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PEST ALERT: Douglas-fir Needle Midge

 

Rayanne D. Lehman, Entomologist
PA Department of Agriculture

Douglas-fir needle midge (Contarinia pseudotsuga) was recently confirmed from Pennsylvania after detection in 2002 at two locations in Lehigh County and one in Northumberland County. Surveys in 2003 have added Berks, Bucks, Dauphin, Montgomery, Schuylkill and York counties to the known distribution of this midge in PA. Heaviest infestations appear to be in Berks, Bucks, and Montgomery counties. Please familiarize yourself with this pest and how to diagnosis midge in Douglas fir plantings.

Douglas-fir needle midge overwinters as larvae in soil under infested trees. In early spring, larvae pupate and adult midges (Figure 1) emerge as buds are expanding. In Oregon and Washington, where this fly has been a pest for many years, emergence may start as early as the beginning of April and continue for 4-5 weeks.

Female midges have long ovipositors with which they place several orange eggs in expanding buds and on elongating needles. Larvae hatching from the eggs chew into the needle, causing elongating needles to form a gall around the larvae. One or more white maggots (Figure 2) can be found inside affected needles during the summer. At the site of the gall, the needle is frequently bent (Figure 3). The damaged area is initially pale in color (Figure 4) but as the season progresses, will darken and eventually turn brown. In late summer, larvae emerge from the undersides of the needle and drop to the soil. The emergence holes are irregular in shape (Figure 5) and may give the appearance of a slit or rupture. Secondary fungus can infect the needle and gall, further complicating identification of the pest (Figure 6).

Rhabdocline needlecast symptoms in late winter and early spring may resemble damage caused by Douglas-fir needle midge during the previous year. However, the margin between the green, healthy needle tissue and red-brown rhabdocline infected tissue (Figure 7) is more distinct than the margins of the gall. Additionally galled areas may have a marginal dark band (Figure 3). The gall appears swollen if the needle is viewed from the side (Figure 8).

Damage to Douglas-fir needles from Cooley adelgid feeding may also superficially resemble Douglas-fir needle midge damage. In summer, adelgid damaged needles frequently have chlorotic areas (Figure 9) where the insects have fed. Later in the year, the chlorosis may not be as evident but the needles may be distorted and bent (Figure 10). To distinguish between midge damage and adelgids damage, look for the cast skins of the adelgids at the needle bend. Again, the galled needle will appear swollen if viewed from the side. In late winter and early spring, these galls will also have the emergence hole on the under side of the needle.

We are interested in determining the distribution of this pest in Pennsylvania. If you think you have the midge, please contact Rayanne Lehman at rdlehman@state.pa.us. Samples to confirm the midge can be mailed to her at Pennsylvania Department of Agriculture, Bureau of Plant Industry, 2301 N. Cameron Street, Harrisburg PA 17110.