The lingering green and white ash trees that have survived the Emerald Ash Borer (EAB) onslaught do not survive by accident. That is according to Penn State researchers who have conducted a six-year study of the ash decline and mortality.
The research shows some ash trees have varying degrees of resistance to EAB. The study is unique because it took place at a plantation of ash trees planted on Penn State’s University Park campus in the mid-1970s.
“We found that genetic variation exists in trees from around the country, and through time, especially as the emerald ash borer population collapses because host trees are rapidly disappearing, the resistance that we observed will likely ensure the survival of the species,” said Kim Steiner, professor of forest biology, College of Agricultural Sciences.
Steiner, who also is the director of The Arboretum at Penn State, collected seeds from wild green ash trees in 27 states and Canadian provinces in the fall of 1975. He grew the seedlings for two years before methodically planting 2,100 of them, all 12 feet apart, in a seven-acre plot. Mixed in were a small number of white ash trees.
This little-known ash plantation off Porter Road near the Penn State’s Swine Research Facility is the largest collection of green ash germplasm in one location in the world.
“We began measuring the decline in 2012, shortly after emerald ash borers arrived in the plantation, and we measured it every year through 2017,” said Steiner. “The effect of the insect was devastating. As of August of this year, only 13 trees remained of the 1,762 that were alive when the emerald ash borer arrived.”
“This suggests that some ash genotypes, especially on favorable sites, will survive with lower densities of emerald ash borer beetles on the landscape,” he said.
“For the first time, this study demonstrated that there is a genetic variation that could be captured in a breeding program to improve resistance to emerald ash borer in both white ash and green ash species,” he said.
Steiner stated there are three kinds of resistance to insects commonly exhibited by trees.
1. Avoidance; when a tree doesn’t attract the adult females that are flying between the trees as they look for a place to lay their eggs. A tree may accomplish this by not emitting a chemical signal the insects are homing in on.
2. Surviving attack; when adults lay insect eggs on a tree, the larvae hatch and the insects grow into adulthood, all the while causing damage, but the tree is vigorous enough to withstand that injury.
3. The tree producing or not producing compounds; this reduces the likelihood of the larvae surviving to adulthood, either by actively killing the larvae or by not offering the nourishment they need.
Stopping the United State’s EAB invasion may lie in a 43-year-old experimental plantation intended for a completely different purpose.
Source: Pennsylvania State University