Predatory beetles can detect the unique alarm signal released by ants that are under attack by parasitic flies, and the beetles use those overheard conversations to guide their search for safe egg-laying sites on coffee bushes.
Azteca instabilis ants patrol coffee bushes and emit chemical alarm signals when they’re under attack by phorid flies. In an article published online July 27 in the journal Ecology and Evolution, U-M researchers and their colleagues show that pregnant lady beetles intercept the ants’ alarm pheromones, which let the beetles know that it’s safe to deposit their eggs.
The findings, which may have practical implications for pest management on coffee plantations, are the first documentation of a complex cascade of interactions mediated by ant pheromones, the authors say.
“It is too often the case that pest management in agriculture focuses on finding a magic bullet solution to every problem,” says ecologist Ivette Perfecto, professor at the School of Natural Resources and Environment (SNRE) and co-author of the Ecology and Evolution paper.
First author of the paper is Hsun-Yi Hsieh, a graduate student at SNRE.
“This research shows that there are very complex ecological interactions that are involved in population regulation, and when the population of concern is a potential pest species, understanding those interactions is key to the long-term sustainability of pest control strategies,” Perfecto says.
Ants and other social insects communicate using chemicals called pheromones. Little is known about insects and other creatures that exploit ant chemical communication systems. A few such cases have been reported, including ant-eating spiders that use ant alarm pheromones to find their prey and parasitic flies that use ant alarm or trail pheromones to find their host.
But complex relationships involving ant pheromone-mediated interactions between multiple species of insects have not been previously reported, Perfecto says.
The cast in this bug-world drama features four main players: the aforementioned Azteca instabilis ant, a tiny insect called the green coffee scale, the predatory lady beetle, and the parasitic phorid fly.
Tree-nesting Azteca ants enjoy a mutualistic relationship with the green coffee scale, which is a coffee pest. The ants protect the scale insects from predators and parasites and in return collect honeydew, a sweet, sticky liquid secreted by the green coffee scale.
The lady beetle eats the scale insects tended by the ants. However, patrolling ants attack and kill adult beetles and remove all beetle eggs laid bare on ant-tended coffee plants.
To get their offspring into food-rich patches among the scale insects, female lady beetles hide their eggs in out-of-the-way places — including the underside of the flat-bodied scale insects. That way, when the beetle eggs hatch they can start eating scale insects immediately, while still being protected from ant predation. A short time later, lady beetle larvae develop a waxy, filamentous coat that offers further protection from ants.
Phorids are a family of small, hump-backed flies resembling fruit flies. The parasitic phorid flies that attack Azteca ants lay their eggs on the ant’s body. Fly larvae develop inside the ant’s head, which falls off when the adult fly emerges. Phorids need to see movement to detect individual ants; therefore moving ants, rather than stationary ones, are their targets.
Needless to say, the ants do their best to avoid becoming phorid-fly victims. When the flies attack, the ants release a phorid-alert pheromone to warn other workers in the vicinity. In response, nearby ants enter a motionless, catatonic state and overall colony activity declines by at least 50 percent. This effect can last up to two hours.
In the Ecology and Evolution paper, Hsieh, Perfecto and their colleagues show that female lady beetles — especially pregnant ones — detect the phorid-alert pheromone and take advantage of the ensuing lull in activity to search out safe egg-deposition sites with plenty of food for their offspring. Interestingly, male beetles did not respond to any of the ant pheromones.
The study results have important implications for the management of the scale insect on coffee plantations. The findings suggest that the conservation of Azteca ants, rather than their elimination, is the best management option, Perfecto says.
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