Ecology meets food security

May 21, 2013

For David Hughes, an associate of Penn State's Center for Infectious Disease Dynamics, the zombie ant phenomenon has implications far beyond the annals of weird science. It's a powerful reminder of the role of behavior in the transmission of disease.

"The other thing I work on is food security," Hughes explains. Doing research in rainforests around the world, he has seen escalating destruction of these habitats, much of it necessary for food production. As the landscape is transformed, however, food plants are increasingly exposed to infectious diseases that emerge from the forest, he says. "These diseases are new, they're evolving rapidly, and we never think about them because we're only ever thinking of the next HIV or ebola, something that will attack humans directly. But equally devastating is a disease of a food plant which feeds 700 million people.

"For me," he adds, "the situation in sub-Saharan Africa today is similar to that in Ireland in 1845, with plant disease threatening to wipe out the food security of vast swathes of the population."

Such is the case with cassava, a dietary staple across much of Africa, and a plant increasingly afflicted by withering viruses including cassava mosaic disease. Ants are a vital component in the spread of these diseases, Hughes says, not because they pass disease directly, but because they protect the pests that do. One such is the whitefly, which feeds by injecting its syringe-like beak into one cassava leaf after another, drawing out nourishment and infecting as it goes.

On its own, the whitefly is an evolutionary weakling, ill-suited to survival in the harsh tropical environment, but with the powerful ant to ward off its predators it can thrive. In return, the ant collects the sweet "honeydew" the whitefly excretes, and carries it back to the nest for food.

Recently, Hughes' knowledge of this mutually beneficial relationship led him to an unorthodox idea for solving the cassava mosaic problem.

"There are many people around the world working on genetically engineering cassava to kill the virus outright," he explains. "What we want to do is modify the plant using RNA interference," inserting a genetic signal to turn off the reproductive genes in the ant queen. The signal would then pass from the leaf through the whitefly and be ingested by the worker ants and eventually the queen, effectively sterilizing her and thus causing a rapid breakdown of the colony.

With fewer bodyguards to protect it, Hughes says, the whitefly would be more vulnerable to both pesticides and natural enemies. As its numbers dwindled, so would the spread of the disease. This approach would specifically target local colonies and not impact the ant ecosystem as a whole, he says.

In 2012, Hughes received a Grand Challenge Exploration grant from the Bill & Melinda Gates Foundation to pursue his idea, working with Penn State colleagues Mark Guiltinan, Siela Maximova and Gary Thompson. "We believe this concept has transformative potential," he said in response, "and could have a major impact on solving some of the world's most important issues, such as food production and agricultural productivity, in a relatively short period of time."

  • zombie ant

    Industrious yet pernicious ants tend viral injecting bugs (flattened bodies under ants) and act as bodyguards preventing effective control.

    IMAGE: David Hughes
Last Updated August 10, 2015