People living near cherry orchards should expect spraying to begin as soon as May 25, in the Mosier area, as orchardists combat the first emergency of the Western cherry fruit fly.

Dr. Ashley Thompson of Hood River-Wasco OSU Extension Service issued a letter this week that states, in part, “according to our cherry fruit fly model, first emergence of Western cherry fruit fly (WCFF) in The Dalles will occur on May 25.”

Thompson reported, “First emergence in The Dalles area, based on work conducted in (Hood River entomologist) Dr. Helmut Riedl’s lab, is assumed to occur at 990 degree-days. (A degree day is the accumulation of heat units based on the life cycle of an insect and the temperature. A certain number of degree days need to be accumulated for pests to reach maturity.)

First emergence in Hood River is assumed to occur at 860 degree-days.”

The discrepancy is due to the fact that the cherry fruit fly emerges earlier where there is more moisture. Based upon this scenario, Dr. Riedl suggested that first emergence for Mosier should be intermediate between The Dalles and Hood River and set the first emergence for Mosier at 925 degree-days.

Thompson’s letter stated, “Cherry fruit flies lay eggs about six to seven days after emergence early in the season; that interval drops to five to six days during 80-plus degree temperatures. First WCFF covers need to be on in early locations within six-seven days of emergence due to the warm temperatures predicted over the next few days.

“If you are using the GF-120 bait, it needs to be applied within a day or two after first emergence, as flies need to find and eat it before they start laying eggs. Dr. Vaughn Walton believes that it is safe to apply control measures that primarily control just western cherry fruit fly (WCFF) early.

“However, when the cherries start to show pink, control measures should shift to chemistries that effectively control both WCFF and SWD.  It is important to remember that SWD can build up very fast once temperatures begin to warm.”

The 2019 Pest management Guide for Tree Fruits in the Mid-Columbia lists pesticides effective for WCFF and SWD. It is available at:

Thompson advises cherry growers, “do not rely on GF-120 for control of SWD (Spotted Wing Drosophila).”


Below are estimated first emergence at several sites in The Dalles area based on the CFF model posted on the OSU IPPC website,

  • Anderson and Steele Roads, May 30
  • Bailey Orchard and Skyline Road, June 2
  • Boyd Road, June 7
  • Cooper Barn, Dry Hollow Rd., May 30
  • Dallesport, May 21
  • Dahle Shed and Cherry Heights Road, May 25
  • Dufur, June 10

Emergence of CFF begins at 990 degree-days in The Dalles (including all of Wasco County and Dallesport) and 860 degree-days in Hood River County. Thompson estimated first emergence for Mosier at 925 degree-days.

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