Causal Organism: Pikonema alaskensis

Crops Affected: all species of spruce

Life Cycle:

• The adult is a stocky, reddish-brown, non-stinging, wasp-like fly about 8-10mm (about 3/8in) long
  • Adults have four transparent wings
• Adults overwinter in the debris under host trees, emerging in late May to mate and lay eggs at the base of the needles in the new growth
  • Sawflies get their name from their egg-laying process; adult females saw through plant tissues to lay their eggs
• Larvae hatch within 5-10 days and begin to feed
  • New larvae have yellowish bodies with yellow-brown heads, and are 3-4mm (about ¼ inch) long
  • Older larvae reach 16-20mm (about ¾in) in length, have brownish heads and shiny, dark green bodies with lighter, longitudinal (lengthwise) stripes
• Larvae are voracious feeders
  • Larvae start feeding on the new needles, then move to feeding on the older needles over time
  • Larvae feed for about 1 month before dropping to the ground to spin a tough cocoon around itself to overwinter in the debris

Symptoms:

• Larvae are visible within the needles, first feeding in the new needles, then older as time passes
  • Larvae can be somewhat camouflaged in the needles
  • Larvae are caterpillar-like
  • Larvae have
    • 3 pairs of legs on the front part of their bodies
    • 6 pairs of stubby leg-like projections in the middle of the body (called abdominal prolegs)
      • You can spell SAWFLY on each pair
    • A single additional pair of prolegs towards the end of the body
    • Larvae rear up when alarmed, raising both ends of their bodies, as well as exuding a liquid from their mouths
  • Larvae chew off the needles, leaving partly chewed needles and needle stubs
  • Trees have a ragged appearance
    • Trees have a brownish appearance in the areas where feeding has occurred
    • Trees may look thin and twiggy, as only the twigs and branches are visible
  • Feeding is typically more extensive in the upper parts of the trees
  • Heavy feeding can result in significant defoliation, leading to majorly reduced outward shoot growth
  • Repeated years of heavy feeding can result in dead branches, top killing, and occasional tree death

Monitoring:

• Trees are most sensitive to attack from 3-5 years after planting until they reach 8m (26ft) in height
• Trees that are alone or that are growing in the open are more likely to be attacked
• Trees should be examined regularly (weekly) starting in early June

Management:

• Ensure that trees are as healthy as possible, as this will help them to resist attack by this pest
• Larval infestations of YHSS on smaller trees can be picked off and destroyed
• Chemical controls may be required on larger trees, however the number of registered options are limited
• Larvae may be blasted with a high-pressure spray of water, which knocks them off of the tree
  • Most will not be able to return and finish their development
• Natural enemies will attack this pest at any stage; this includes insects, rodents, and birds
  • These natural enemies will not be able to completely control a population of the sawfly
• Over time, feeding and defoliation of the trees may work against the sawfly, with a natural decline in the population as they can’t find suitable egg-laying sites