Insects and Diseases in the Woodlot

Waiting it out is often the only option, but management can help
By Patrick White

Managing insects and diseases in the forest is not the same as managing them in agriculture.
Photo by MemoryCatcher/

Managing insects and diseases in the forest is not the same as managing them in agriculture. With row crops, it's much easier to scout for problems and much more practical to apply treatments when necessary.

When managing Christmas trees or ornamental trees in a landscape setting, for example, chemical applications are often used to control insect and disease outbreaks. "That's really not the case in a woodlot. The approach is very different," explains Barbara Schultz, forest health program manager with the Vermont Department of Forests, Parks and Recreation. "There is nothing in this part of the country where people take out sprayers and start treating."

Woodlot owners are often at the mercy of nature, and sometimes the only thing you can do when a tree insect or disease shows up is to wait for it to run its course and hope for the best. That said, there are certain steps that can be taken to manage - as much as possible - some insects and diseases in your woods.

To slow the spread of beech bark disease, you can remove scale-infested trees. The disease occurs when fungus infects wounds left by scale insects.
Photo by Linda Haugen, USDA Forest Service/

"With the cankers on hardwoods, for instance, that's really just a case of getting rid of the worst ones - the theory is that helps prevent the spread, and it certainly improves the quality of the stand," says Schultz. "If you cut them out, you will have a greater percentage of trees without cankers and without stem decay."

The "sanitation" that comes along with crop tree release, or single tree selection - always making the stand a little better - can help prevent some insect and disease problems from becoming too bad, says Schultz. In other words, a woodlot with a greater percentage of healthy trees, which can handle additional stress, may stand up better against some insect and disease outbreaks than a woodlot with unhealthy trees. "For example, shoestring root rot (Armillaria) is more virulent on unhealthy trees," she explains.

Every insect and disease "has its own little quirk," says Schultz. "Some we can maybe do a little something about, and some we just watch them take their course." Native pests and diseases are typically not as dangerous to woodlots as invasive and exotic threats. "They may take out a tree here or a tree there, or make a tree here or there lose its value, but the native ones tend not to be devastating," Schultz explains.

Insects like the gypsy moth come in cycles. When populations are rising, it's often prudent to avoid thinning and harvesting.
Photo by Hannes Lemme/

Bill Ostrofsky, forest pathologist with the Maine Forest Service, says there are many preventive steps that can be taken to manage diseases in trees. "If you're doing work on your woodlot - taking out trees, or doing a firewood harvest, etc. - probably the single most important thing you can control is damage to the residual stand," he says. Reducing damage to the trees left behind improves their health (and timber value), allowing them to stand up better to disease pressures. "Maintaining tree vigor is important, because when trees respond to injury, there's a cost in terms of their energy," he says. Injury and the resulting loss of vigor can very easily make trees more susceptible to disease.

One management technique that can be used to promote tree health is to grow trees on appropriate sites. "Each species has their own ideal situation, and most tree species are fairly adaptable to a lot of sites, but you can push species onto sites that they're not well-suited to," says Ostrofsky. "If trees are introduced into sites that are less than ideal, then they become more susceptible to insects and diseases."

Given the number of different diseases that can impact trees in our region, it's impossible to give blanket advice, and a forester should be consulted when strategizing how to handle specific outbreaks. However, Ostrofsky offers some advice on a few of the more common diseases. "White pine blister rust is a complex disease. The fungus has a complex life cycle that requires infection of ribes plants - currants and gooseberries - and then the spores that are produced on the currants and gooseberry plants can infect the white pines," he explains. "So if you have white pine and you're concerned about white pine blister rust, the recommendation is to make sure you remove the ribes plants in the very near proximity to your white pine stand."

Another example Ostrofsky provides relates to beech bark disease. "The disease is initiated by a scale insect, and then the bark becomes predisposed to infection by a fungus. One of the best things you can do is examine the trees: If you can capture and remove those trees as they become scale-infested, you can reduce the population and slow down the spread of the beech bark disease in the stand." That doesn't mean removing all beech, but focusing on salvaging the infested trees, he stresses.

Removing ribes plants from the vicinity of white pines can help prevent the spread of white pine blister rust.
Photo by Joseph O'Brien, USDA Forest Service/

Careful scouting of the trees is important. "There is a low percentage, probably between 1 and 2 percent, of the beech tree population that is actually resistant or immune to infestation by the scale insect and development of the beech bark disease, so if those trees are in the stand, those are good ones to keep," Ostrofsky explains. "They will be disease-free, so you can tailor forest management to make sure those disease-resistant trees stay in the stand as part of the population." The resistant trees usually stick out like a sore thumb in the presence of an infestation because they have nice, smooth, clean, disease-free bark, he adds.

Sometimes it's weather that helps to bring about, or magnify, tree diseases. "In the last decade, we've seen an enormous explosion in not just conifer needle diseases, but also in hardwood leaf diseases. We're pretty confident that the main driving factor in this is the extremely wet weather that we've had; we've had six or eight years now of way-above-normal precipitation during the summer months," notes Ostrofsky. "With most of the needle and foliage diseases we have, the leaves and needles need to be wet for the spores of the pathogens to germinate and cause infection." During long periods of wet weather, there's a big buildup of these, he says.

Just one example is white pine needle cast disease, which has been common in recent years. "We get a big browning and needle drop in the month of June of 1-year-old needles that were infected the prior year. This should not be occurring. If the tree were healthy, there should be no needle drop in June," says Ostrofsky.

One action that may help with these types of diseases is to ensure good airflow through the woodlot. "We've been recommending that, at least on a small scale, if you have trees in stands that are overly dense and need to be thinned, that it may be a good idea to open them up a little bit. I think giving the advantage to microclimate conditions that will be a little bit more dry will help reduce infection levels," he explains.

One other recommendation Ostrofsky offers when this particular disease is present is to discourage regeneration cuts: harvesting trees with the main objective of getting regeneration growth under existing trees. "The inoculum that's in the bigger trees is just going to rain down on the smaller trees and cause problems, so you may want to hold off on your timing of doing regeneration cuts so that you're not putting the regeneration that you're trying to grow in jeopardy," he explains. A couple of dry years should help to clear the situation up, he notes.

Woodlot owners with noninfested forests should consider cutting hemlock back away from any public roadways to prevent vehicles from inadvertently spreading hemlock woolly adelgid.
Photo by Fungus Guy, CC-BY-SA-3.0, via Wikimedia Commons.

Charlene Donahue, entomologist with the Maine Forest Service, says a similar management approach can be helpful when dealing with some insect outbreaks. "There are some insects that come in cycles. So if you know that something is on the rise, it's usually best to not do any thinning or harvesting during the time period that the population is building," she says.

Examples when this strategy is advisable include outbreaks of hemlock looper, saddled prominent and gypsy moth. Conducting thinnings or timber harvests when populations are high can further stress good-quality trees that will be left, says Donahue. "Those trees will also start to grow really fast and put on some lush foliage, which is exactly what defoliators would like to see. And there's also fewer trees [after the cutting], so you're concentrating the outbreak on your fewer, better trees," she explains. Once the outbreak is over and the trees have recovered, there is usually a window of several years to conduct harvests before the cycle will resume, she notes.

While emphasizing that invasives are "a whole different ball game," Donahue says there are some steps woodlot owners can take to help slow their spread. The first, obviously, is routine scouting and reporting of any suspicious signs. "And leave your firewood at home," she stresses. Cutting firewood and bringing it to a different area can inadvertently bring invasive pests along for the ride. "You never know what's inside the wood; a lot of what we're dealing with are wood borers that you can't see," she notes.

Somewhat related advice is given as far as hemlock woolly adelgid. "In Maine, we still don't have it everywhere. One of the strategies is just to reduce human movement of it," says Donahue. "Hemlock woolly adelgid can be moved on vehicles and equipment and people's clothing." If you (or your logging contractor) are going to be visiting both infested and noninfested stands in a given day, a good approach is to visit the noninfested stand first, she says. Also, woodlot owners with noninfested forests should consider cutting hemlock back away from any public roadways to prevent inadvertent spread from vehicles traveling through, adds Donahue.

Balsam woolly adelgid requires a different approach. "If you're in a stand and it looks like balsam woolly adelgid is just moving in, sometimes taking [the infested trees] out can make a difference. It depends on the scale of things, but that's the recommendation given to a small woodlot owner who is regularly in his stand and is managing fairly intensively," says Donahue.

Again, because there are so many different diseases and insects that can attack trees, Donahue says there is no one single approach to help manage them all. Consulting with the experts about specific outbreaks is important, she says.

"Probably the best thing you can do is stay vigilant. Very often, especially with diseases such as decays, there's not much you can do once the tree has decayed. It has to be a preventive mentality to try to reduce the effects of these insects and diseases," adds Ostrofsky. Some diseases take a while to build up, so there may be a window of opportunity to proactively take measures - perhaps removing diseased trees - to reduce the impact, he notes. "Get out in your woods. Know what they look like, and if you see signs of trees that are declining or dying for some reason, then you can do a further investigation."

Schultz says it can be easy for woodlot owners to become overwhelmed by the number of diseases and insects that can attack their trees. "There's a pest for every tree; in fact, there's a bunch of pests for every tree," she says. "But the good thing is, if you look out the window, we have a bunch of green trees around us. For the most part, the woods can handle that."

Patrick White is a freelance writer based in Middlesex, Vt. He has covered a wide range of agricultural operations around the Northeast and is always on the lookout for interesting and unusual stories. Comment or question? Visit and join in the discussions.