Arborvitae (Thuja) are favored landscape plants known for their dense, evergreen foliage, often used in hedges and screens. While generally robust, these plants are susceptible to specific pests that can compromise their health and appearance. When an Arborvitae begins to show patchy browning, unexplained needle drop, or a general decline in vigor, a pest infestation is often the underlying cause. Recognizing these symptoms early is crucial for protecting these valuable conifers. Understanding the life cycle and habits of the most common pests is necessary for successful intervention.
Bagworms Identification and Targeted Removal
The Bagworm (Thyridopteryx ephemeraeformis) is a caterpillar pest that causes significant damage to Arborvitae by feeding on the foliage. The insect creates a unique, spindle-shaped bag constructed from silk and pieces of the host plant’s needles, which serves as a protective casing. These bags are typically one to two inches long and hang from the branches. Defoliation is the main sign of infestation, which can cause the evergreen to turn brown or appear bronze.
Bagworms are often not detected until late summer when the damage is severe and the caterpillars have matured. The female remains in the bag for life, laying hundreds of eggs inside the sealed structure to overwinter. Treatment during the dormant season, from fall through early spring, should focus on mechanical removal. Hand-picking the bags from the branches and destroying them before the eggs hatch in May or June prevents a new generation of larvae.
For larger infestations, Bacillus thuringiensis (Bt) is highly effective, but timing is crucial. Bt is a stomach poison that must be ingested by the caterpillar while it is actively feeding. Applications should occur in late June through mid-July, when the larvae are still small and have emerged from the overwintering bags. Chemical sprays become ineffective once the bag is sealed and the larvae stop feeding.
Spider Mites Detection and Miticide Application
Spider mites are common pests of Arborvitae, but they are arachnids, meaning standard insecticides will not control them. These tiny, eight-legged pests feed on individual plant cells, causing a characteristic fine yellowish speckling or stippling on the needles. A severe infestation can lead to a noticeable fine webbing between the branchlets, giving the foliage a discolored appearance.
A simple method for detection is the “tap test,” where a branch is sharply struck over a sheet of white paper. The small mites fall onto the paper and can be seen crawling. Spider mite populations thrive in dry, hot conditions, making drought-stressed plants susceptible to outbreaks.
Non-chemical management involves dislodging the mites with a strong, steady stream of water. This physical removal can be repeated every few days during peak activity to keep populations low. When chemical intervention is necessary, horticultural oils or specific miticides are appropriate choices. Horticultural oil works by smothering the mites and their eggs, requiring thorough coverage of all foliage surfaces. Rotating the type of miticide used is a good strategy to maintain effective control.
Arborvitae Scale Recognizing Infestation and Dormant Oil Use
Scale insects are stationary, sap-sucking pests that attach themselves to the stems and needles of the Arborvitae. They appear as small, waxy, or crusty bumps on the plant tissue, which are the protective coverings of the insect. As the scale feeds, it excretes a sugary substance called honeydew, which coats the foliage and provides a substrate for black sooty mold to grow.
The impact of a heavy scale infestation is a general weakening of the plant, leading to yellowing, sparse foliage, and eventual branch dieback. Effective management relies on targeting the overwintering stage, usually the nymph or egg stage. This is best accomplished through the application of dormant horticultural oil.
Dormant oil is applied in late winter or early spring, before the plant breaks dormancy and when temperatures are above 40°F. The oil acts as a physical control, smothering the overwintering pests and their eggs by blocking their respiratory systems. Complete coverage of the plant’s branches and stems is required for this method to be successful. Insecticidal soaps or summer-rate horticultural oils can also be used to target the mobile, newly hatched “crawler” stage during the growing season.
Cultural Practices for Long-Term Health
Preventative care is the most effective defense against pest infestations, as a healthy Arborvitae is naturally more resilient to attack. Proper planting is foundational, ensuring the top of the root ball is level with the surrounding soil to prevent moisture stress and root rot. Adequate spacing between plants, generally three to seven feet for a hedge, promotes good air circulation, which discourages certain pests and fungal issues.
Consistent, deep watering is important, especially during periods of drought or high heat, to prevent the stress that makes plants vulnerable to spider mites. Water should be applied directly to the root zone to saturate the soil twelve inches deep. Applying a layer of organic mulch around the base of the plant helps to retain soil moisture and regulate soil temperature. Mulch should be kept a few inches away from the trunk to prevent moisture buildup that can lead to decay.