Yellow-Headed Spruce Sawfly: The Facts

The yellow-headed spruce sawfly (Pikonema alaskensis) is a species native to Alberta, although the introduced European spruce sawfly (Gilpinia hercyniae) is also found in the Prairie Provinces. Sawflies attack native white, black and Engelmann spruce trees,  as well as variety of non-native spruce trees, such as Colorado and Norway spruce. They have been known to create significant damage to many young spruce trees in the Prairie Provinces. They prefer younger trees which provide tender needles that are easier for the larvae to consume. Shelterbelts, windbreaks, forest edges, clearings, and urban parks are common
habitats where infestations can occur. The yellow-headed spruce sawfly is considered a major
pest due to its ability to cause severe defoliation, which can lead to reduced growth rates,
weakened trees, and mortality, especially in younger trees.

Pest ID

As larvae blend very well with spruce green needles, the close-up inspection of branch or shoot is
necessary in locating and identifying this pest. Another effective method would be putting a white sheet of paper under the branch and shoots and shaking the branch which will cause larvae to drop on the paper. Adult yellow-headed spruce sawflies emerge in the spring, just as the buds on spruce trees are beginning to swell. Females deposit eggs into the base of new needles and larvae emerge within two weeks and begin feeding on the succulent needles. By early to mid-July, the larva is fully grown and about 20 mm in length.
In the early-stages, larvae is yellowish and 4-8 mm long. Grown larvae have brownish-orange heads
and green bodies marked with six stripes. Fully developed larvae drop to the ground and
overwinter in cocoons in the soil. The sawfly completes one generation per year. Adult sawflies
are wasp-like, small, (8–10 mm) and reddish brown.

Symptoms

This insect can cause significant damage and mortality during repeated defoliations. Sawfly is a defoliator and consumes host needles. It reduces growth, kills branches, and eventually kills trees. Young trees are most susceptible to attack after planting and until the trees reach up to 30 feet or more in height. Spruce trees that grow in open areas, such as shelterbelts and windbreaks, and are fully exposed to sunlight are the preferred choice for this insect to lay eggs. Spruce trees growing in natural forest with aspen in the understory are rarely infested by this sawfly.

There are several symptoms to recognize that spruce trees are under the attack of this insect:

• Newly hatched larvae feed first on the new needles and then on older foliage.
• By mid-June, loss of needles is most noticeable on top of trees and on the tips of the branches and shoots.
• After larvae chew needles the remaining twigs and shoots are brownish in colour.
• By mid-July, due to heavy defoliations trees appear ragged, yellowish, and bare.
• Three to four years of repeated defoliation can lead to tree mortality.

Management and Control

There are several management options for the control of the yellow-headed spruce sawfly:

• Look for damage and groups of larvae from late-May through mid-July.
• Hand picking is recommended on small trees.
• Use insecticidal soap as it is the most effective against young larvae.
• Use a high pressure water gun with insecticide soap to blast and remove larvae. Knocking
  the larvae off will disrupt larvae development and may reduce population.
• Keep spruce trees healthy by regular watering and, if necessary, adding fertilizer to boost
  growth and vigour of affected trees. Watering is critical during a drought period
• Biological control involves the use of natural predators and parasitoids to control sawfly
  populations. Birds, such as chickadees and warblers, feed on sawfly larvae. Parasitic
  wasps lay their eggs inside sawfly larvae, ultimately killing them. Encouraging the
  presence of these natural enemies through habitat enhancement can be an effective
  strategy.
• Chemical control may be necessary in cases of severe infestation. Timing is critical when
  applying insecticides - they should be applied when larvae are small and actively feeding.
• Use only a registered chemical spray (e.g. Malathion or permethrin) when the larvae are
  small to reduce the amount of damage.