Machiavellian Milkweed Doesn’t Play Fair with its Insect Partners

By Andrew Moseman | June 25, 2008 5:56 pm

Ants, aphids, and milkweed all have to live togehter Who’s the boss? Milkweed is the boss.

Milkweed plants engage in a helpful bit of mutualism with the aphids and ants who take up residence on them. Aphids feed on the milkweed’s sap, then secrete honeydew, which ants eat. The ants, in turn, are the muscle of the operation—they help both the plants and the aphids by fighting off potential predators like caterpillars. The partnership goes three ways, but the power is not equal—milkweed is in control.

Researchers Kailen Mooney and Anurag Agrawal recently found that the milkweed can manipulate the relationship between ants and aphids, altering the dynamics for its own good. The scientists planted 32 groups of milkweed, with each group containing 10 siblings from the same family, in a field full of ants. In 20 of the 32 milkweed groups, the presence of ants was a big boon for aphids—the aphid population increased by 150 percent compared to plants with no ants to protect them. But in the other 12 groups, the numbers of aphids actually decreased by more than half when ants were around.

So why should the three-way arrangement benefit all parties in most cases, but not in others? Since the researchers divided milkweed into groups according to genetic families, they say the plants’ genetics must be controlling the relationship. Mooney and Agrawal aren’t exactly sure how, but they speculate that the composition of milkweed sap can vary from family to family. Perhaps some families of milkweed just didn’t need as much protection, and therefore allowed fewer aphids to live on them by producing a less inviting sap. After all, this arrangement is not free for the plants—they give up lots of sugar and water for the aphids to live on the plant and induce the ants to come.

However it happens, the scientists say, finding that plants can manipulate the ant-aphid relationship helps to explain why the numbers of aphids and ants can vary so much between one plant and another, a problem that had puzzled biologists before. Now they know—milkweed is making a power grab.

Image: flickr/ViaMoi

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