How a Wasp and Geographic Data are Improving Surveillance for the Emerald Ash Borer
By Paige Embry
When Claire Rutledge, Ph.D., an entomologist with the Connecticut Agricultural Experiment Station, first moved from Indiana to Connecticut, she says she spent years “running around with my hair on fire saying ‘It’s coming! It’s coming!'”
“It” was the emerald ash borer (Agrilus planipennis), which was first discovered in the U.S. in Michigan in 2002. The emerald ash borer did show up in Connecticut—as it has in 36 states and five Canadian provinces, killing ash trees and causing financial havoc as it spreads.
The emerald ash borer (EAB) is stealth beetle. Its larvae live inside ash trees (Fraxinus spp.), making galleries in the sapwood, destroying the phloem, and eventually killing the trees. The adults flit about high up in the tree canopy and are essentially invisible. Often, the first sign of the beetles’ presence is when a tree begins to die. A key to managing this pest is finding it early in an infestation. Rutledge uses an established surveillance method involving a predatory wasp, groups of citizen scientists called “Wasp Watchers,” and baseball fields in combination with geographic information system (GIS) data on tree cover to determine specific areas where EAB is hiding. The results are detailed in a study published in May in the Annals of the Entomological Society of America.
The most common surveillance tool used to find EAB are pheromone-baited traps. They can be placed anywhere, but their average reach is only 30 meters. A U.S. Department of Agriculture ash conservation program puts the number of ash trees in the U.S. in the neighborhood of 8 billion—that’s a lot of traps that need to be deployed. However, the surveillance method Rutledge uses in her study involves the predatory wasp Cerceris fumipennis, which some call the smoky-winged beetle bandit. This wasp collects adults in the Buprestidae family (metallic wood-boring or jewel beetles) to feed to its young. Although thousands of species of buprestid beetles exist, if the wasp brings home EABs, people know nearby ash trees are under attack—but exactly how nearby? What is the foraging range for the wasp? Tens of meters? Kilometers?
To assess this, Rutledge used beetles collected from 37 nest sites over seven years. These wasps don’t sting people and are aggregate nesters. In fact, one of their favorite nesting grounds is the packed sand of baseball fields, which makes accessing nest sites simple for researchers. Only sites yielding at least 100 beetles were included in the study. Wasp Watchers—the citizen scientists recruited to assist with the project—used one of two methods to collect beetles: hunting or gathering. Hunting involves using a net to catch a wasp returning to the nest with its paralyzed prey. Rutledge says, “When you catch these wasps, they freak out and drop the beetles.” The wasps fly off and the catcher is left with a conveniently immobilized beetle. Gathering involves scouring the nesting area for beetles dropped by the wasps when something else freaks them out, like a predator or a kid running to first base.
For the next stage of the study, the beetles were identified and put into two groups: those that eat deciduous trees and those that eat coniferous trees. (Beetles that eat both weren’t included in the study.) For each site, Rutledge calculated the ratio of conifer-feeders to deciduous-feeders. Using the National Land Cover Dataset 2016, she calculated the proportion of conifers in the area between 500 and 3,500 meters of the nest sites. She assumed that the proportion of conifer-feeding beetles collected by the wasps would be related to the percentage of conifers in the surrounding area. The data bore this out and showed the highest correlation between 1,000 and 1,500 meters of the nest site, giving an effective range for this method of surveillance that is significantly higher than for pheromone-baited traps.
Better defining the reach of this surveillance method will aid in a variety of settings, as ash aren’t just forest trees. Fraxinus species are also commonly planted in parks and used as street trees and may make up more than 20 percent of the trees in some municipalities. Knowing when EAB arrives and exactly where it is gives municipalities, power companies, homeowners, and departments of transportation time to act—set aside money, use pesticides to protect important trees, take others down over time—if they choose to. It’s hard to comprehend the extent of the devastation an essentially invisible, exponentially growing pest can have, so it’s no surprise that some cash-strapped municipalities decide to wait to act, but waiting means all the destruction may descend suddenly.
“So what will happen,” says Rutledge, “is that you won’t notice anything is wrong, and then a couple of trees are dying and that’s not too hard to deal with. Then all of a sudden 60 percent of the trees [or] 80 percent of the trees are dying and you really need to deal with that.”
And the costs are huge. Fort Wayne, Indiana, had 9000 dead trees to deal with between 2011 and 2012. They couldn’t even find enough arborists to take them down. And the trees have to be taken down because, says Rutledge, the same straight-grained wood that makes ash a good choice for baseball bats and tool handles “also means that when the tree dies they become very brittle, so they will drop limbs without warning or fall over—so they’re a real liability. At least 15 people have been killed by falling ash trees in Michigan.”
Meanwhile, EAB continues to spread. It was recently found for the first time on the Pacific Coast, in Oregon. At the moment, people trying to figure out where EAB is in western North America will have to rely on the pheromone-baited traps as their early warning system. Sending out Wasp Watchers to scare Cerceris fumipennis into dropping its prey won’t work there—it doesn’t live in west, and so far no easily found and abundant substitute has been identified.
Annals of the Entomological Society of America
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.