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Use degree days to effectively control fruit tree pests

Two insects tend to be the most prevalent pests in our area’s fruit trees: the codling moth (apples) and the western cherry fruit fly (cherries). Both create “wormy” fruit, and both, once the “worm” (larval stage) gets deep into the fruit, can be tough to control.

One way to monitor the presence of these two insects is to use a combination of pheromone sticky traps (use of a lure that mimics the female sex pheromone) and degree-day accumulation. Sticky traps can be found online, just make sure you get the right one for the insect you are trying to control.

Degree days are determined by use of the average temperature for a day (maximum temperature + minimum temperature/2) and subtracting it from the base temperature at which the insect does not develop.

For more information

Codling moth: https://bit.ly/3yvTOry.

Western cherry fruit fly: https://bit.ly/3PeiPxh.

Historic weather data: https://bit.ly/3Phwbss.

For the codling moth, a base temperature of 50 degrees Fahrenheit is used, and for the western cherry fruit fly, 41 degrees is used.

As an example for the codling moth, a day when the high temperature was 80 degrees and the low temperature was 40 degrees, then 10 degree days would accumulate [(80 + 40/2) - 50]. I recommend starting the accumulation process in April.

Codling moth

Typically, the timeline for codling moth starts at “biofix,” which is the first sustained capture of moths in pheromone traps. But if you do not have traps set up, then you can also go off the rough estimate of 206 degree days accumulated as your base number. My guess is that will be around the third week of May.

  • Egg hatch begins after biofix has been established (220-250 degree days).
  • Peak period of 1st generation egg hatch/critical control period (340-600 degree days).
  • First egg hatch, 2nd generation (1,000-1,100 degree days).
  • Peak period of 2nd generation egg hatch/critical control period (1,320-1,720 degree days).
  • End of 2nd generation egg hatch (2,100 degree days).

The control method you use – either organic or not – is going to be based off the stage the pest is in. Refer to the Colorado State University fact sheet listed in the inset box as to which insecticide, if any, you decide to use.

Western cherry fruit fly
  • Place traps on tree or in orchards (750-800 degree days).
  • First adult flies expected on traps. Treat 5-7 days or 190 degree days after first fly is caught if cherries have developed a salmon-colored blush; otherwise wait for fruit to turn color (900-950 degree days).
  • Typically, the first spray is applied – can be organic or not (1,060 degree days).

For more information about western cherry fruit fly control, visit the website listed in the inset box.

Get Growing, written by the La Plata County Extension Office’s Master Gardener Program, appears during the growing season. Darrin Parmenter is the director and horticulture agent of the La Plata County Extension Office. Reach him at darrin.parmenter@co.laplata.co.us or 382-6464.