Willow Leafblotch Miner

Micrurapteryx salicifoliella (Chambers)

Host(s) in Alaska:

Salix spp. (≥ 37 willow species in Alaska)

Habitat(s): larvae mine the inner tissue of willow leaves

Current Status (2023 Update)

About 7,400 acres of willow leafblotch miner damage were recorded during ADS in 2023, down from over 16,000 acres recorded in Interior Alaska in 2022. Damage was scattered across the greater Fairbanks North Star Borough, Minto Flats, and adjacent to the Alaska Highway from Dry Creek to Northway Junction. Acres of recorded willow leafblotch miner damage may be lower in 2023 because survey coverage did not include the Yukon Flats, where the majority of willow leafblotch miner has been traditionally observed.

Ground detection surveys found willow leafblotch miner along every major roadway in the Interior, from the Brooks Range south to the Alaska Range, up the Steese Highway, and to the Canadian border. In Southcentral, some willow leafblotch miner damage was detected along the Parks Highway near Talkeetna. In iNaturalist, seven research grade observations of willow leafblotch miner were recorded around Fairbanks, one research grade observation in Salcha, and one research grade observation in Lake Minchumina.

Historic Activity

Historically, ground observations demonstrate that willow leafblotch miner is present throughout the Interior and more widespread than is indicated during aerial surveys. In 2020, ground surveyors recorded over 400 acres of willow leafblotch miner activity along all Interior roads north of the Alaska Range where willow was present, though it proved to be difficult to locate during the scan and sketch survey. Willow leafblotch miner was also observed in some areas in the Copper River Valley. Damage appears as blotchy orange and brownish-grey foliage and was observed on shrub-size willow with stems up to 4” in diameter. At most sites, damage was recorded on multiple stems, with some sites showing over 30 stems affected. Levels of observed damage ranged from trace to moderate with a small number of sites featuring heavy damage.

In 2022, over 16,000 acres of willow leafblotch miner damage were recorded in Interior Alaska during ADS, similar to 2021. Of those 16,000 acres, almost 14,000 acres were mapped in areas that have been traditionally affected by the agent, such as the Yukon Flats, along Beaver Creek, and the Yukon River; just over 1,000 acres of damage were also mapped along the Yukon River between Circle and Eagle and in the Yukon-Charley Rivers National Preserve.

Willow leafblotch miner are moths (Lepidoptera: Gracillariidae) and observed throughout Interior, Southcentral and Western Alaska. Larvae feed within leaves, creating areas of necrotic blotches (i.e. dead and discolored tissue) on upper surfaces of willow (Salix spp.) leaves. Severe damage can defoliate entire plants, kill leaves and branches, and can result in willow mortality. Willows are well-adapted to disturbances, and can often recover unless defoliation persists for several consecutive years. In Alaska, willow is a critical food source for several wildlife species, including moose (Alces alces). During winter, moose feed on woody stems of multiple species, including willow. Willow leafblotch miner negatively affects willow biomass production, but stem quality (nitrogen concentration and/or protein precipitation capacity) is unaffected. High willow leafblotch miner population levels do not reduce nutritional quality of willow, but can reduce resource availability for overwintering moose. Effects of willow leafblotch miner activity on moose populations, if any, have not been quantified.

Willow leafblotch miner eggs are pale green, satiny with finely veined surfaces, and adhered singly on bottom surfaces of willow leaves. Larvae undergo five developmental stages (instars), and are hypermetamorphic (early larval stages are distinct from later stages). First and second instars are small, have forward facing mouthparts, and lack legs and maxillae (mouthparts used for chewing). Later larval stages (instars 3-5) have downward facing mouthparts and legs and maxillae are present. Adults are small gray moths with mottled areas of light and dark gray to brownish gray on the forewings. Wingspans are 10-11 mm and antennae are approximately as long as wings.

Willow leafblotch miners are univoltine (one generation a year). In typical years, adult moths emerge and oviposit eggs in May. Larvae hatch in early June, and develop through the first four instars within four weeks. Fifth instar larvae may be present in June, but are observed primarily in July. Later instar larvae feed on mesophyll within leaves, producing characteristic blotches along upper leaf surfaces. Individual blotches are initially distinct, but coalesce as larval feeding continues. Later instar larvae are mobile, frequently exiting established mines, and initiating new mines within younger leaves. When larval stages are complete, larvae emerge from mines and pupate in silken cocoons often at distal ends of shoots. Estimates for survival from egg to pupae are less than 10%. Adults emerge in late July or early August, and overwinter in this stage.

Willow leafblotch miner is known to affect at least ten of the 37 species of willows found in Alaska. High egg densities have been recorded on Park’s (S. pseudomonticola), pacific (S. lasiandra), and diamond-leaf (S. pulchra) willows. Willows with dense trichomes (hairs) on undersides of leaves, such as felt-leaf willow (S. alaxensis) and gray-leaf willow (S. glauca), are rejected with higher frequency by ovipositing moths than willow species with smoother surfaces. Dense trichomes restrict egg attachment and prevent eggs from sinking into leaves prior to larval emergence. First instar larvae require eggs to sink allowing a direct lateral transition from eggs to leaves. Forward facing mouthparts of early instars restrict dorsal-ventral rotation, preventing initiation of mines from the leaf’s surface.

Willow leafblotch miner is native to North America, but damage due to this insect was not reported by FHP staff in Alaska until the 1990s. From 1991-1993, over 300,000 acres were mapped, predominantly along the drainages of the Yukon and Kuskokwim Rivers. Another outbreak occurred in the late 90’s. Centered in the Yukon Flats National Wildlife Refuge, 300,000 acres of damage was recorded from 1998-1999. Since its initial discovery, willow leafblotch miner, has been consistently mapped each year at various intensities, with the highest single year damage (500,000 acres) recorded in 2010.

Arctos, a collaborative museum collection, reports a single specimen of willow leafblotch miner recorded in Alaska prior to the 1990s outbreaks. In 1980, a specimen was collected from Bonanza Creek Experimental Forest, south of Fairbanks, AK, but there are no records of damage occurring during this decade. Willow leafblotch miner may be a relatively new immigrant to Alaska. It is unknown if willow leafblotch miner has historically avoided detection by remaining at low levels, or if the 1980 record and subsequent 1990s outbreaks mark a new introduction to the state.

Control methods on a landscape scale are expensive and impractical. In urban areas where trees are grown in more isolated, artificial conditions homeowners may wish to select one of the following control methods:

Method 1: If leaf miner feeding is low to moderate and willows are vigorous and showing little leaf discoloration. The use of pesticides is not warranted, but the following steps should be taken:

Care should be taken to avoid damaging trunks, injuring roots, altering drainage patterns, or severely compacting soils. Make sure trees receive adequate water throughout the growing season. Remove excess soil from areas over root zones. These actions will reduce water stress and/or soil oxygen depletion to trees.
Spring fertilization helps promote tree vigor. Use lawn or garden fertilizers high in phosphorus. Fertilization should begin in spring and continue through summer. Stop fertilization before trees begin fall dormancy. Feeding programs may not be necessary every year. Fertilizer uptake, soil type, rainfall, weather, and grass cover will determine the frequency of reapplication.

Method 2: If the willow shows signs of heavy damage (extensive leaf discoloration, leaf drop, etc.), chemical control may be warranted (see the Pesticide Caution Statement below). In the current year of defoliation, it is best to ensure that the trees receive adequate moisture in order to re-flush leaves. Foliar and systemic pesticides available for controlling leaf miner populations are available. When using pesticides it is important to read and follow all label directions. Contact the University of Alaska Cooperative Extension Service for recommended insecticides and information on how to apply them.
Method 3: When planting consider using species of willows less targeted by the leafblotch miner or consider replacing current susceptible willows with less susceptible species. Felt-leaf willow (S. alexensis) and gray-leaf willow (S. glauca) are less susceptible due to higher density of trichomes (small hairs) on leaves compared to more susceptible willow species.
Pesticide Caution Statement

Pesticides can be injurious to humans, domestic animals, desirable plants, and fish or other wildlife—if they are not handled or applied properly. Use all pesticides selectively and carefully. Since approved uses of a pesticide may change frequently, it is important to check labels for current approved and legal use. Follow recommended practices for disposal of surplus pesticides and pesticide containers. Mention of a pesticide on this website does not constitute a recommendation for use by the USDA, nor does it imply registration of a product under Federal Insecticide, Fungicide, and Rodenticide Act, as amended. Mention of a proprietary product does not constitute an endorsement by the USDA.

Damage from willow leafblotch miner is typically mapped by aerial detection survey and roadside/ground surveys. Willow leafblotch miner is a moth (Lepidoptera: Gracillariidae) native to North America and observed throughout Interior, Southcentral, and Western Alaska.

Willow leafblotch miner damage mapped by aerial detection survey 2015-2019 and gound observations 2018-2019.

Kruse, J. and N. Lisuzzo. May 2010. Willow Leafblotch Miner. Alaska Region Forest Health Leaflet. Available here.

Furniss, M. M., Holsten, E. H., Foot, M. J. and M. Bertram. 2001. Biology of a Willow Leafblotch Miner, Micrurapteryx salicifoliella, (Lepidoptera: Gracillariidae) in Alaska. Population Ecology. Entomological Society of America. Abstract or full text available here.

Allman, B. P., Kielland, K., and Wagner, D. 2018. Leaf herbivory by insects during summer reduces overwinter browsing by moose. BMC Ecol. 18:38. here

Content adapted from Willow Leafblotch Miner Forest Health Leaflet by James Kruse and Nicholas Lisuzzo. For more information, contact Entomologist Dr. Sydney Brannoch at sydney.brannoch@usda.gov.