The fungus-farming polyphagous shot hole borer (Euwallacea sp. or, in other descriptions, Euwallacea aff. fornicatus) is wreaking havoc on trees in California, and it is difficult to control. A new study on several insecticides, fungicides, and combinations found “limited performance” in controlling the pest among all the formulations tested. (Photo credit: Jiri Hulcr, Ph.D.)

By Meredith Swett Walker

A spate of tree deaths in California has arborists, park managers, and home owners alarmed. Keen-eyed tree caretakers might notice a cluster of tiny holes in the bark of affected trees—almost as if the tree had been shot with a Lilliputian shotgun. But the culprit is not a tiny tree hitman but rather a small, fungus-farming beetle known as the polyphagous shot hole borer.

Meredith Swett Walker

The polyphagous shot hole borer, or PSHB, gets its name from holes it makes in tree bark as it burrows and the broad range of hosts it infests. It is a species of ambrosia beetle, similar in appearance to the tea shot hole borer (Euwallacea fornicatus), and PSHB is still in the process of being formally recognized as a distinct species from E. fornicatus. For now, the PSHB’s scientific name is Euwallacea sp. (or, in some descriptions, Euwallacea aff. fornicatus). The beetle is not native to North America and was first reported in California in 2003. PSHB is indiscriminate and will attack more than 200 species of trees across 20 plant families. But PSHB does not eat the trees it infests; it eats fungus. The tree is only the fertile field in which it grows its food.

PSHB has a symbiotic relationship with several species of fungus. When a female beetle disperses to find a site to lay her eggs, she brings fungal spores nestled in special pockets in her mandibles. After boring through the tree’s bark, the female beetle digs out tunnels, called galleries, and seeds them with the fungal spores and her eggs. When the beetle larvae hatch, they eat the fungus growing in these galleries. It is the fungus, not the beetles, that actually kills the tree. When the fungus spreads in the tree’s tissues, it can cut off flow of water and nutrients, killing the affected branch or, in some cases, the whole tree. This is known as Fusarium Dieback disease.

The entry hole made by the polyphagous shot hole borer (Euwallacea sp.) is about the size of the tip of a ball-point pen. (Photo credit: Monica Dimson, University of California Cooperative Extension)

A team of researchers from the University of California set out determine which of the available pesticide treatments was most effective for controlling PSHB, evaluating the ability of various insecticides, fungicides, and insecticide-fungicide combinations to control PSHB on infested California sycamore (Platanus racemosa Nutt.) trees. Their study was published last week in the Journal of Economic Entomology.

The study included one systemic insecticide, one contact insecticide, three chemical fungicides, and one bacterial fungicide, applied both individually and in combination. The systemic insecticide, emamectin benzoate, is injected into the tree and spreads by the tree’s vascular system. Contact insecticides, such as the bifenthrin used in this study, are sprayed on the tree’s trunk. Two of the chemical fungicides, tebuconazole and propiconazole, were injected into the tree, and the third, metconazole, was sprayed on the tree. The bacterial fungicide, Bacillus subtilis, was sprayed on the tree’s trunk. The researchers then counted PSHB holes in the tree bark at one month, three months and six months following treatment.

Their results were not encouraging: PSHB are very difficult to control with pesticides. “We saw limited performance for all our pesticides; none completely stopped the beetles. However, it is likely that the best performing treatments may be effective for trees that are more lightly infested and for tree species that are less favored by the beetles,” says lead author of the study Michele Eatough Jones, Ph.D. The beetles spend very little time on the bark surface of the tree, which means they have minimal contact with spray-on insecticides or fungicides. And, because they do not eat the tree’s tissue, they also have limited interaction with systemic (injected) insecticides.

The polyphagous shot hole borer (Euwallacea sp.) digs tunnels, or “galleries,” into the wood of a tree, as shown here in a box elder trunk. (Photo credit: Monica Dimson, University of California Cooperative Extension)

A multipronged strategy seems to be the most promising. Jones and colleagues found that only the combination of emamectin benzoate (systemic insecticide), bifenthrin (contact or spray-on insecticide), and metconazole (the spray-on fungicide) was effective at reducing PSHB attacks at the six-month assessment. But the authors note that not all possible combinations were evaluated due to a limited number of trees; other three-agent combinations might also prove to be effective.

For homeowners or land managers who wish to avoid chemical treatments, the bacterial fungicide, B. subtilis, may be an attractive option. It was effective at reducing PSHB attacks, but only for a short period—less than 3 months—so frequent reapplication may be necessary. But it may be possible to extend B. subtilis‘s effect: “We are currently researching surfactants that will hopefully increase the effective time for microbial agents such as B. subtilis,” says Jones.

A heavily-infested sycamore tree shows multiple entrance holes and characteristic “staining” resulting from infestation by the polyphagous shot hole borer (Euwallacea sp.). (Photo credit: Monica Dimson, University of California Cooperative Extension)

Pests that affect plants used in agriculture often receive the most attention from pest control scientists because they have an obvious, direct economic impact on farmers. While PSHB will infest avocado trees, the majority of species it attacks are trees used in landscaping like the native sycamores in this study. But, these ornamental trees have economic and ecological value in addition to their aesthetic value. Urban and suburban trees provide shade that helps to cool cities, habitat for wildlife, carbon storage, and enhanced property values.

Biological control options to combat PSHB are being explored but will take several years to develop, according to Jones. For now, “the impact of PSHB on trees in southern California is severe enough that pesticide options for management are needed,” she says.

Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.