The ants have a cozier relationship with the green coffee scale, a flat, featureless insect that is a serious coffee pest in some regions, but not on the farm where the study was done. Azteca protects the scale from predators and parasites in return for honeydew, a sweet, sticky liquid the scale secretes. One of those predators is the lady beetle (Azya orbigera), whose adult and larval forms both feed on scale. When an adult beetle tries to attack a scale insect, the ants chase it away. But beetle larvae, which are covered with waxy gunk that gums up the ants' mouthparts, are able to polish off plenty of scale. The ants even aid the murderous larvae, albeit inadvertently. In the course of shooing off parasitic wasps that attack scale, the ants also scare away bugs that parasitize beetle larvae.
The beetles also seem to influence the ants' distribution patterns by preying on the scale, on which the ants depend for honeydew. The researchers explored the relationship using theoretical modeling and found that if ants take over the whole plantation, the beetle goes extinct because adult beetles can't get enough to eat. If the ants disappear from the farm, the beetles go extinct because the larvae starve. But if ants are confined to clusters, due to the influences of both beetles and parasitic flies, the beetles thrive and keep the scale insects under control.
"The interesting thing is that the beetles could not exist except for the highly patterned ant population, but it could be those very same beetles causing the pattern formation in the first place," said Vandermeer, who is the Asa Gray Distinguished University Professor of Ecology and Evolutionary Biology. "The beetle creates the conditions for its own survival."
The white halo fungus, a disease of scale insects, also enters in. The disease occurs here and there throughout the farm but runs rampant only where la
|Contact: Nancy Ross-Flanigan|
University of Michigan