UNIVERSITY PARK, Pa. -- 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.