Host:  White fir

Figure 17. Defoliated white fir caused by the white fir needleminer.

Symptoms/Signs:  Larvae mine the needles, resulting in bleached-yellow needles from late spring to early fall. Mature larvae are about 8 mm long, yellowish-green to cream colored, with brown to black heads. Pupae are orange to dark brown, about 5.5 mm in length, and can often be seen protruding from the hole in the mined needle prior to emergence. The adults are 11 mm dusty gray moths. They can be seen swarming around host trees in late June and July.

Biology:  The white fir needleminer has one generation per year in New Mexico and Arizona. Eggs are deposited, usually one per needle, in late June and July. Eggs hatch in August and September. Young larvae immediately bore into the needle where they overwinter. As weather warms in the spring, feeding begins. Each larva can mine several 1-year-old needles. Two- and three-year-old needles are also mined during outbreak conditions when insect populations are high. Two to six needles are usually webbed together and remain on the tree for the duration of the summer. Pupation occurs within the last mined needle in June or early July. Adult moths emerge in 10 to 14 days after pupation. Mating occurs within 2 to 3 days of emergence.

Figure 18. Closeup of white fir needleminer defoliation to white fir.

Effects:  After several consecutive years of heavy defoliation, a high proportion of limb mortality and widespread tree mortality may occur. This can give individual trees or even entire stands a silvery appearance. These heavily defoliated stands decline in vigor and may be predisposed to attack by fir engraver beetle.

Similar Insects and Diseases:  Similar defoliation may be caused by western spruce budworm or Douglas-fir tussock moth; however, the larvae of white fir needleminer only mine inside the needles.

References:  24, 110