Douglas-Fir Needle Midge

Douglas-fir needle midge overwinters as a mature larva in soil beneath an infested tree. In March–April, it pupates in the soil. Damage includes needle discoloration and injury.
Douglas-Fir Needle Midge - Articles
Douglas-Fir Needle Midge

Kinking caused by Douglas-fir needle midge larvae. Courtesy of Tracey Olson, PDA

Contarinia pseudotsugae Condrashoff

Hosts

  • Douglas-fir

Damage Potential

  • Moderate–high

Symptoms and Signs

Summer Through Fall

  • Swollen areas on current year’s needles; needle may bend at gall
  • Pale or yellow spot(s) on both sides of needle; needle may turn purple or brown in fall
  • Premature needle loss

Causes of Similar Symptoms

  • Rhabdocline needle cast
  • Cooley adelgid

Identification

The Douglas-fir needle midge is a tiny, orange-colored fly, approximately 1⁄8 inch (5 mm) long. During the emergence period in spring, look for minute, orange flies resting on the tips of needles. Females can be distinguished by a very long ovipositor with which they probe between bud scales and into partially opened buds. This elongate ovipositor enables the female to deposit the long, narrow, orange-colored eggs in protected areas. A hand lens is needed to view the eggs.

A white, legless maggot lacking a distinct head hatches from each egg and bores directly into the needle. In response to the maggot’s feeding, the needle produces excessive tissue, resulting in one or more swollen areas on the needle. There may be a minute, orange spot visible on the underside of the needle at the larval entrance site. When mature galls are carefully sliced open, a pale maggot may be found, generally under the entrance spot. At maturity, the maggot is about 1⁄8 inch (5 mm) long. After the maggot has dropped out of the needle to pupate in the ground, an irregular, three-sided exit hole can be seen in the epidermis on the underside of the affected needle. In fall and winter, damaged needles drop from the tree or break at the gall location, making winter scouting more difficult.

Biology and Life Cycle

Douglas-fir needle midge overwinters as a mature larva in soil beneath an infested tree. In March–April, it pupates in the soil and the adult midge emerges (Figure 1). The emergence period is generally from mid- April through early May and depends on the weather conditions at each site. Rain and cool temperatures delay emergence. Mating and oviposition occur immediately after emergence and adults die shortly thereafter (Figure 2). The peak emergence period lasts about 7–10 days.

Figure 1. Female Douglas-fir needle midge. Courtesy of Sandy Gardosik, PDA

Figure 2. Adult midge (circled) resting on a Douglas-fir bud. Courtesy of Sandy Gardosik, PDA

Calendar of Activities

Eggs (Figure 3) hatch within a few days and larvae (Figure 4) bore directly into the young needles to feed throughout the summer. As the larvae feed and grow, galls form in the needles and cause them to kink (Figure 5). By late summer, the galls will begin to turn yellow and then brown (Figure 6). In late fall, the full-grown larvae (Figure 7) exit the needles and drop to the ground to overwinter, leaving a distinct triangular exit hole in the underside of the needle (Figure 8). A single generation occurs each year.

Figure 3. Eggs found tucked in the needle bud before buds are fully broken. Courtesy of Rayanne D. Lehman, PDA

Figure 4. Young larva found inside a needle. Courtesy of Rayanne D. Lehman, PDA

Figure 5. Early evidence of galls; needles are beginning to kink (late July). Courtesy of Tracey Olson, PDA

Figure 6. Advanced gall symptoms (November). Courtesy of Tracey Olson, PDA

Figure 7. Mature larva still in the needle (December). Courtesy of Tracey Olson, PDA

Figure 8. Needle showing larva exit hole. Courtesy of Tracey Olson, PDA

Monitoring and Management Strategies

Plantation Establishment

  • Plant late breaking tree varieties to decrease midge damage.
  • Remove overgrown Douglas-firs from the perimeter of the block. Damage is often more severe on older trees.

Preseason

  • Place emergence traps under the north side of previously infested trees by April 1 or before the daily temperature reaches 60°F for several days (Figure 9). See Appendix E: Insect Trap Use and Construction for information on trap construction.
  • Place at least three traps per field and check every day until the first midge appears. Record the number of midges in the trap jar before emptying and replacing the jar. Continue monitoring until no midges are present for several days.

Figure 9. Emergence trap placed under a previously infected tree. Courtesy of Sandy Gardosik, PDA

Growing Season

  • Growing degree days: The adult midges emerge from the soil beneath trees at 200–400 GDDs (based on midge population monitoring in Pennsylvania).
  • Threshold level: No established threshold level exists for this pest.
  • Scout in August for infestation. Mark trees that exhibit symptoms to aid with trap placement and distribution of control chemical next spring.
  • At the end of the season, evaluate results and update records.

Control Options

Biological

  • Encourage and protect natural predators such as tiny parasitic chalcid wasps. Ill-timed chemical application is not effective and may kill natural enemies.

Mechanical

  • Remove heavily infested trees in early fall before larvae exit the needles.

Biorational

  • No recommendations are available at this time.

Chemical

  • Base application on collection of adults in emergence trap. Chemicals will not be effective against eggs, only adults. First application should be made within a day of collection of the first midge.
  • Consider a second application in 2 weeks if adults are continuing to emerge.

Next Crop/Prevention

  • Purchase and plant pest-free nursery stock from a reputable company.