How to Identify and Manage Tree Pests in Late Fall

Aphids feed on tree sap by piercing the plant tissue, which stunts growth and causes leaves to curl, wilt, or yellow. The honeydew they excrete can lead to sooty mold, which reduces photosynthesis and weakens the tree further. Large infestations can significantly impact the health of young trees or stressed mature trees.

Identification: Aphids are tiny (1-2 mm) and can be green, black, brown, or pink, depending on the species. They gather in large colonies on the undersides of leaves and along stems. Sticky honeydew on leaves or nearby surfaces is another telltale sign.

Management: Encourage natural predators like ladybugs and lacewings, which feed on aphids. Use horticultural oils during late fall to smother overwintering eggs on tree bark. Insecticidal soaps or neem oil can be used in the growing season, though these are less effective in late fall.

Scale insects cause damage by sucking sap from trees, leading to yellowing leaves, reduced growth, and dieback of branches. Like aphids, they also produce honeydew, which encourages sooty mold development. Heavy infestations can stress and kill small trees and shrubs over time if left untreated.

Identification: Scales appear as small, immobile bumps on twigs, branches, or leaves. Some species are covered in a waxy or cottony coating. They can be found clustered on twigs or stems, and large infestations may result in sticky leaves or a black, moldy residue from honeydew.

Management: Use dormant oils in late fall to smother overwintering scale insects. Pruning heavily infested branches can also help, and systemic insecticides may be necessary in severe cases. For minor infestations, natural predators like ladybugs and lacewings can help control populations.

Spider mites are not true insects but arachnids that feed on the sap of leaves, causing stippling, bronzing, and leaf drop. This weakens trees, especially during periods of drought, making them more vulnerable to other stressors. Heavy infestations can lead to defoliation, particularly in conifers.

Identification: Early signs include tiny yellow or white stippling on leaves, webbing on the underside of leaves, and overall yellowing or bronzing of foliage. Spider mites are difficult to see with the naked eye but appear as tiny moving dots if observed closely.

Management: Apply dormant oils in late fall to kill overwintering mites. Maintaining tree health by proper watering and avoiding over-fertilization helps reduce mite populations. Insecticidal soaps or miticides may be necessary for severe infestations.

Gypsy moth caterpillars are voracious feeders, stripping trees of their leaves during their larval stage. This defoliation weakens trees, making them susceptible to secondary pests and diseases. Deciduous trees can recover with a second flush of leaves, but conifers, which cannot regrow needles as easily, may die after just one or two years of heavy defoliation.

Identification: Gypsy moth egg masses are large, tan, and fuzzy, found on tree trunks, branches, and even nearby objects. Defoliation often starts at the crown of the tree and progresses downward.

Management: Scrape egg masses off surfaces into soapy water to prevent hatching in spring. Biological control methods include using Bacillus thuringiensis (Bt) sprays or introducing natural predators like parasitic wasps. Insecticide treatments can be timed to target larvae early in the growing season.

Emerald ash borers (EAB) are highly destructive to ash trees, killing them within 3-5 years of infestation. The larvae burrow through the tree’s cambium layer, where nutrients are transported, disrupting its water and nutrient flow. This leaves the tree vulnerable to dieback and eventual death. EAB infestations spread rapidly, causing significant losses in ash tree populations.

Identification: D-shaped exit holes from adult borers are a key sign. Other symptoms include canopy thinning, bark splitting, and shoots growing from the base of the tree. Woodpecker activity may also increase as they feed on larvae under the bark.

Management: Systemic insecticides like imidacloprid or emamectin benzoate can be injected into the tree’s trunk or applied to the soil around the tree’s base. Treatments are most effective in spring, but planning for it during late fall can help prevent further damage. Removing infested trees and quarantining firewood helps limit the spread.

Bagworm larvae feed on leaves and needles, especially on evergreens, leading to defoliation. If a tree is heavily infested, the damage can be fatal, particularly to conifers, which do not recover as easily from lost foliage. Deciduous trees may tolerate bagworm damage better, but repeated infestations weaken the tree over time.

Identification: Look for small, spindle-shaped bags made of silk and plant debris hanging from branches. These bags contain larvae that feed on tree leaves and needles.

Management: Physically remove and destroy bags during the fall and winter months before the larvae emerge in spring. Insecticides, such as Bt, can be applied in early spring when larvae are small and vulnerable.

Fall webworms build silk nests at the ends of branches and feed within these nests, defoliating trees. While not typically fatal, large infestations can weaken trees and reduce their aesthetic value. This defoliation can stress young or already weakened trees, leaving them more vulnerable to diseases and pests.

Identification: Web-like nests at the ends of tree branches are the most visible sign. Inside, small, hairy caterpillars can be seen feeding on leaves.

Management: Prune and destroy nests in fall to prevent the larvae from continuing to feed and overwintering in the area. Natural predators, such as birds and parasitic wasps, can help control populations. Chemical treatments are rarely needed.

Tent caterpillars defoliate trees by feeding on leaves in their larval stage. While healthy trees can typically recover from one season of defoliation, repeated infestations weaken the tree, making it more susceptible to other pests and diseases. Severe infestations can cause significant aesthetic damage.

Identification: Tent-like silk structures are visible in the forks of branches, especially in late fall or early spring. Caterpillars are often visible inside the tents.

Management: Prune and destroy tents before larvae emerge. Biological treatments like Bt sprays can be used in early spring to control caterpillar populations. Insecticides can also be applied, but are best used in the larval stage.