Utah State University Extension
Insecticides that are applied for a perennial insect pest based on a calendar date often result in poor insect control and a waste of resources. Insect activity varies from year to year depending on weather. For example, in Logan, Utah, eggs of the apple pest codling moth began hatching on May 15 in 2005, May 5 in 2006, and April 30 in 2007.
Murray, Marion, "Using Degree Days to Time Treatments for Insect Pests" (2008). All Current Publications. Paper 978.