Leafminers are serious pests of boxwoods. Boxwood leafminers prefer American boxwood but will attack English and Japanese boxwood.
Leafminer Description and Their Damage
Leaf mining by larvae (small maggots) creates oval yellow areas similar to blisters on infested leaves (Figure 1). When abundant, damage can cause premature leaf drop, twig dieback, and increased susceptibility to winterkill.
Leafminer maggots (Figure 2) are present in leaves from June through March. They pupate in late spring; adults emerge, mate, and lay eggs for 10 to 14 days from late May to early June (about the time Weigela blooms). There is only one generation each year.
Adult boxwood leafminers are small orange gnats. Females use sharp egg-laying organs to create tiny puncture wounds in leaves into which they insert their eggs.
- Shearing off and destroying infested leaves before adults become active is one approach.
- Insecticidal control options include applying a 12-month tree and shrub systemic insecticide (containing imidacloprid or dinetofuran) after bloom to control the larval stages in leaves. Alternatively, a general tree and shrub insecticide can be applied to leave a residue on the foliage that kills females before they lay their leaves. This should be done in late April to coincide with Weigela bloom.
By Lee Townsend, Extension Entomologist