Forest & Grassland Health

Larch Sawfly

Pristiphora erichsonii (Hartig) 

Primary Hosts in Alaska: Tamarack/Eastern larch (Larix laricina) and ornamental Siberian larch (Larix sibirica) throughout Interior and Southcentral Alaska. 

Damage: Needle defoliation.

Photos

Click on the image for a larger version.

Photograph of an adult female larch sawfly specimen Photograph of an adult larch sawfly resting on a larch branch Photograph of larch sawfly larvae feeding on larch needles in front of a home Photograph of a larch with crown defoliation caused by larch sawfly

 

Adult female larch sawfly, dorsal view

 

 

Adult larch sawfly on a larch branch, lateral view

 

Larch sawfly larvae aggregation

 

Larch crown defoliation caused by larch sawfly attack

Current Status in Alaska (2020 Update)

Larch sawfly has been detected in small to trace amounts in isolated areas in Southcentral Alaska in 2020. In July 2019 and June/July 2020, there were positively identified occurrences of the larch sawfly in the Fairbanks area through social media, however FHP staff recorded no direct observations of the species themselves. 

Historic Activity 

Significant larch sawfly defoliation was observed in Alaska in the mid-1990s. Large outbreaks (>150,000 hectares) developed in Interior Alaska and lasted for more than five consecutive years. Scattered larch mortality was especially apparent on sites with poorer health. In 1993 12,000 acres of tamarack (eastern larch) defoliation was observed in Interior Alaska. By 1996, this outbreak encompassed more than 600,000 acres and continued, although at a slower pace. Tamarack mortality has been observed over extensive areas of the Interior as a result of more than seven consecutive years of feeding. In 1999, the larch sawfly was recorded from the Mat-Su valley and the Anchorage Bowl feeding on ornamental Siberian larch. This is the first time that this defoliator has been recorded south of the Alaska Range and is likely the result of accidental introduction. By 2001, larch sawflies were observed feeding on Siberian larch on the Kenai Peninsula. 

Symptoms, Biology, & Impacts 

The eggs are translucent and are approximately 1/16 inches long. The eggs are laid under host bark from mid-June to mid-August. Newly hatched larvae have brown heads and cream-colored bodies. Mature larvae are about 1/2 inches long and have shiny black heads with bodies that are grayish-green along the upper dorsum and whitish below. Larvae feed from mid-June to early September on tamarack needles and generally feed in groups. From mid-July to early September, larvae will drop from the trees and begin forming their cocoons on the ground, where they will overwinter. The papery brown cocoons are capsule-shaped and measure about 7/16 inches by 3/16 inches. Adults will emerge the following spring from late May to early July. Reproduction is largely parthenogenic as few males are found. Adult females are about 3/8 inches long with black antennae, head, and thoraxes. The abdomen is orange and tapers sharply towards the rear. Males have yellowish antennae, an orange abdominal band, but the abdomen is cylindrical and rounded at the rear. There is substantial overlap of life stages for larch sawfly during the growing season. 

Larch sawfly can be responsible for heavy defoliation. Heavily defoliated trees commonly refoliate after a few weeks. Repeated defoliation, however, can result in trees with thinned foliage, branch mortality, significant growth loss, and, in severe cases, tree mortality. Tree stress from heavy defoliation can make tamaracks more susceptible to other damaging insects, including deadly attacks from larch beetle (Dendroctonus simplex). Tamarack mortality has been observed over extensive areas of the Interior as a result of more than seven consecutive years of sawfly feeding. Ornamental Siberian larch can withstand consecutive years of defoliation better than native larch. 

Guidelines for Reducing Damage 

Larch sawflies are a natural part of Alaskan forests and management of these insects on a landscape scale is costly and impractical. For small scale control on for specimen trees, or those of particular value, chemical control can be an option; however, to avoid impacts on non-target species, mechanically removing larvae from trees is best. This can be done with a high pressure water spray on needles to dislodge larvae, shaking trees to dislodge larvae, or hand picking larvae from trees. Raking and disposing of needles and litter beneath the infested tree in the fall can remove the overwintering pupae and minimize sawfly defoliation the following year. Actions can be taken to improve tree vigor to make them more resilient to sawfly damage, including fertilizing, adequate watering, and protecting roots from injury. 

Resources and Publications 

Holsten, E. H. 2001. The Larch Sawfly. USDA Forest Service, Alaska Region, State and Private Forestry. Leaflet R10-TP-101. Available here or here

Holsten, E. H., Hennon, P., Trummer, L., Kruse, J., Schultz, M., & Lundquist, J. 2008. Insects and Diseases of Alaskan Forests. USDA Forest Service, Alaska Region, State and Private Forestry, Forest Health Protection. Publication R10-TP-140, 55. Available here.  

Turnock, W. 1960. Ecological Life-History of the Larch Sawfly, Pristiphora erichsonii (Htg.) (Hymenoptera: Tenthredinidae), in Manitoba and Saskatchewan. The Canadian Entomologist, 92(7), 500-516. doi:10.4039/Ent92500-7. 

 

Content adapted from Insects and Diseases of Alaskan Forests (2008) p.48 (available here) and Larch Sawfly Forest Health Leaflet (available here or here). For more information on this agent, please contact Entomologist Dr. Sydney Brannoch at sydney.brannoch@usda.gov. Content prepared by Dana Brennan, Biological Science Technician, Forest Health Protection. 

 

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