Chewing Insects

Chewing insects have mouth parts that enable them to bite into and chew plant tissue. Some insects will burrow into soft leafy tissue and consume the fleshy material below the leaf surface. This type of feeding activity is known as mining. Another type of chewing damage is call skeletonizing. A skeletonizer consumes most of the fleshy part of a leaf leaving only the solid, midrib and veins of the leaf. Finally there are some insects that consume the entire leaf, flowers, or small twigs. Here are a few of the more common chewing insects that impact trees in Frederick County.


Eastern Tent Caterpillar: Eastern tent caterpillar is one of the first insects to become active in the spring and they begin building their silk tents in late April. The larvae of this insect begin spinning their nests in the branch forks and will spend most of their time in these webs. This insect is sometimes confused with gypsy moth, but gypsy moth does not build tents. The eastern tent caterpillar is a native insect that normally does not display large population explosions since there are many environmental factors that keep populations in check. However, there were some large infestations of this insect throughout Frederick County in 2015. Eastern tent caterpillar normally weaves its nest and attacks trees in the Prunus family such as wild black cherry, crabapple, apple, hawthorn, plum, cherry, and peach trees etc. The insect can consume most leaves causing defoliation especially with small trees. This insect can be a serious pest to fruit trees and it controlled in orchards and some landscapes. It seems like many small wild black cherry trees are defoliated every year only to bounce right back no worse for wear.


Forest Tent Caterpillar: Forest Tent Caterpillar is another native insect with a somewhat misleading name in that this insect does not build large distinct nests. The web of this insect is built during pupation and it normally encircles a leaf on the tree. This insect hatches soon after the eastern tent caterpillar and like other insects enlarges as it sheds its exoskeleton going through various instars. The young larvae climb to the top of the tree and congregate in large groups feeding on one branch at a time. Forest tent caterpillar is a forest pest and it sometimes can cause complete defoliation of trees. Some of the preferred species include the oaks, black gum, ash, birch, elm, and basswood.

Gypsy Moth: Gypsy moth is by far and away the most serious pest that we have that feeds on leaves. Gypsy moth first appeared in our area in the late 1980’s and stayed active until about 1991. Over that time period thousands of acres of forest were defoliated by this insect and Maryland like the rest of the region initiated aerial control of the insects. The aftermath of this defoliation was that thousands of acres of forest witnessed profound mortality mostly in the oak component. Gypsy moth hatches in early April and the larvae go through a number of instars. The early instars spin silken threads which enables them to spread by floating through the air, this is known as ballooning. The early instars have fairly selective food requirements preferring mainly oak leaves and they create tiny “shot holes” in the leaf as they feed. The later instars grow to a length of about 3 inches and they consume most of the leaf. These later instars become much less choosy about what they will eat going after many more species besides the oaks. The larvae goes into a pupal state in late June, emerges as an adult in July and by August the females have laid their small football shaped egg masses on trees and other objects. Over the years various insect and animal predators and dieses have begun to impact gypsy moth populations. This environmental resistance has kept the moth in check somewhat. Gypsy moth numbers appear to run in cycles where there are 8 - 9 down years between population spikes, then we witness 3 up years when the insect is a problem. The last up year for gypsy moth in Frederick County was 2009 so if the bug exhibits these trends we might see a population spike in 2017 or 2018. Recent surveys by the US Department of Agriculture indicate that gypsy moth numbers are building up in mountains of Virginia and Pennsylvania. These populations could potentially migrate into Maryland. Stay tuned!


Spring and fall canker worm: There are two species of this native insect that are differentiated mostly by their egg laying activity. The fall canker worm lays its eggs in the late fall while the spring cankerworm lays its eggs in late February and March. The larvae of these insects hatch at about the same time in April and May right about the time that trees begin leafing out. These green and brown colored larvae are sometimes called inch worms because they appear to inch along as they move about. Cankerworms defoliation looks a lot like the damage of gypsy moth. They can defoliate large expanses of forest especially oaks, elms, linden, apple and beech. The larvae stay active for about a month then they pupate. Fortunately canker worm populations do not spike very often. However, significant cankerworm defoliation has occurred over the last two years around Sugarloaf Mountain in southern Frederick County and the southern part of Washington County.

Bag Worm: Bagworms get their name from the bag like structure that the larvae assembles around itself for protection. This bag is made up of silk and bits and pieces of leaves, needles, and other tree parts. The bagworm larvae spends most of its time in the bag, the larvae pupates in the bag and the adult female remains in the bag during mating. Once mating occurs the female lays eggs in the bag and perishes. Bagworms are predominantly pests or ornamental evergreen shrubs such as junipers, cedars, Leland cypress, and spruce. Heavy bagworm infestations can completely defoliate some of these evergreens causing the death of the host plant.


Fall Web Worm: Fall webworms is a native insect that produces distinctive nests in the fall of the year appearing in late August throughout September. This insect encircles sections of the tree with these nests devouring the leafy material inside the nest. Fall webworms can attack most trees but they seem to prefer hickory, walnut, maple, and fruit trees in our area. Fortunately this insect usually does not completely defoliate large trees and the timing of their defoliation is during the fall when most trees have finished growing and produced enough energy reserves to ensure that they leaf out next spring. The fall webworm can weaken immature, and fruit bearing trees and is more serious pest of nurseries and orchards than it is of forests. The fall webworm has become a serious defoliator of trees throughout Eastern Europe.

Locust Leaf Miner: The adult locust leaf miner is a beetle that looks a little bit like an orangish black firefly. The larvae of this beetle chews into a leaf and feeds on the soft tissue between the surface creating a mine under the leaf, these mines merge together giving the leaf a brown appearance that usually shows up around late July early August in our area. Like clockwork locust leaves turn brown in late summer and the locust spring back to life the following spring.


Black vine weevil: The black vine weevil is an example of an adult insect that inflicts most of the damage on plants versus most other insects where the larvae inflicts most of the damage. There are numerous species of weevils that impact trees, crops, fruits, etc. The black vine weevil is an important pest to ornamental plantings such as rhododendron, azalea, and many flowers. The weevil does most of its damage during the evening when it will emerge and feed on various plant parts particularly the leaf. Oftentimes the plant looks like something came by and took a large bite out of the leaf.


Japanese beetle: Japanese beetles were introduced to this country in the early 1900’s and have been a problem ever since. The larvae of the beetle lives underground where it feeds on grass roots sometimes causing large dead spots in a lawn. The adult beetle emerges in mid-June early July and begins feeding on a number or ornamental plants and trees. Japanese beetle is an example of a skeletonizer an insect that feeds on the fleshy part of the leaf leaving only the veins intact. Japanese beetles feed on many species of trees but they seem to prefer lindens, elms, maples, and fruit trees. There was an uptick in Japanese beetle numbers in 2015 around Frederick County.


Sawflies: Sawflies encompass a large group of insects. In most cases the adult sawfly looks a lot like a bee or wasp. The larvae of most sawflies is a caterpillar. Sawflies can skeletonize, mine, or consume leaves, needles, and other plant parts, depending on the particular species of sawfly. There is a group of sawflies such as the European sawfly, red head sawfly, white pine sawfly, and yellow headed spruce sawfly that devour the needles of young evergreens. The larvae of these insects congregate in large numbers on evergreens and they can quickly partially or fully defoliate the tree. These sawflies are of particular concern to Christmas tree growers.

Orang Striped Oak Worm: The larvae of the orange- striped oak becomes active in late summer. As these larvae enlarge they tend to travel out to branch tips where they congregate in large numbers and skeletonize the leaves. Despite the menacing look of these larvae their damage will not impact large, well established oak trees. Recently planted oak seedlings and saplings on the other hand may suffer from defoliation.