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.
This diagram shows how wasps, lice, bacteria and trees interact with methyl salicylate (wintergreen oil). Image: Adapted from Xavier Martini, Kirsten S Pelz-Stelinski and Lukasz Lech Stelinski, Frontiers in Ecology and Evolution
Quiz questions
Is the lens in a magnifying glass convex or concave?
How many toes do cats have, in total?
Which is the only planet to spin horizontally?
Which chemical gives parmesan cheese its smell?
What is the waxy coating of a leaf called?
Try this: Ants smells
You will need these things.
Put some of each essence on cotton wool.
Put each bit of scented cotton wool at an equal distance to the ants.
Watch how the ants behave. This ant is climbing over the lemon essence!
Safety: This activity involves ants, which should be treated with care. They may bite. Children should ask an adult for help. For more information, go to the Double Helix safety page.
You will need
Cotton wool
Peppermint oil
Food flavouring essence, like strawberry, vanilla, coconut and lemon
Ants
What to do
Find some ants. Some ants bite, so make sure not to get too close.
Put peppermint oil on a piece of cotton wool. Do the same for each of the food flavouring essences.
Place each bit of scented cotton wool at an equal distance to the ants.
Observe the ants’ behaviour. What do they do?
What’s happening?
We observed that when ants walked close to the cotton wool with peppermint oil, they turned and walked away. They were more likely to get close to the food essences. The first time we did the experiment, the ants crawled all over the lemon essence. The second time, they walked on the vanilla essence instead.
Ants use their antennae to smell. Researchers at Vanderbilt University found that ants have more odour receptors than silk moths, fruit flies or even mosquitoes. A good sense of smell can help them find food and receive chemical messages from other ants.
Applications
Some people use peppermint oil to keep ants out of their house. Other strong smelling plants, including cloves and bay leaves, are also used.
By studying the chemicals that ants produce to communicate with other ants, we can learn about how the colony works. We can also get clues to how other animals might communicate. Many different species of animals, not just ants, use the same chemicals to communicate different things.
Watch it! Take a journey with this MinuteEarth video about ocean confetti, supported by CSIRO.
Quiz answers
A magnifying glass has a convex lens.
Cats have 18 toes, five on each front paw and four on each back paw.
Uranus is the only planet to spin horizontally.
Butyrate, a short-chain fatty acid, gives parmesan cheese its smell.
The waxy coating of a leaf is called a cuticle.
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