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Sweet deal. Nectar provided by these cacti feeds ants, but it also gives them an appetite for insects.

Building Better Bodyguards

PORTLAND, OREGON--In their fight against herbivorous insects, some plants don't have to go it alone. By producing a nectarlike substance, they attract ants that help keep enemies at bay. Now, researchers have found that the sweet concoction they serve up may be just what's needed to turn ants into voracious insect hunters. The findings were presented here on 4 August at the annual meeting of the Ecological Society of America.

Various plant species invest in extrafloral nectaries--organs that exude a sugary substance at sites other than the flowers--to reward ants that also eat plant-eating insects. This classic case of mutualism had seemed straightforward, but ecologist Joshua Ness of the University of Arizona in Tucson and colleagues thought the relationship might be more subtle. Ness wondered if the high-carb confection might actively make ants crave the proteins that insects offer, in the same way that consuming too much soda pop causes humans to long for something of substance.

To find out, he set out baits of meat and sugar in pairs beneath the barrel cactus Ferocactus wislizenii, whose extrafloral nectaries attract four species of ants in the Arizona desert, and beneath nearby plants without nectaries. Regardless of species, ants near the Ferocactus showed a stronger preference for meat than those near the plants without nectaries.To test the ants' insect-fighting prowess, Ness's team also placed lab-reared caterpillars on cacti and watched as the ants attacked, usually within minutes. The most effective attackers were the two species that had shown the greatest preference for meat baits under cacti, they found. The team then looked at insects occurring naturally on the cacti and estimated the threat of ant attack based on numbers of ants and the aggressiveness those species showed in the caterpillar trials. They found that the greater the threat of ant attack, the fewer insects were found on the plant. Together the results suggest that pumping ants up on sugars makes them better bodyguards, Ness says.Ecologist Colby Tanner of the University of Utah, Salt Lake City, agrees--and the interplay may be even more complex, he says. It would be interesting to determine the chemical makeup of the exudates, Tanner says, to see whether the cacti are fine-tuning the ingredients to cater to the ant species that offer the best protection.Related sites
Ness's Web site
Guide to cactus extrafloral nectaries in the Desert Southwest