plant lice, and that causes big headaches for the agriculture industry.
Australia is lucky to be free of jumping plant lice. Curiously, a century ago these lice were in the Northern Territory – there are sap-sucking specimens from Australia in the collection at the Natural History Museum in London. But when scientists went back to the Northern Territory in 2002, they couldn’t find any sign of the plant lice anywhere.
Jumping plant lice are bad news because they don’t just suck plant sap; they also spread bacteria. Candidatus Liberibacter asiaticus causes citrus greening disease which weakens and kills trees. The bacteria have a neat trick to infect new plants. They attract plant lice to infected trees by making the trees produce minty wintergreen oil. The lice fly in and land for a taste. When they fly away again, the bacteria hitch a ride and spread to more trees.
Another insect is attracted by the smell of wintergreen oil – a small, parisitoid wasp. Tamarixia radiate lays its eggs underneath young jumping plant lice, so when a new wasp hatches it has a lice meal ready. The wasp is attracted to the smell produced by trees infected by bacteria to attract lice. Phew!
Unravelling this complicated web is a team at the Citrus Research and Education Center at the University of Florida. They tested how wasps behaved when exposed to natural plant odours, caused by either bacteria or munching plant lice. Then the scientists compared the effect of using pure methyl salicylate (wintergreen oil), a lemony smell, a flowery smell and no smell at all.
Wasps prefer wintergreen. Many of us like communications delivered with minty freshness, but it’s amazing that complex communication across species can happen with a single, scented message.
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People brew coffee and bake bread to leave a nice smell in the house when they are looking for buyers. What other smells attract humans? Are roses attempting cross-species communication?
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