Douglas fir needle midge damage showing up in Christmas tree plantations
Douglas fir needle midge larvae cause galls to form on current-year needles. Infested needles drop from the tree in the fall, usually just before harvest. Severe infestations can cause needle loss, a serious problem for Christmas trees.
Over the past few years, we have seen an increase of damage to Douglas fir in Christmas tree plantations caused by the Douglas fir needle midge, Contarinia pseudotsuga Condr (Cecidomyiidae). Severe infestations can cause unacceptable needle loss, an especially serious problem for Christmas trees (Photo 1). Even moderate needle loss can reduce the value of Christmas trees and heavily damaged trees may be unsalable.
Douglas fir needle midge over-winters as larvae in the soil under infested trees. In early spring, larvae pupate and adult midges emerge to mate and lay eggs in the needles of the expanding buds. Infested needles appear yellow (Photo 2) by late summer and will contain one or more white maggots (Photo 3). Sometimes the needle may bend at the site of the gall. Initially, the damaged area is pale-yellow, but as the season progresses, will darken and eventually turn brown. These needles may shed or remain on the tree.
Damage from Cooley spruce gall adelgid may resemble damage caused by Douglas fir needle midge. Adelgid-damaged needles frequently have chlorotic areas (Photo 4) where the insects have fed. To distinguish between midge damage and adelgids damage, look for the small cottony balls and cast skins of the adelgids at the yellow spot or needle bend. Again, with Douglas fir needle midge the galled needle will appear swollen if viewed from the side.
It is too late to control this pest this year. If you are having a problem with Douglas fir needle midge, next spring use yellow sticky traps or box traps to determine when adult midges are beginning to emerge. Set out at least 10 traps per field by mid- April. Check the traps every other day until your area accumulates roughly 160-180 growing degree days (GDD) base 50 degrees Fahrenheit, then monitor the traps daily. Time insecticide applications when adult begin to emerge in the spring. Adult midges must encounter treated foliage before they lay eggs. Insecticide treatments should be timed when the adults emerge around 200-225 GDD50.