Spruce cone maggot (Strobilomyia neanthracina)

(Written by Dr. Herb Cerezke)

Last Review/Updated: January 6, 2003

The spruce cone maggot (Strobilomyia neanthracina Michelsen) is a major pest of spruce cones and seeds. Its larvae tunnel within cones, feeding at the base of scales and consuming seeds.

Primary Hosts and Distribution

  • White spruce (Picea glauca (Moench) Voss))
  • Engelmann spruce (P. engelmannii Parry)

The spruce cone maggot occurs throughout the range of white and Engelmann spruces in Alberta.

Life Cycle

The spruce cone maggot has a one-year life cycle, but a variable portion of the overwintering population may diapause for an additional year or more.

Larvae

Larvae appear within the young female conelet shortly after pollination. There are three larval instars, the first instar develops within the egg while the second and third instars construct spiral feeding tunnels around the central cone axis. Feeding occurs from about mid-May to mid-July, after which the mature larvae tunnel out of the cone and drop to the ground. Mature larvae are 5-7 mm long, creamy white and without a definite head capsule.

Puparium

This stage develops in the soil under the tree, is reddish-brown in color, ovoid in shape, and measures 4-6 mm by 1.2-1.4 mm. It overwinters in the soil and may undergo extended diapause for one or more years.

Adults

These are small black flies, 3.9 - 4.6 mm long, and resemble houseflies. They are present in the spring about the time conelets are open for pollination.

Eggs

Eggs are deposited singly between scales of conelets during or shortly after pollination. They are pearly white, ovoid in shape and measure 1.6 x 0.5 mm.

Detection and Damage


Look for the following signs and symptoms:

  • white pearly eggs deposited between scales in May;
  • in bisected cones after mid-summer, note presence of holes infiltrated with resin and destroyed seeds;
  • look for a small hole ringed with brown discoloration on the exterior of the cone after mid-July.

Cones with a single larva present may have 55-65% of the filled seeds consumed, while the presence of two or more larvae may destroy 100% of the seeds per cone. Percentage of cones infested from year to year varies widely but may sometimes exceed 80%. Seed losses tend to be higher in established seed orchards than in natural wild stands.

Management

  • A sequential sampling method has been developed and field-tested to classify spruce cone maggot infestation levels in white spruce seed orchards and to predict expected levels of seed loss in the fall. The method is applied in the spring by sampling young conelets and can serve as a basis for decisions on control strategies.
  • Sampling of mature cones in the fall can serve to estimate and verify infestation levels and percentage seed losses.
  • Systemic insecticides applied as a foliar spray and as tree implants or as stem injections can be effective in reducing populations in seed orchards.
  • By observing and monitoring annual cone crop size and cone maggot infestation levels, years of maximum seed yield can often be predicted.