Language English Arborvitae Leafminer. Photo: Connecticut Agricultural Experiment Station Archive, Bugwood.org OngletsDescriptionSummaryIn Quebec, there are four species of leafminers, small moths that lay their eggs mainly in arborvitae trees. The tiny caterpillars burrow into the young shoots, causing them to die back. About 3 cm at the tip of mined leaves appears bleached, turning yellow and then brown. A heavy infestation may cause many of a tree's leaves to drop. Signs and symptomsThe first signs of damage appear in early spring; at first glance, the terminal shoots may appear frostbitten or parched. In the first year of an infestation, about 3 cm at the tip of the terminal shoots appears bleached, turning yellow and then brown. The following year, the withered brown foliage drops. Many of a tree's leaves may drop after several consecutive years of severe infestations. Around mid-May, the small green caterpillars, which have overwintered in the terminal shoots, begin tunnelling into the scales, from the tips to their base; at this time of year, the leafminers can still be seen in the galleries. In late May, the caterpillars build cocoons inside the scales or under the foliage, depending on the species. From mid-June to mid-July, small greyish moths emerge from the cocoons; they can be seen at nighttime flying around the trees to feed, mate and lay their eggs. A heavy infestation may affect most of a tree's branches, but arborvitae can withstand a few infestations without suffering any real damage; the twigs die back only after several consecutive years of severe infestations. Latin name (genus)Argyresthia spp., Coleotechnites thujaellaHost plantsMainly arborvitae and its cultivars. Name of host plants Development cycleDescription and life cycleLeafminers are members of the order Lepidoptera, like butterflies and moths. They undergo complete metamorphosis (holometabolic insects) and pass through the egg, larva and pupa stages. Eggs: Greenish yellow, 3 mm long. Larvae: Black head and a light to dark green body tinged with red, 5 mm long. Pupae: Leaf green with a brown head, 3 to 4 mm long. Adults: Small moths (2.5 mm long, with an 8 mm wingspan), grey with brown wingspots. Leafminers produce a single generation each year. They overwinter as larvae, inside arborvitae scales. In early spring (mid-May), the larvae wake up and begin feeding, tunnelling into twigs. In late May, they build cocoons inside scales or under the foliage, depending on the species, and pupate. The adult moths emerge from the cocoons and are active from mid-June to mid-July. After mating, the females lay their eggs (about 25 eggs per female) around the edges or tips of the tender new leaves. As soon as they hatch, about 20 days later, the tiny caterpillars start mining into the leaves. In fall, the larvae stop feeding and overwinter in the tunnels. Prevention and controlFavourable conditionsLeafminers cause more damage to unhealthy trees with weak shoots, like neglected cedar hedges. IdentificationIn early spring (mid-May), inspect your arborvitae trees for any bleaching of the terminal shoots. Examine a yellowed or bleached twig under a magnifying glass. At this time of year, you may either see tiny green larvae in their galleries, or find that the branch has already been mined. From mid-June to mid-July, you will be able to see small moths flying about, mating and laying eggs on new shoots. PreventionKeep trees healthy by fertilizing them adequately, pruning them properly and watering them during dry spells. Do not install any lights that will attract the moths to lay their eggs in arborvitae trees. Physical controlIn early spring, cut off and dispose of the tips of yellowed or withered branches to break the caterpillars' life cycle. In summer, prune hedges to keep them dense and compact and to remove any newly infested shoots. Never compost infested plant material. Biological controlLeafminers have a number of natural predators and parasites (wasps, ladybird beetles and praying mantises), which help to keep populations under control; attract these predators and parasites by growing a wide variety of plants. Chemical controlThe Montréal Botanical Garden does not recommend the use of pesticides to control these insect pests.