The boxwood leafminer is originally from Europe. It was accidentally imported to the United States and has since spread across the country. It is the most serious pest boxwoods have. Leafminers attack all species of boxwood, but the slower growing English boxwood is not as severely affected as the American and Japanese boxwoods.
The leafminer is a small orange looking fly. It lays its eggs on the underside of boxwood leaves. The larvae hatch and eat the interior of the leaf, between the two exterior walls. This causes a blister on the back of the leaf. The leaf also turns yellow and in severe infestations, looks completely unhealthy.
Boxwood leafminers survive the winter as a partially grown larvae in the blister that has formed on the infected leaf. When the temperature warms, the larvae become active and continue eating the boxwood leaves. They then pupate into an orange looking cocoon which darkens as the leafminer becomes mature. It forces its way out of the leaf blister as an adult.
When the boxwood starts putting on new growth, the adults swarm and mate. The female then puts an egg on the underside of a boxwood leaf. It hatches in fourteen to twenty-one days. The maggot feeds on the boxwood leaves the rest of the summer. The adult fly dies soon after mating. Only one generation of boxwood leafminers occur per year.
There are several ways you can fight the boxwood leafminer. First, you can plant resistant varieties. Culltivars of English boxwood such as Buxus sempervirens ‘Pendula,’ ‘‘Suffruticosa,’ ‘Handworthiensis,’ ‘Pyramidalis,’ ‘Argenteo-varigata’ and ‘Varder Valley’ are more resistant than other boxwoods.
Encouraging green lacewings and spiders to live on your boxwood can help as these predators eat leafminer larvae. If you are careful to maintain the health of the plant, it will better tolerate the leafminer. Boxwoods in poor health are especially vulnerable to dying from a leafminer infestation.
You can prune the boxwood in the spring right before the leafminer emerges as an adult. You can then dispose of these pruned branches by burning them or putting them in a secure bag and throwing them in the trash. This reduces the volume of the leafminers on the boxwood. You can also pinch the blisters on the leaf hard enough to kill the maggot to reduce the population and damage to the boxwood.
Pesticides have a hard time reaching the maggot because the outer parts of the leaves shield it from the pesticide. However, you can apply a pesticide when the new leaves are fully formed. Make a second application between six weeks and ten weeks later. You can use carbaryl to control the adult flies. Acephate can be applied about three to four weeks after the adults emerge to control the larvae developing in the new leaves. In February to April, the systemic insecticide imidacloprid can be applied around the base of the shrub to kill the larvae.