Yellowheaded spruce sawfly

Distribution

Canada

The yellow-headed spruce sawfly is native to North America. In Canada, its range extends from coast to coast and into the Northwest Territories.

Damage, symptoms and biology

The yellow-headed spruce sawfly damages its hosts by causing repeated defoliations. Trees become susceptible to attack 3-5 years after planting and may be reinfested annually until they are 8 m or more in height. Young open-grown trees are preferred as egg-laying sites; this sawfly rarely causes injury to naturally grown and closed forests.

Egg-laying females attack trees in early- to mid-June, soon after the bud caps fall. Newly hatched larvae feed first on the new needles and then on older foliage. This feeding damage appears more intense in the upper portion of the tree crown, and may result in complete defoliation of the upper shoots and branches. After the feeding cycle is completed, the remaining partly chewed needles and needle stubs impart a brownish colour and ragged appearance to the tree. One year of severe defoliation causes a reduction in shoot growth and stem thickness, while two or more years result in dead branches and top kill, and sometimes the tree dies.

The adult yellow-headed spruce sawfly is wasp-like in form, reddish brown in colour, and 8-10 mm long. Females begin laying eggs in June when new shoot growth is 2-3 cm long. A single egg is deposited in a slit in the bark at the base of each needle; it hatches after 5-10 days into a small yellowish larva, 4-5 mm long, with a yellow-brown head. It feeds and develops to maturity over the next 30 to 40 days. The mature larva is 16-20 mm long, with a reddish-brown head and a dark green glossy body with lighter longitudinal stripes along the back and sides of the body. When disturbed, the larva exudes a liquid from its mouth while arching both ends of its body in a characteristic alarm reaction.

When mature, the larva drops to the ground and spins a cocoon in the soil where it over-winters. In the following spring, the larva transforms to a pupa and emerges from the cocoon a few days later as an adult sawfly, completing a one-year life cycle.

The yellow-headed spruce sawfly has many natural parasitic and predaceous insect enemies that attack it in the egg, larval, and cocoon stages. Small rodents and shrews may prey upon the sawfly in the cocoon stage, while feeding larva and adult sawflies may fall prey to birds. These natural enemies, however, seldom appear able to keep sawfly populations in check.

Defoliated trees may produce fewer shoots in the following year, thereby causing a reduction in egg-laying and feeding sites and subsequent starvation of larvae. Ornamental trees that have been severely attacked for one or more years may require some pruning to encourage new growth and reshape the tree crown.