Yellowheaded Spruce Sawfly
Hosts: White, black, red, blue, Norway, and Engelmann spruces
Evidence: In July, look for groups of olive green larvae with reddish-yellow to chestnut brown heads (a) on branches of open-grown spruce. Look for defoliation of current year's foliage; older foliage may be eaten when new foliage is completely gone (b). The ground beneath trees may be littered with partially consumed needles and frass. When disturbed, larvae have a tendency to rear up both their head and tail ends, and they may give off a viscous fluid from the mouth. After larvae complete feeding, their droppings may still be seen and dark brown cocoons with bits of soil around them will be present in the litter.
Life Cycle: Winter is spent in a prepupal stage in a cocoon in the litter. Pupation takes place in the spring, and adults emerge in early- to mid-June. Eggs are laid singly in slits at the base of new needles in the crown of the tree. Newly-hatched larvae begin feeding on the edges of new needles, and later, when half-grown, begin to consume entire current-year needles before moving to older foliage. Larval feeding lasts for about a month. Then larvae drop to the ground and spin cocoons.
Management: Parasites and predators are known to kill significant proportions of yellowheaded spruce sawfly populations, but control using insecticides is frequently necessary. Insecticides should be applied as soon as the tiny larvae begin to feed (about 10 days after bud caps have been shed). Young, open-grown and ornamental trees seem to be preferred. Defoliation often begins on single branches but may spread to the entire tree. Trees may be killed if completely defoliated for more than one season. Trees that experience severe needle loss may survive, but lose branches and radial growth as a result. If there are just a few, young trees infested, larvae can be handpicked or dislodged into a bucket of soapy water.
Figure a: E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, Waterbury, VT.
Figure b: E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, Waterbury, VT.
Drooz, A.T. 1985. Insects of Eastern Forests. USDA Forest Service Miscellaneous Publication 1426. p 404-405;
Johnson, W.T. and Lyon, H.H. 1991. Insects That Feed on Trees and Shrubs. 2nd edition. Cornell University Press. p 18;
Martineau, R. 1984. Insects Harmful to Forest Trees . Agriculture Canada Government Publishing Centre, Supply and Services, Ottawa. p 4-6;
Rose, A.H. and Lindquist, O.H. Revised by Syme, P. 1994. Insects of Eastern Spruces, Fir and Hemlock. Canadian Forest Service Publication. p 77, 79-80.