Yellow-headed spruce sawfly (Pikonema alaskensis) is native species to Alberta. Also introduced European spruce sawfly (Gilpinia hercyniae ) is found in Prairie provinces. Sawfly attacks native white, black and Engelmann spruce trees as well as variety of non-native spruce trees( Colorado, and Norway spruce). It can create significant damage to many young spruce trees on the Prairie Provinces. 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. Larvae emerge within two weeks and begin feeding on the succulent needles. By early to mid- July the larvae are fully grown and are about 20 mm in length.
As larvae very well blend with spruce green needles, the close-up inspection of branch/shoot is necessary. Also putting white sheet of paper under the branch and shoots and shaking branch will cause larvae to drop on the paper. At early-stage larvae are yellowish and 4-6 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.
This insect can cause significant damage and mortality during repeated defoliation’s. Sawfly is defoliator and consumes host needles. It reduces growth, kills branches, and eventually kills trees. Usually, young trees are susceptible to attack after planting and until trees are up to 30 feet or more in height. Spruce trees that grow in open areas and fully exposed to sunlight are preferred choice for insect to lay eggs. Spruce trees growing in natural forest with aspen in 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, on tips of the branches and shoots
- After larvae chewed needles remaining twigs and shoots are brownish colour
- By mid-July due to heavy defoliation’s tree appears ragged, yellowish and bare
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 most effective against young larvae.
- Use 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 vigor of affected trees. Watering is critical during a drought period
- Use ONLY a registered chemical spray(eg. Malathion) when the larvae are small to reduce the amount of damage.
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