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Boxwood Pests: How to Keep Your Boxwood Healthy

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Boxwoods are a popular evergreen choice for landscaping but can be susceptible to several pests. In St. Louis and the surrounding areas, two of these common pests can quickly damage or even kill your plants if left unmanaged. Here’s what you need to know to fight these pests.

Photo Courtesy of Missouri Botanical Garden

About Boxwood Leafminer

Boxwood leafminer, when left untreated, can cause serious injury to a boxwood plant. These pests cause damage to the boxwood by feeding between the upper and lower portions of the leaves. This causes the leaf to blister and become yellow, leading to eventual leaf drop. While all boxwood are susceptible to leafminer, English boxwood (Buxus sempervirens) and its cultivars are more resistant to Boxwood Leafminer.

The Life Cycle of Boxwood Leafminer

Early in spring, the previous year’s larvae will pupate, causing affected leaves to blister and swell. Adult leafminer, technically a gall midge, will emerge by May. This adult leafminer resembles an orange-red mosquito. 

Once the adult leafminer has mated, the female will lay an average of 20 eggs into the undersides of a single boxwood leaf. These eggs will hatch in 14 to 21 days.

After the eggs hatch, the larvae will feed on the inside of the boxwood leaf, “mining” out the leaf until it is hollow. This mining causes the leaves to blister and turn yellow with the damage occurring during the summer and fall months. On heavily infested plants, you can hear the larvae feeding, a sound similar to Rice Krispies crackling.

How to Treat Boxwood Leafminer

Controlling adult leafminers can be difficult because of their short lifespan.

  • When to Spray: The best time to spray for adults is when Weigela blooms. This is often around the first part of May.
  • What to Spray: Systemic insecticides are the most effective way to eliminate Boxwood leafminer. Apply these products in late June throughout the summer.

 

Photo Courtesy of Missouri Botanical Garden

About Boxwood Psyllid

A common symptom of psyllid (Psylla buxi) damage is the cupping of young leaves in the spring. This happens when the psyllid pierces and sucks the sap from the buds of young leaves. These leaves will typically fall off within a year.

The Life Cycle of Boxwood Psyllid

Boxwood psyllid overwinters as eggs in between the bud scales. As fresh growth appears in the spring, the nymphs will feed on the buds, causing the cupping at the tips of the new season’s growth. Mature psyllids appear in May to early June. They are 1/16 inch long and look like miniature cicadas. The adult psyllid will lay its eggs in the bud scales in June and July.

How to Treat Boxwood Psyllid

Boxwood psyllid damage is often aesthetic and the plant’s health is seldom affected.

  • Pruning: Where psyllid damage is minimal, prune out the damaged tips before the psyllid has matured into adults.
  • Oils and Soaps: Apply horticultural oils or insecticidal soaps in early May. Severe infestations may require the application of systemic insecticides. 

Taking the time to diagnose and manage insect problems on your boxwood will keep them looking great for years.