Posted: September 1, 2021

Defoliation can adversely impact a tree’s health. When pest infestations are high, intense defoliation of trees already stressed by other factors can potentially lead to mortality. However, it isn’t just how much these insects eat or how often they dine, but also when during the growing season.

If you have driven along Interstate 80 this summer, passing through the ridges and valleys of Central Pennsylvania and onto the Allegheny Plateau, you may have noticed long bands or patches of trees looking rather bare among their flush, green neighbors. If you spent time in some of these forests in early summer, you might have noticed bits of munched-on leaves scattered on the ground or even heard the dripping of rain while there were no clouds overhead. In that moment, you were equivalent to an ant on the floor of the Lymantria dispars’* cafeteria; the leaves were their food scraps, and that pitter-patter you heard…indeed, it was excrement, commonly and more kindly referred to as frass. L. dispar is one of a number of forests pests that, in their larval (e.g., caterpillar, inchworm) stage, feed on the leaves of trees. This is a process known as defoliation, where a tree loses some or all of its leaves due to stress or disturbance from insects, disease, or a natural event such as wind or rain. The severity (i.e., how many leaves are lost) and frequency (i.e., how many times in one season and how many seasons in a row) of defoliation can adversely impact a tree’s health and vigor, and repeated yearly defoliation can lead to death. In years such as this, when pest infestations are high, intense defoliation of trees which are already stressed by other factors – like changing weather or a lack of forest management – can potentially lead to tree mortality in one year. However, it isn’t just how much these crawly insects eat or how often they dine that can impact trees, but also when during the growing season. 

Trees need their leaves in order to survive. During the growing season, green leaves capture sunlight and carbon dioxide and when combined with water from roots, glucose is produced, and clean oxygen is released back into the atmosphere. This is the process of photosynthesis. Glucose, a sugar, provides energy to the tree to survive and grow, is vital for creating flowers and fruit, and is imperative to deciduous trees that store it away in reserve for survival during a leafless dormant season. Essentially, as long as the sun is up in the sky, leaves are working in overdrive to photosynthesize and create enough energy to perform essential life functions now and store energy away to simply survive the winter later. So, when a tree is defoliated, it loses its most important means by which to create energy and it must act fast to solve such a huge problem. Refoliation is that very solution. Refoliation is just as it sounds – the process by which a tree regrows leaves. To refoliate, a tree must dip into that pot of reserved energy to develop a new flush of leaves and break through the buds, and then must attempt to quickly replenish that energy store before the dormant season arrives. Repeated defoliation and subsequent refoliation throughout a season deplete those energy stores even further. The capacity of a tree to successfully refoliate and replenish reserved energy is dependent on a number of things, like species (oaks are well adapted to survive defoliation because they allocate a lot of energy into reserve in their root system) and tree vigor (which is decreased with stress and age), but it is also related to the timing of a defoliation/refoliation event. At the beginning of the growing season, typically April–May, trees use large amounts of what is left of their overwinter stored energy to break bud and send out a new flush of green leaves. If trees are defoliated by insect pests in this early part of the season, they must again use even more of what is left of that stored energy to try and refoliate so that they can simply survive the coming months, let alone try to replenish their reserves. As you can imagine, this is stressful for a tree and as it attempts to allocate all its energy to survival, its other functions – such as growth, pest and disease resistance, and reproduction – are hampered. This is so different from a tree that is defoliated in June or July. A tree defoliated during these months has had weeks and months to photosynthesize and grow, and has adequate energy stored up to bounce back at a lesser expense. And, as you may expect, a deciduous tree that is defoliated late in the season, typically August–September, has a different response than it would have to an early or mid-season defoliation event. While this tree has had the opportunity to perform all of its seasonal functions and build up its energy reserve, proximity to the dormant season can lead to 1) a lack of refoliation for the remainder of the growing season, or 2) attempted refoliation without enough time for leaves to harden, leaving the tree susceptible to frost injury. Late season defoliation thus provides more opportunity for stress to the tree by reducing the amount of time for photosynthetic activity or introducing injury, requiring energy to heal just as the tree is headed into dormancy. It is evident then that the seasonality of forest insect and pest life stages and feeding habits plays a role in their effect on trees. Let’s take a look at a few examples and what you can do to help.

Eastern tent caterpillar (Malacosoma americanum), forest tent caterpillar (Malacosoma disstria Hübner), and fall cankerworm (Alsophila pometaria) are all native moth species that have a larval stage that is active and feeding on leaves early in the season. Eastern tent caterpillars emerge in early spring as tree buds are breaking (mid-March to early April) and are known for the silk tents or nests that the larvae – caterpillars – weave in forks or branch crotches of trees in early spring. For six to eight weeks, caterpillars feed on the new, tender leaves of cherry, crabapple, and apple trees, as well as other deciduous tree species, and hide away in their nests during poor weather. The forest tent caterpillar is in the same family as the eastern tent caterpillar and is similar in many ways, except it does not spin a silky tent or nest. Forest tent caterpillar larvae emerge at about the same time as the eastern tent caterpillar and get to work eating the new leaves of deciduous hardwood trees for about five to six weeks. Fall cankerworm is also a moth species, but is a bit different in that its larval stage is something we might recognize as an inchworm or looper. Fall cankerworm larvae emerge late March and early May and feed on the leaves of deciduous forest trees, preferring ash, basswood, beech, cherry, maples, and oaks. This feeding lasts for five to six weeks. Perhaps our most well-known and widely discussed early-riser is an invasive moth species we touched on earlier – Lymantria dispar. L. dispar larvae begin to emerge in late April and continue emergence into late May and can be carried on silk strands in the wind from one area of the forest to another. For about seven weeks, larvae feed on leaves, preferring oak, apple, alder, basswood, hawthorn, poplars, and willows, but also willingly feeding on a variety of other hardwood and even coniferous species. Heavy infestations of L. dispar occur periodically, and this year, high populations of L. dispar left some parts of the state with swaths of defoliated forest stands by late June. Each of these moth species will spin a cocoon, pupate at the end of their larval stage, and emerge as an adult moth either late in the summer or, in the case of the fall cankerworm, late in the fall. The adults lay egg masses on trees; these egg masses overwinter on these trees and hatch in the spring to begin the cycle all over again. Because eastern tent caterpillar, forest tent caterpillar, fall cankerworm, and L. dispar all emerge as larvae by mid- to late April, they defoliate trees before there has been enough opportunity for adequate photosynthesis to replenish the energy used to leaf out after a long dormant season. This in and of itself is stressful for an already weakened tree – but more stress can certainly come to these same trees if they are defoliated again, at the end of the season, by this next cast of characters.

The fall webworm (Hyphantria cunea Drury) and the orangestriped oakworm (Anisota senatoria) are native moth species that have larval stages active in the later portion of the growing season. Fall webworm has a life cycle somewhat backwards from the other moths we just discussed. It pupates and remains in a cocoon in the soil over winter, and adult moths do not emerge until late June. Adult females lay their eggs on leaves, and within a week, larvae emerge and begin feeding on leaves. Similar to eastern tent caterpillar, fall webworm larvae spin a silken tent around the tips of branches and leaves. Larvae feed on hardwood trees from July until mid-late September. Orangestriped oakworm function similarly, with an adult female laying eggs in early summer, and larvae consuming leaves from midsummer through September, when the larvae return to the soil to pupate again. Certainly, oaks are a preferred host of the orangestriped oakworm, but they also feed on maples, hickory, and other common hardwood species. Recall, now, that late season defoliation can be detrimental because a tree may successfully refoliate without enough time for the tender, new leaves to harden before a frost. Frost injury can seriously damage the same buds and shoots that will attempt to produce new leaves next season. Because fall webworm and orangestriped oakworm larvae feed into late September and the median timing for first fall freeze/frost in Pennsylvania is within the first few days of October, defoliation by these pests poses intense risk of harm and challenges to heal as trees head into their dormant season.  

As a landowner or homeowner, the time has passed to intervene in this season’s early and late infestations and feeding. Biological and chemical insecticides typically must be applied in April or May in order to deplete the number of larvae on the landscape. Sticky band traps on trees, which are commonly used in small, backyard woodlots, are applied in June or July to capture larvae as they move along a tree. But just because the summer? has almost passed does not mean that there aren’t vital steps you can take to protect and prepare your trees for the next season. Fall and winter are vital times to 1) mitigate, 2) monitor, and 3) make a plan.

  1. Recall that the early-emergent moths we talked about earlier lay their eggs on trees in the fall and winter. As you spend time in the forest or in your backyard this fall, identify and count the egg masses you see on your trees. Invite your local DCNR service forester to perform counts with you and talk about what egg masses might need to be removed to mitigate intense infestation next spring. L. dispar egg masses can be scraped from trees and soaked in soapy water for a few days to ensure that all eggs are destroyed. Note that the other moths we talked about above are native and thus have effective native predators, decreasing their likelihood of having as intense or as frequent infestations as L. dispar. Be sure to chat with your forester or other natural resources management professionals before taking action on identified egg masses on your trees.
  2. Pay attention to your trees this winter and into next spring. Are they showing signs of stress, like limb dieback? Are they struggling right away to send out new leaves or are they developing lots of little sprouts down the stem? These could be signs that a tree is experiencing elevated stress and can indicate that you may need to take some more active steps in the spring and summer to protect trees from pests. Talk with your forester about what those options might be.
  3. Make a plan. Our forests are more resilient to pests and pathogens when they are healthy. They are healthy when they are actively and scientifically managed. Prepare your trees to better face defoliation or any other disturbance with active forest management, reducing their stress from other factors like competition. Again, contact your local service forester to learn more about the value of a management plan and to talk about your options in beginning active management of your trees or forest.

For more information on identifying and understanding the life stages of these insects, click on the Penn State Extension and other informational links attached to their names above. To find your local DCNR service forester and a list of local consulting foresters, visit the DCNR’s Managing Your Woods page. To learn about the PA DCNR Division of Forest Health’s L. dispar aerial spraying program, and the potential for contracting for private spray, visit their comprehensive platform.

*The historically used common name for this insect is "gypsy moth." This release utilizes the insect's scientific name L. dispar, reflective of the recent recommendation from the Entomological Society of America to discontinue use of this common name. 

Center for Private Forests

Address

416 Forest Resources Building
University Park, PA 16802

Center for Private Forests

Address

416 Forest Resources Building
University Park, PA 16802