The larch casebearer is an introduced species from Europe. It was first reported in Massachusetts in 1886. Until the late 1950's, it was only found in the eastern United States. Following that time, it spread to the west. The common name of this insect comes from the case constructed by the larva (caterpillar). This case allows the larva to be highly camouflaged, and detection of this insect on its host can be very difficult. Cases are formed by part of a mined-out larch needle that is lined with silk spun by the larva. Cases are the color of a dead needle and cigar-shaped. The insect overwinters as a larva in its case, firmly attached to a branch or near the base of a bud. Feeding by the larvae begins again in the spring. This begins as soon as foliage begins to appear. Fully grown larvae are approximately 6 mm in length. By late May or June, larvae complete their feeding and pupate within their case. Adults emerge by June and early July and are winged to disperse. Adult moths have a wingspan of approximately 8 mm and are silvery/gray in color. Females lay tiny eggs that are attached to each needle, approximately 1 per needle. Eggs are described as reddish in color, and shaped like inverted jelly molds with 12-14 ridges when viewed with magnification. Upon hatching from the egg, larvae burrow directly into the host plant needles. Larvae start their feeding as needle miners for approximately 2 months.
While larch is a conifer that naturally loses its needles before the winter (unlike most conifers), the larch casebearer can sometimes cause its hosts to lose their needles prematurely. All species of North American larch as well as European species can be damaged by this insect. Damage may be severe, whether the host is found in a forest or a managed landscape. The needle tips will appear bleached or scorched and then turn brown. Feeding occurs on newly emerged foliage in the spring, and this is when the most conspicuous damage occurs. In heavy populations, trees may appear bronzed by mid-June. Heavy defoliation can slow both diameter growth and height growth of the host plant. If defoliation occurs during two or more consecutive years, host mortality may occur.
Monitoring host plants for fresh injury from caterpillars feeding in the spring may be most obvious, however keep in mind the activity of this insect is often cryptic. Scout plants as soon as new foliage emerges. During the flight period of the adult moths, shake foliage and look for flying insects (starting in June and early July). Some private companies sell larch casebearer lures and traps for detection of the adults. Possible sex pheromones exist for this species and have been studied (McMillian and Borden, 1974; Priesner et al, 1982).
Cultural management options for the larch casebearer in ornamental landscapes are limited.
There are natural enemies that reduce larch casebearer populations, however they are not always successful in preventing damage to host plants in managed landscapes. Some of these natural enemies are native and others were introduced as biological control agents of the larch casebearer. Parasites such as Agathis pumila, Bassus pumilus, and Chrysocharis laricinellae are important parasites. In some geographic locations, birds are also important predators of this insect. (Additionally, abiotic conditions such as sudden temperature change and late frosts are known to impact larch casebearer populations.)
Abamectin (NL)
Acephate (NL)
Azadirachtin (NL)
Bacillus thuringiensis subsp. kurstaki
Bifenthrin (NL)
Carbaryl (L)
Chromobacterium subtsugae (NL)
Cyantraniliprole (NL)
Cyfluthrin (NL)
Deltamethrin (L)
Emamectin benzoate (L)
Flonicamid + cyclaniliprole (N)
Gamma-cyhalothrin (L)
Horticultural oil (L) (eggs)
Indoxacarb (L)
Lambda-cyhalothrin (L)
Malathion (L)
Permethrin (L)
Pyrethrins+piperonyl butoxide (L)
Spinetoram + sulfoxaflor (N)
Spinosad (NL)
Tebufenozide (NL)
Active ingredients that may be applied systemically include: Acephate (injection), azadirachtin (injection, soil drench), and emamectin benzoate (L).
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
