Predatory Beetles “Listen” To Ants Conversations To Find Safe Egg-Laying Spots

By Mo Mozuch on August 6, 2012 3:16 PM EDT

photo: Ivette Perfecto, University of Michigan
photo: Ivette Perfecto, University of Michigan

Researchers at the University of Michigan recently discovered that a species of predatory beetle can detect a pheromone used by ants to communicate danger.  The study, published by the journal Ecology and Evolution, provides the first hard evidence that ant alarms set off a chain reaction of behaviors in other insects. Scientists believe the work can lead to improved pest-management techniques on coffee plantations in Mexico, one of the world's leading coffee-producing countries.

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"It is too often the case that pest management in agriculture focuses on finding a magic bullet solution to every problem," said University of Michigan ecologist Ivette Perfecto, professor at the School of Natural Resources and Environment and co-author of the Ecology and Evolution paper. "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 said on the University of Michigan News Service website.

Like a juicy Mexican soap opera, this saga involves more than the Azteca ants and the female coccinellid beetles that live among Mexico's many coffee bushes. Two of nature's tiniest insects are also involved: the coffee-bush eating green coffee scale and the parasitic phorid fly. Azteca ants have a mutually beneficial relationship with the green coffee scale. The green coffee scale insects secrete a sugary liquid the ants can eat, and the ants protect them from predators like the coccinellid beetle.  As a result, pregnant coccinellid beetles lay their eggs in some creative places, including the underside of the green coffee scale. However, any journey into scale-rich territory is treacherous for the beetles; the ants are larger, faster and essentially fearless. Unless the phorids are in the air.

Phorid flies are tiny, humpbacked flies that see the Azteca ants as a combination babysitter/wet nurse for their offspring. They lay their eggs on the ant's body and the fly larvae migrate to the brain where they grow, and eat. Eventually, the ant's head falls off. Phorid flies across the world do this, and many species of ants have developed a fast alert response pheromone. In the Azteca ant, the alarm pheromone causes all nearby ants to remain motionless for up to two hours. Phorid flies locate ants based on motion, so while the ants remain motionless they are virtually invisible. However, they are also unable to patrol for beetle eggs, and researchers discovered that when these alarms are raised the coccinellid beetles, especially pregnant ones, head into dangerous territory to scout for safe egg-laying locations.

Researchers suggest that the lack of studies involving these types of relationships between insects is "the product of investigators failing to search for them in the first place." However, their findings suggest that conserving Azteca ants actually helps control the scale population. Azteca ants and lady beetles favor the same types of trees so destroying the Azteca's home also eliminates the lady beetles. Any decrease in lady beetle numbers leads to more green coffee scales and, ultimately, to more coffee bushes destroyed by the scales.

"This is counterintuitive because the ants protect the scale insects," Perfecto said. "However, the ants are distributed in a patchwork on the plantations, and the patches of Azteca are essential for the survival and reproduction of the predatory lady beetle."

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