Boxwood leafminers (Monoarthropalpus flavus syn. Monarthropalpus buxi) are orangish gnat like flies that are a major pest of boxwood. Their larvae, which are small worm like maggots, feed inside of boxwood leaves causing blisters and puckering on the backside of the leaf and brown blotching on the upper surface. Premature defoliation of infected leaves occurs. In large numbers they can cause extensive and unsightly damage even the death of the plant.

Identification and Damage

Eggs: Eggs are white to transparent.

Larvae: small whitish to lemon yellow, 3 mm long larvae commonly called maggots.

Fly: The adult leafminer is a yellow to orange-red gnat like fly that looks like a mosquito. They measure about 2.5 mm (Hoover GA. 2001)

Damage Boxwood leafminer larvae feed on the inside of the leaves causing blisters and puckering to appear on the underside of the leaf. Infected leaves yellow and may appear smaller in size with brown blotching. Some leaves, even entire plants (depending on the severity of the infestation) may shed prematurely.

Life Cycle

The larvae of boxwood leaf miners overwinter inside the leaves. In spring the larvae begin feeding again for a brief period before molting into pupa. At the same time the blisters develop a translucent window. In late May or June, after boxwoods have completed flowering and when weigela shrubs are in bloom (Sadof C. 2018), the pupa wriggle through the blisters. From here the adult flies emerge and begin mating shortly after. This mating period lasts for about two weeks, during which you will see swarms of these little gnat like flies hoovering around the boxwood plants. Adult emergence occurs predominantly in the early morning hours (Kaur B. and Dale A. 2020 citing Brewer 1981).

The impregnated females then use their needle-like ovipositor to deposit eggs into the underside of recently expanded leaf tissue (Kaur B. and Dale A. (2020). On average, a female will lay about 20 eggs per leaf ((Kaur B. and Dale A. 2020) cited Hamilton 1925). The adult fly dies soon after. The eggs hatch in about 14-21 days into the larval stage (a maggot) that grows and feeds for the rest of the summer (Missouri Botanical Garden (n.d.). The leaves develop the characteristic blisters as the larvae feed. The larvae overwinter inside the leaf and the cycle begins again. There is one generation per year.

Control

Physical Control:

• When possible choose resistant varieties. (Kaur B. and Dale A. 2020 cited Thurn et al. 2018)
  • Buxus microphylla var. japonica
  • Buxus microphylla ‘Green pillow’, ‘Grace Hendrick Phillips’
  • Buxus microphylla var. sinica ‘Franklin’s Gem’
  • Buxus sempervirens ‘Pendula’, ‘Argenteo-varigata’, ‘Handworthiensis’, ‘Pyramidalis’, ‘Suffruticosa’, ‘Vardar Valley’, ‘Justin Brouwers’
  • Buxus harlandi ‘Richard’
  • Buxus sinica var. insularis ‘Nana’
• For a more complete list see The Boxwood Bulletin, Vol. 54 No.1, page 9, Evaluation of Boxwood Species/Varieties to Boxwood Leaf Miner by Robert A. Dunn R.A. & Saunders B. (2014).
• Prune off infected leaves and stems before the adults emerge.
• Cleanup all the leaf litter and destroy it.
• Encourage natural predators such as green lacewings and spiders.
• Keep your boxwood healthy through proper irrigation, fertilization, and pruning practices. Healthy plants are more resistant to insect attacks.

Chemical Control:

• Apply an insecticidal soap spray during the 2 week fly stage or other approved of insecticide.
• Apply an approved of systemic insecticide as a soil drench in mid- to late April to target the overwintering larvae.

Summary

Boxwood leafminers are a serious threat to boxwood shrubs. The insects lay their eggs on the inside of the new growth of boxwood leaves in June. By July the eggs have hatched and the larvae remain inside the leaf feeding. This feeding creates dead patches on the leaves that seriously mar its appearance and causes defoliation. The larvae remain inside the leaf until the following May, safe from birds and insect predators, safe from some unfavourable environmental conditions, and safe from insecticidal sprays. They only make a brief appearance for about 2 weeks around the end of May to mate and lay their eggs once again in the new growth.

Photo Credits: all photos taken by the author.

References:

Dunn R.A. & Saunders B. (2014), The Boxwood Bulletin, Vol. 54 No.1, page 9, Evaluation of Boxwood Species/Varieties to Boxwood Leaf Miner, Retrieved on Jan 30, 2021 from: 54_1_2014_Summer.pdf (boxwoodsociety.org)

Finneran R. (2018), Michigan State University Extension, ‘Boxwood Leafminer a Serious Pest of a Favorite Landscape Plant’, Retrieved on Jan. 30, 2021, from: https://www.canr.msu.edu/news/boxwood_leafminer_a_serious_pest_of_a_favorite_landscape_plant

Hoover GA. 2001. Penn State College of Agricultural Sciences Research, Boxwood leafminer, Retrieved on Jan. 30,2021 from: https://ento.psu.edu/extension/factsheets/boxwood-leafminer

Kaur B. and Dale A. (2020), Entomology and Nematology Department, University of Florida, Retrieved on Jan. 30 2021 from: http://entnemdept.ufl.edu/creatures/ORN/SHRUBS/boxwood_leafminer.html

Missouri Botanical Garden(n.d.), Retrieved on Jan. 30, 2021 from: https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/insects/flies/boxwood-leafminer.aspx

Sadof C. (2018), Purdue University. retrieved on Jan 30,2021 from: Don’t Be Bewildered by Brown Boxwood Leaves – Purdue Landscape Report

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