Arborvitae (Thuja) are popular evergreens, valued for their dense foliage and ability to form privacy screens. When these plants show signs of decline, homeowners often wonder if they attract unwanted insects. Arborvitae are not inherently pest magnets, but they are susceptible to common garden pests that can cause visible damage. Understanding the relationship between the plant’s health and pest presence is the first step toward maintaining a healthy landscape.
Why Arborvitae Become Vulnerable to Infestation
Arborvitae become highly susceptible to infestation when under environmental stress. Pests target weakened specimens because their natural defenses are compromised, making them easier to colonize. This susceptibility often traces back to poor growing conditions or improper care practices.
Drought stress is a common issue, exacerbated by the arborvitae’s shallow, fibrous root system. When the soil dries out, the plant cannot absorb enough water, leading to dehydration that signals a vulnerable host to pests. Improper planting depth is another major factor; planting too deep can suffocate roots by limiting oxygen exchange, leading to a slow decline in tree vigor.
Poor soil drainage also contributes to stress, causing waterlogged roots that can lead to root rot and an inability to take up necessary nutrients and water. Physical damage, such as winter burn or excessive shearing, can also weaken the tree. These stressful conditions provide an opening for insects to establish themselves.
Key Pests That Target Arborvitae
Insect damage on arborvitae is typically caused by a few specific pests, each leaving unique and identifiable symptoms.
Spider Mites
Spider mites are tiny arachnids common during hot, dry periods. They feed on foliage cells, resulting in stippling or bronzing, which gives the plant a dull or dusty appearance. In heavy infestations, fine webbing may be visible on branch tips. To check for them, use a “tap test”: shake a branch over white paper, and the mites will appear as tiny moving specks.
Bagworms
Bagworms are highly destructive pests identified by the spindle-shaped bags they construct from silk and foliage pieces. The caterpillars live and feed inside these bags, which grow up to two inches long. Significant defoliation occurs as the larvae feed, and a heavy, untreated infestation can completely strip the tree of foliage, leading to death.
Leafminers and Scale
Arborvitae leafminers are moth larvae that tunnel and feed within the foliage tips, causing them to turn brown. This damage is often mistaken for winter injury since browning appears in late winter or early spring. Damaged tips may be hollowed out, and small exit holes may be visible where adult moths have emerged. Scale insects, such as Fletcher scale, are sap-feeding pests that drain fluids, leading to foliage yellowing and often leaving a sticky residue called “honeydew.”
Proactive Health Strategies to Deter Pests
Maintaining the general health of an arborvitae is the most effective long-term defense against pest infestations. Healthy plants are better able to withstand and recover from insect feeding.
Watering and Mulching
Proper watering is paramount, requiring deep, infrequent irrigation rather than shallow, frequent sprinkles. The goal is to keep the soil consistently moist but never waterlogged, requiring about one inch of water per week during dry periods. Correct mulching helps regulate soil temperature, reduce water evaporation, and minimize weed competition. Spread a two to four-inch layer of organic mulch around the base, keeping it several inches away from the trunk to prevent moisture buildup and decay.
Pruning and Planting
Pruning should be done carefully to maintain the natural shape and improve air circulation within the canopy. This helps reduce the damp, stagnant conditions favored by many pests and diseases. When planting new trees, select a suitable site with well-drained soil and ensure adequate spacing. Proper spacing prevents competition for light, water, and nutrients as the trees mature, reducing stress that makes them easier targets for insects.
Targeted Methods for Pest Eradication
Once a pest has been correctly identified, targeted intervention is necessary to eliminate the established infestation and prevent further damage.
Physical and Biological Control
For bagworms, the simplest method is physical removal: hand-picking the silk bags from branches during fall, winter, or early spring before the eggs hatch. The bags must be destroyed, not just dropped on the ground, because the eggs inside will still hatch. Biological control involves releasing or encouraging natural predators that feed on the pest population. For example, specific beneficial mites can prey on pest spider mites, helping to keep their numbers in check without the use of broad-spectrum chemical sprays.
Chemical Treatments
Horticultural oil and insecticidal soap sprays are effective tools against soft-bodied pests like spider mites and scale insects, working by smothering them upon contact. Timing is important: dormant oil can be applied in late winter or early spring to target overwintering eggs and nymphs before new growth begins. During the growing season, a lighter application of summer oil or insecticidal soap can be used, but it requires thorough coverage to be effective.
Chemical treatments, such as those containing Bacillus thuringiensis (Bt), are highly effective against young bagworm caterpillars shortly after they hatch in late spring or early summer. They must be applied before the larvae seal themselves inside their protective bags. Systemic insecticides applied as a soil drench can offer season-long protection against pests like leafminers and scale by moving through the plant’s vascular system.