May 16, 2007 | By: Carol Lea Spence

No one wants to be trapped inside by a rainy summer, but frequent, deep-soaking rainfalls might be just what the landscape needs to relieve the stress caused by the Easter freeze. With borers on the lookout for weakened trees to serve as host to their eggs, homeowners may find that practicing good plant health care – with a little help from Mother Nature – might be just the thing to keep the pests at bay.

“If there is abundant rainfall this summer, we may see fewer problems with borers, because the trees won’t have any further stress,” said Dan Potter, University of Kentucky professor of entomology. “But if we have a normal summer or a dry summer, it could be a bad year for borers.”

Two main groups of borers, flatheaded and clearwing, attack recently transplanted or stressed landscape trees. Flatheaded borers are larvae of metallic-colored beetles. Adults of clearwing borers are day-flying, clear-winged moths – although they look more like wasps. Adults of both groups lay their eggs on the bark of trees and shrubs. After hatching, the larvae tunnel under the bark, girdling the tree and preventing water and nutrient movement in the tree by disrupting the plant’s “plumbing,” said Lee Townsend, UK entomology professor.

“A few of them aren’t going to hurt very much, but if you have a good number of them or small trees that are newly established that are trying to get a foothold in the landscape, those are going to be ones that are going to be susceptible to borers,” he said. “It’s not something that necessarily will show up this year or maybe even next year. The effects can be delayed.”

Healthy trees are able to fight off most borer attacks. But if the tree is under stress – for instance, expending extra energy to leaf out a second time after a damaging late freeze – its natural defenses against insects and disease will be impaired. Townsend thinks damage from the borers may go unnoticed this year, but emergence holes in trees and trunks will be apparent next year.

Particular borers that might be seen in more profusion over the next few years are the dogwood borer, which affects dogwood trees; the lilac borer, which hones in on ash trees as well as lilacs; and the flatheaded appletree borer that targets red maples, hawthorns, apple and 
crabapple trees. There are also the peachtree borer and lesser peachtree borer, which attack not only peach trees, but both fruit-bearing and ornamental cherries and plums.

It’s not common for borers to kill a tree in one year, Potter said. 

“Usually there’s a spiral of decline, where one year you get a few borers in there, and they stress it more, and then the next year more come,” he said. “But on a small tree, a newly transplanted tree, something like a flatheaded appletree borer can really do a number on it.”

But homeowners have options to protect their landscape trees and shrubs from infestations. 

“With borers, we always want to keep the trees as healthy as we can. It’s much like catching a cold. If you get rundown and don’t get enough sleep, you’re more vulnerable,” he said. “Healthy trees rarely get borers. Homeowners should practice good plant health care.”

UK Horticulture Professor Bill Fountain concurs and stresses the importance of regular watering and a healthy restraint when it comes to fertilizing.

“It’s important that we do not over-fertilize in the spring. Nitrogen in the fertilizer causes the plants to use even more of their stored energy reserves to make new leaves,” he said. “We want the plant to be able to recover slowly at a very gradual rate.”

He recommends thoroughly watering trees and shrubs once a week throughout the spring and summer as a panacea to the spring’s stress.

“Watering during dry periods is more important than adding fertilizer,” he said. “We generally don’t recommend fertilizing trees and shrubs in the landscape in the spring, and now with the freeze damage, it’s even more important that we not fertilize in the spring.

“Remember Mother Nature doesn’t go out into the woods with a sack of fertilizer. She fertilizes with little bits of compost and leaves and twigs and branches,” Fountain said. “Do what Mother Nature does. Follow the advice that Mother Nature gives us.”

Fountain points out that most defenses that will help the plant are protectants. 
“We can’t cure the disease or kill the insects or the borer once it gets into the plant. So we’re going to have to prevent it from getting in,” he said. 

Townsend says a number of preventive products are available to homeowners. He recommends consumers look on the main part of the insecticide’s label to make sure the spray is for use on landscape shrubs and trees. The list of pests on the label should include borers. 

When should the homeowner spray the trunks and limbs of their trees? According to Townsend, some of the borers are pretty predictable in terms of when they’re going to be active. 

“The dogwood borer is an example of that,” he said. “It’s going to be here the very last of May to the very early part of June, just about like clockwork. It has a short flight period, so it is something that a homeowner could do something about conceivably. Other borers are active over a much longer period of time, or they may have two or three generations a year, so there’s not a single focused time that homeowners can do something about it.”

It’s important to apply the spray when the insect is most vulnerable, preventing it from laying its eggs beneath the bark. That’s why timing of the application is critical. Townsend recommends using what he calls “indicator plants” to figure out when borers will be most susceptible, rather than going by set calendar dates.

“Temperature really drives insect activity and if it’s warmer than normal, they’re going to show up before the calendar date says. If it’s cooler than normal, they’ll lag behind,” he said. “So if you use the blooming of these indicator plants, it can help you really time your treatment.”

For dogwood borers, he said about a week after first bloom of little-leaf linden or northern catalpa would be a good time to spray tree trunks. This year that window will fall around the middle of May. Potter said homeowners could consider spraying susceptible trees for dogwood borers, flatheaded appletree borers and the peachtree borer then and again three weeks later. 

A Cooperative Extension Service publication entitled “Insect Borers of Trees and Shrubs” is available at no charge through local extension offices. There is also one pertaining to borers that attack landscape pine trees. The publications include schedules for spraying, lists of indicator plants for each type of borer and tips for preventive management.


Lee Townsend, 859-257-7455, Bill Fountain, 859-257-3320, Dan Potter, 859-257-7458