Arborvitae are popular landscape plants valued for their upright, evergreen foliage, often used to create dense privacy screens. When sections of these trees begin to discolor or thin out, it is a common sign of a pest infestation. Protecting these trees requires accurately identifying the specific pest and applying the correct treatment method at the optimal time. Understanding the pest’s life cycle is crucial for effective elimination.
Common Pests and Damage Signs
Accurate identification is the first step toward effective pest management, as different pests require unique removal strategies. One common issue is the Spruce Spider Mite, an arachnid that thrives in cooler weather. These tiny pests pierce the foliage cells to feed, resulting in fine yellow or gray speckles on the leaves, known as stippling. Continued feeding causes the foliage to turn bronzed or dull brown, and severe infestations may show fine webbing stretched between the branchlets.
The Bagworm, the larval stage of a moth, is a highly destructive pest that can cause rapid defoliation. The telltale sign is a spindle-shaped silk bag, typically 1 to 2 inches long, which the caterpillar covers with bits of foliage for camouflage. The larvae consume leaves and twigs, and if left unchecked, a heavy infestation can completely strip a tree, leading to branch death or loss of the entire plant. Bagworms overwinter as eggs inside the female’s bag, with young larvae hatching and emerging from late May through early June.
The Arborvitae Leafminer is the larva of a tiny moth that tunnels inside the tips of the leaves. Damage appears as browning or bleaching on the terminal 1 to 2 centimeters of the foliage, often starting in the spring after the larvae overwintered. When damaged tips are held up to the light, they may appear hollowed out, and a tiny exit hole may be visible where the adult moth emerged in mid-summer. This internal feeding causes the leaf tips to die, giving the tree a scattered, scorched look sometimes mistaken for winter burn.
Immediate Eradication Strategies
An existing infestation requires swift, targeted action, using physical or chemical methods. For bagworms, the most direct approach is hand-picking the bags during late fall, winter, or early spring before the eggs hatch. Each bag removed contains hundreds of overwintering eggs, stopping the next generation. The collected bags must be destroyed, typically by soaking them in soapy water or sealing them for disposal, to prevent larvae from crawling out.
For spider mites and aphids, a strong, direct spray of water from a garden hose can physically dislodge them, reducing the population. Horticultural oils and insecticidal soaps offer a low-impact control option, working by suffocating or disrupting the pest’s cell membranes upon contact. These products are effective against soft-bodied pests like mites and young bagworm larvae. They must thoroughly cover the pest to be successful and should not be applied when temperatures are above 85°F to prevent foliage damage.
When infestations are severe or trees are too tall for thorough coverage, chemical controls become necessary, categorized as either contact or systemic. Contact insecticides (like Spinosad or Bifenthrin) kill the insect immediately upon being sprayed and are the best choice for newly hatched bagworms. Timing is crucial: application must occur in late May or early June, when bagworms are small and actively feeding outside their bags, before they seal themselves inside their casing in mid-August.
Systemic insecticides (such as soil drenches containing Dinotefuran or Imidacloprid) are absorbed by the roots and distributed throughout the plant’s vascular system, killing pests that feed on the treated foliage. This method is effective for pests like leafminers, which are protected inside the leaf tissue, and for treating very tall trees. Since systemic treatments require several weeks to reach effective concentrations, they are typically applied as a preventative measure in the spring or as a long-term control for internal feeders.
Long-Term Health and Monitoring
Maintaining robust plant health is the most effective defense against future pest problems, as stressed arborvitae are more susceptible to attack. Consistent, deep watering is foundational, especially during dry periods, since drought-stressed trees are highly vulnerable to mites and other insects. Once established, arborvitae generally require about one inch of water per week, delivered slowly and deeply to encourage a healthy root system.
Applying a 2- to 3-inch layer of organic mulch around the base of the tree helps conserve soil moisture and moderates soil temperature, reducing stress. Ensure the mulch is pulled back a few inches from the trunk to prevent moisture buildup against the bark, which can invite disease or rot. Well-hydrated and properly mulched trees possess stronger defenses and are better equipped to tolerate low levels of pest activity.
Routine inspection allows for the early detection of pest problems before they escalate into major infestations. A simple monitoring technique for mites involves holding a white sheet of paper beneath a branch and tapping it sharply. If tiny specks begin to move across the paper, a mite population is present and requires immediate action. Combining proactive health practices with attentive monitoring minimizes the need for reactive treatments and keeps arborvitae healthy.