Boxwood Leafminer Info
Boxwood Leafminer is the most destructive insect pest of boxwood. The adult insect is a small mosquito-like fly. The adult insect does little to no damage to the plant. The larvae, though, are another story. The adult females lay eggs on the boxwood leaf surface. These eggs hatch and the larvae will start feeding typically from June through early fall. The larvae spend their winters inside the leaf (between the upper and lower leaf surface). The larvae pupate (changes form) in April and the adult Leafminer emerges in early to mid May to start the cycle over again.
These pests are particularly destructive as they feed on the “meat” of the leaf while leaving the upper and lower surface intact but no longer productive. How can you manage them? You can prune the plants back by about 10-30%. Dispose of the branches and leaves that you trim off (burn or bag them). This will help remove many of the insects currently on the plant.
Next, you will want to protect the plant so that the Boxwood Leafminer does not return. Chemical controls are the best protection for complete control of the insect. The most effective controls of Boxwood Leafminer target the larval stage, since it is a long window and control is likely effective. Systemic insecticides should be applied just after the eggs hatch (typically mid to late June in Northern Illinois). Since boxwood flower in spring, wait until the eggs hatch before spraying to avoid harming visiting pollinators.
Because the larvae are feeding and growing during the summer and early fall, research has found that a systemic insecticide spray can be very effective when applied during that same time period. When these treatments are applied properly and thoroughly to all boxwood in a given area, they eradicate nearly the entire population and it takes several years for a population to return to noticeable levels. Wasco Nursery recommends Bonide Systemic Insect Control or Bonide Systemic Insect Spray as the systemic insecticide for treating Boxwood Leafminer.
This photo was taken by Matt Zerby and shows a Boxwood Leafminer larvae inside of the leaf of a boxwood.