Researchers studying walnut twig beetles (Pityophthorus juglandis) discovered a potential repellent for the pest when they ran out of one adhesive for sticky traps and switched to another, and suddenly the beetles were no longer attracted to the traps. (Photo by Steven Valley, Oregon Department of Agriculture, Bugwood.org)

By Paige Embry

Paige Embry

Walnut twig beetles (Pityophthorus juglandis) slip into their host trees via tiny access routes: lenticels, leaf scars, and other small inlets through the bark. Once inside, they invade the phloem, and the males send out a chemical signal that proclaims that they’ve found a home and others should join them. The beetles look like shiny brown armored personnel carriers, which is strangely appropriate because they carry a load of “personnel”—fungal spores of Geosmithia morbida. Together, the two tiny invaders cause thousand cankers disease, which can lead to the decline and death of various walnut (Juglans spp.) and wingnut (Pterocarpa spp.) trees.

Walnut twig beetles are native to the southwestern U.S. and parts of Mexico, where their host is the Arizona walnut (Juglans major), but the beetles have increased both their range and the types of host trees. The beetle/fungal duo threatens native stands of walnuts and wingnuts, along with agricultural and timber crops in various parts of the U.S. and  Italy.

Jackson Audley, Ph.D.

California’s edible walnut crop (Juglans regia), valued at more than $1 billion in 2019, is one of the crops in potential peril. Thousand cankers disease was first noted in California in the late 2000s, and shortly afterwards, scientists began flight monitoring studies in various habitats in northern California. During the 2011 monitoring season, a small snafu led to an unexpected finding. A new paper exploring the results of that serendipitous finding was published in November in the Journal of Economic Entomology.

During the monitoring study, scientists put out yellow cards coated with a sticky substance and baited them with the male aggregation pheromone (the “everybody come on in, I’ve found a good place” signal). In 2011, partway through the trapping season, they ran out of Stickem, the sticky substance they’d been using, and replaced it with Tanglefoot. Jackson Audley, Ph.D. the lead author on the paper and currently a postdoc with the U.S. Forest Service in Davis, California, says, “Without thinking too much about potential confounding effects, they put the new adhesive onto the traps only to find that over several months of intensive trapping not a single twig beetle was caught.”

Jackson Audley, Ph.D. (left), and the late Steven Seybold, Ph.D., pose in front of a northern California black walnut tree infested with walnut twig beetles (Pityophthorus juglandis) in downtown Davis, California. Seybold and his lab pioneered much of the early research on the walnut twig beetle and were responsible for the discovery of the male-produced aggregation pheromone. (Photo by Kathy Keatley Garvey)

Funnel traps like this four-unit Lindgren funnel trap are often used for trapping bark beetles, as their shape superficially mimics the silhouette of a tree’s trunk. (Photo by Jackson Audley, Ph.D.)

After discovering that walnut twig beetles (Pityophthorus juglandis) were repelled by the sticky substance known as Tanglefoot, researchers identified the chemical ingredient apparently responsible: limonene. To test its repellency, they added it to funnel traps that would otherwise attract the beetles using their aggregation pheromone. In the closeup shown here, the large bottle contains the pheromone attractant, and the vial contains limonene, set to release into the trap at approximately 700 milligrams per day. (Photo by Jackson Audley, Ph.D.)

Walnut twig beetles are native to the southwestern U.S. and parts of Mexico, where their host is the Arizona walnut (Juglans major), but the beetles have increased both their range and the types of host trees. The beetle/fungal duo threatens native stands of walnuts and wingnuts, along with agricultural and timber crops in various parts of the U.S. and  Italy. Infestations can cause significant dieback in trees such as shown here. (Photo by Jackson Audley, Ph.D.)

This startling finding led to a new round of experiments, looking for what it was about the Tanglefoot that repelled the beetles. They focused on semiochemicals—chemicals that transmit information between organisms that leads to behavioral changes—rather than other options like visual cues. Audley says that focus made sense, because “bark beetles have a very chemically driven biology.” The group found that the most abundant volatile compounds emitted by the Tanglefoot were limonene and pinene (both a and b). The Stickem exuded almost no limonene and much lower levels of total pinenes. The next step was testing the repellent effects of those compounds on the beetles. They found that adding limonene (approximately 700 milligrams per day) to pheromone-baited sticky traps reduced catches by 91–99 percent. The pinenes were much less effective, reducing catches by 40–53 percent.

The accidental discovery of the repellent effect of limonene on P. juglandis shows how seemingly innocuous changes in an experiment can have a large impact. This time, it was a useful one, because finding effective insect repellents can be a long, involved process. “Oftentimes,” Audley says, “the game for finding repellents, especially within bark beetle systems, is almost like a fishing expedition—of throwing a whole bunch out there and then slowly whittling down.” The next step is to figure out whether, and how, limonene can be used to protect individual trees and orchards—presumably something short of covering trees from top to bottom in Tanglefoot.

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.