Beetles with unusual "green thumbs" for growing fungi are threatening avocado crops and could transform into a more destructive pest, according to an international team of researchers.

Ambrosia beetles are insects that bore into trees and cultivate fungi to use as a food source for their young. The fungi -- species of Fusarium -- carried by types of the Ambrosia beetle can damage or even kill trees, making the beetle and its fungi a threat to avocado production in the U.S. and Israel, according to Matthew Kasson, who recently received his doctorate in forest pathology from Penn State.

Kasson, currently a post-doctoral researcher in plant pathology, physiology and weed science at Virginia Tech, said the ambrosia beetle that is threatening avocado crops is similar to many other ambrosia beetles, including one he discovered attacking the invasive Ailanthus altissima trees -- Tree of Heaven -- in the Northeast.

While the type of beetle threatening avocado crops attacks living trees, the ambrosia beetle associated with the Ailanthus is less of a problem because it is only known to attack trees that are dying or already dead, Kasson said.

However, researchers are worried that hybrid versions of either the beetle or fungus could pose a larger threat to farms and forests.

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

Evidence that the fungi associated with the beetles easily form hybrids is one reason for the alarm, according to Geiser.

"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 partnership between the fungi and ambrosia beetles may be an example of co-evolution, in which beetles essentially domesticated the fungi, analogous to how people domesticated crop plants. 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.

Both of these adaptations are indications that the beetles and fungi co-evolved, according to Geiser. A total of seven evolutionary lines -- lineages -- of the Fusarium have an unusual club shape that is distinct from the canoe-shaped spores typical of other Fusarium. The club shape may be an adaptation for serving as a food source, he said.

"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."

The researchers, who published their findings in a recent issue of Fungal Genetics and Biology, identified nine lineages of Fusarium associated with ambrosia beetles. The fungi, which look similar, but are genetically distinct from each other, include four lineages of Fusarium that are currently threatening avocado crops in Israel, Australia and in the U.S., specifically in California and Florida.

Because the Ailanthus is an undesirable, noxious tree and the beetles are already present in Pennsylvania, Kasson said researchers could use this system as a good research model for studying how the beetles and fungi interact.

Kasson 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.

Geiser and Kasson also worked with Don Davis, professor of plant pathology, as well as an international team of researchers from the U.S. Department of Agriculture, the University of Florida, the Ministry of Agriculture and Rural Development, Israel; the University of South Bohemia, Czech Republic; Queensland Department of Agriculture, Australia; the University of California, Riverside; the Tea Research Institute, Sri Lanka; and the National Institute of Agrobiological Sciences, Japan. The U.S. Department of Agriculture and sources in the Czech Republic, Norway, Israel, Australia, Sri Lanka and Japan supported this work.