Beetles with green thumbs attack avocados

PENN STATE  (US) — Beetles with an unusual talent for growing fungi are threatening avocado crops and could become an even more destructive pest, researchers report.

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 US and Israel, says Matthew Kasson, who recently received his doctorate in forest pathology from Penn State.

A cross-section of the paired mandibular mycangia which house the symbiotic fungus in the beetle head. (Credit: Matthew Kasson/Penn State)


The ambrosia beetle in question is similar to many other ambrosia beetles, including one 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.

But 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,” says 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.”

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

“There is already strong evidence for genetic exchange between fungi from different beetles,” Geiser says. “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?”

Fungi-beetle partnership

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, Geiser says.

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.

“We think this fungus actually co-evolved with the beetle,” Geiser says. “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 US, specifically in California and Florida.

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

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.

Don Davis, professor of plant pathology, contributed to the research. Other contributors are from the University of Florida, the University of California, Riverside; the US Department of Agriculture, the Ministry of Agriculture and Rural Development in Israel, the University of South Bohemia, Czech Republic;  Queensland Department of Agriculture, Australia; the Tea Research Institute, Sri Lanka; and the National Institute of Agrobiological Sciences, Japan.

The US Department of Agriculture and sources in the Czech Republic, Norway, Israel, Australia, Sri Lanka, and Japan supported the work.

Source: Penn State