Alder Defoliators

In 2023, alder defoliation was observed over almost 11,100 acres of forests. No damage agent has been confirmed for this activity, but it is presumed to be a generalist-feeding moth species as other hardwood defoliation occurred in the area. Damage was observed on the southwestern tip of the Kenai Peninsula between English Bay and Sadie Cove (3,400 acres). In 2022, extensive alder and other hardwood defoliation was observed in this area; however, weather conditions in 2023 partially limited aerial survey efforts to follow up on this activity. The unobscured inlets that were flown suggest that the defoliation event continues but appears to have declined in some areas. Scattered alder defoliation was observed near Halibut Cove, between Swift Creek and Moose Creek, near Nellie Juan Lake, along the South Fork Snow River (300 acres), and near treeline on the western slopes of Sheep Mountain and Andy Simons Mountain. Thin alder crowns (5,200 acres) were observed west of Sevenmile Lake and north of the Denali Highway on the West Fork Maclaren River. In Southeast Alaska, alder defoliation was observed on Sokolof Island (780 acres) and near Haines. Green alder sawfly and woolly alder sawfly were observed during ground survey causing heavy defoliation of red alder in Juneau, especially the Eagle Beach area. 

Alder defoliation from insect pests was not detected during 2020 scan and sketch surveys, which replaced traditional aerial detection surveys due to the safety limitations imposed by the pandemic. Alder defoliation was observed during ground surveys across all regions where surveys were conducted in 2020. A variety of agents were attributed as the causal agents.  

During the 2019 aerial detection survey, alder defoliation was observed across 2,600 acres, with the highest concentration of damage (1,100 acres) between the McArthur and Susitna Flats and several patches of damage along the Tanana River between Fort Wainwright and Healy Lake (700 total acres). Small patches of damage were observed spread over several locations throughout Southeast Alaska. 

Several species of sawflies (Hymenoptera) and caterpillars (Lepidoptera) feed on alders throughout the state, with varying abundances from year to year. Caterpillars of the fingered dagger moth (Acronicta dactylina) were observed feeding on Sitka and red alder in Ketchikan, Petersburg, and Wrangell. Native to North America, their range was thought to end in southern British Columbia. First reported in Southeast Alaska in 2015, this caterpillar has become increasingly common, suggesting that this may be an example of range expansion. Spotted tussock moth caterpillars (Lophocampa maculata) continue to be abundant in Southeast, Alaska. Spotted tussock moth caterpillar feeding occurs late in the season causing minimal damage. Green alder sawfly (Monsoma pulveratum) was observed feeding on thin-leaf alder near the Deshka River and was the most commonly encountered sawfly on alder in Southeast, Alaska.

Alder defoliation can be caused by several insects, which often occur together in the same location. Alder defoliators are predominately sawflies (Hymenoptera: Tenthredinidae) or caterpillars (Lepidoptera: Geometridae, Tortricidae, Erabidae). Fluctuations in defoliator populations can be attributed to multiple factors such as climate, predation, and disease. See below for more information specific to alder sawflies.

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The green alder sawfly is the first of the three sawflies to emerge in the spring. Egg-laying occurs in mid-May, at approximately 60°F, when the alder leaves are just beginning to flush. Both woolly alder and striped alder sawfly adults emerge from mid to late June. Egg-laying commences immediately and continues for several weeks as new adults emerge. Egg deposition for green alder sawflies is on the upper leaf surface, woolly alder sawflies deposit eggs on the underside of the leaf along and into the midrib, and striped alder sawflies deposit eggs along the leaf petiole. Embryonic sawfly development for all three species is 1-2 weeks. Upon hatching, green alder sawfly larvae migrate to the lower leaf surface. All three species feed openly on the leaves, consuming (in most cases) all the soft leaf tissue, leaving behind only the leaf veins and midrib. Striped alder sawfly larvae feed gregariously, while the alder woolly and green alder sawflies feed individually. However, in high larval density situations, it is not uncommon to find five or more green or woolly alder sawfly larvae per leaf. At times, only one species might be found on an individual alder but it is possible to have two or even all three species feeding on the same plant, or even on the same leaf. 

Green alder sawfly larvae are cream colored to very pale green upon hatching. As they grow and develop, they change to a vibrant bluish-green. Full grown larvae can exceed 18 mm in length. Woolly alder sawfly larvae secrete a white, waxy, hairlike covering on their bodies. As they grow, this covering becomes denser. In their last instar, the covering is shed, exposing a very pale green body. The larvae of striped alder sawfly are yellowish- to greenish- brown with two black lateral stripes on either side of their body. They are quite robust, though slightly shorter in length at full development than woolly or green alder sawflies. When larval development is complete, the woolly alder and striped alder sawflies drop to the ground and burrow into the soil beneath the leaf litter where they excavate a small chamber 1-4.5 cm beneath the soil surface. The larvae then enter a pre-pupal state and remain in this condition throughout the winter. In spring, when the ground begins to warm and reach the threshold temperature for each species to continue development, the pre-pupae change to pupae, and after some time, they emerge as adults. By contrast, the green alder sawfly typically burrows into woody material and spends the winter in stumps, broken branches, and down logs. This behavior is believed to be unique to this genus of sawfly and may account for their early emergence in the spring.

The damage to alder due to sawfly feeding is quite obvious. Early in the season, as active feeding begins, numerous small holes appear in the leaves as the small larvae feed. This results in a characteristic “shotgun” blast appearance. In time, these small feeding holes grow even larger; over time, the will larvae consume all the soft tissue between the leaf veins leaving behind only the stouter veins and leaf midrib. Attacked shrub takes on a thin, brownish appearance easily seen from the ground or air. Alder, like most hardwood tree species, is able to withstand several seasons of severe defoliation, suffering nothing more than growth loss and occassionally branch dieback. However, severe defoliation in alder can reduce its nitrogen contribution to the soil by more than 70% annually. Alder mortality has been observed, in other defoliator outbreaks, after 5 or more consecutive seasons of severe defoliation.

Since thin-leaf alder is primarily a riparian species and an important component of many salmon spawning streams, it can be difficult to implement sawfly control measures. Some insecticides have deleterious effects on aquatic insects as well as the terrestrial insects they specifically target. Many infested alders lie within an area normally excluded from operational insecticide spray programs because of their ecological sensitivity. Homeowners do not normally encounter problems with these alder-defoliating insects. If an individual is concerned about a small group of infested alders on their property, effective control can be achieved either through hand-picking the larvae from the leaves or treatment with a strong jet of water. Repeating these activities several times throughout the summer may be required.