A species of boring beetle with a knack for growing fungi are threatening avocado crops and could transform into an even more destructive pest, according to an international team of researchers.

Ambrosia beetles are adept at boring into trees to cultivate fungi to use as a food source for their young, but the relationship between the beetles and the trees is not symbiotic -- the fungus, a species of Fusarium, can damage or kill avocado trees, which the researchers say makes the little beetle a big threat. If future hybrid generations of the insect or the fungus develop, researchers say more than just avocado trees will be threatened.

"This really wasn't on the radar screen of too many researchers," said David Geiser, professor of plant pathology at Penn State. "But, over the past four or five years, ambrosia beetles seem to be really out of control."

Gesier said mounting evidence of the intermixing of hybrid beetles and hybrid fungi is one reason for alarm.

"There is already strong evidence for genetic exchange between fungi from different beetles," said Geiser. "We want to know if a beetle of one species bored into the same tree as another beetle species, can the fungi they maintain mate and produce new genotypes that are even more problematic?" 

The destructive Fusarium fungus is believed to be the product of co-evolution, wherein the ambrosia beetles essentially domesticated the fungus in the same fashion the humans domesticate crops.

The beetles carry the Fusarium and other fungi in specialized pockets in their heads, and the beetle-associated fungi have evolved a unique spore shape, the researchers report. Geiser believes that the beetles' pocket and the shape of the fungi are adaptations indicative of co-evolution.

"We think this fungus actually co-evolved with the beetle," said Geiser. "There are no other examples of this in Fusarium, which is mostly known as an associate of plants and soil."

Nine lineages of Fusarium were found to be associated with ambrosia beetles, and four of those nine have been linked with the destruction of avocado trees in the Florida, California, Israel and Australia.

Matthew Kasson, who recently received his doctorate in forest pathology from Penn State, said that the ambrosia beetle infestation is a global concern because the beetles can be introduced into wood pallets that are transported around the world by cargo ships.