An In-Depth Look at the Boxwood Leafminer

Boxwood Leafminer

The boxwood leafminer, a notorious fly maggot known as Diptera, has struck a chord as the most formidable pest to evergreen plants. From the East to the West coast, this pest infests boxwood varieties across the United States, wreaking havoc on their delicate leaves.

Symptoms and Diagnosis

When the larvae feed between the upper and lower layers of the leaf, blisters appear on the underside. The affected leaves turn yellow and become significantly smaller than normal, leaving the plant looking alarmingly unhealthy.

Life Cycle

Throughout winter, partially-grown larvae hunker down in leaf blisters. As the days grow warmer in spring, the larvae spring into action, rapidly feeding between the upper and lower leaves throughout the summer. In May, adult boxwood leafminers, resembling yellow to orange-red mosquitoes, emerge from the pupal skin. At this time, they swarm around boxwood plants, coinciding with the flowering of Weigelas. After mating, the female flies insert their eggs into the underside of the leaves before perishing. The eggs hatch into maggots within 14-21 days, initiating a period of larval growth and feeding throughout the summer. As the larvae feed, blisters develop on the leaves. Eventually, the larvae transform into orange pupae that darken over time before giving rise to adult flies. This pest follows a single generation per year.

Integrated Pest Management Strategies

1. Select resistant cultivars.

Certain English boxwood cultivars, such as Buxus sempervirens ‘Pendula,’ ‘Suffruticosa,’ ‘Handworthiensis,’ ‘Pyramidalis,’ ‘Argenteo-varigata,’ and ‘Varder Valley,’ exhibit higher resistance to the boxwood leafminer.

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2. Natural controls.

Encouraging the presence of natural predators like green lacewings and spiders can help control the population of boxwood leafminers. Additionally, maintaining the overall vigor of the plants ensures they are more tolerant to insect damage.

3. Mechanical controls.

Pruning the foliage either before the adults emerge or soon after the adult flies lay their eggs in May can reduce the overall population of leafminers. In cases where feasible, pinching the leaves forcefully enough to kill the maggots within the infested leaves can also be effective.

4. Chemical insecticide control.

Should you choose to employ pesticides, it is advisable to apply them when the new leaves have fully formed, around May 1st, coinciding with the blooming of Weigelas. A second application between mid-June and mid-July is recommended. Carbaryl (Sevin) or malathion can be used to control adult flies, while Acephate (Orthene) applied about 3-4 weeks after the emergence of the adults in mid-May effectively targets the larvae developing in new leaves. Between February and early April, imidacloprid (Merit), a systemic insecticide, can be applied around the shrub’s base.

Organic Strategies

In pursuit of organic pest management, the following strategies are highly recommended:

  • Select resistant cultivars: Opt for English boxwood cultivars known to exhibit higher resistance against boxwood leafminers.
  • Natural controls: Encourage the presence of natural predators like green lacewings and spiders, and prioritize maintaining the vigor of your plants.
  • Mechanical controls: Prune the foliage prior to the emergence of adults or closely after the eggs are laid in May, reducing the population of leafminers. Additionally, consider pinching leaves to eradicate maggots when feasible.
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As you delve into the world of boxwood care, remember that combating the boxwood leafminer requires a multifaceted approach. By incorporating integrated pest management strategies and exploring organic options, you can ensure the longevity and health of your treasured boxwood plants.

To learn more about the topic or find reliable boxwood sources, visit Ames Farm Center.