How To Predict the Date of An Insect Outbreak

Have you ever heard someone say, “After a mild winter, insects will hatch early?” Is this a gardening myth? Can scientists forecast when particular insect will emerge? See the answers below!

Idaho State Department of Agriculture Entomologist Paul Castrovilla points to Phenology. The study of relationships between weather and biological activities, it is also known as the Degree-days Model (DDM).

When Insects Make Their  Appearance: Insects are cold-blooded. Their development is linked directly to external temperatures. Each requires a defined amount of heat accumulation to reach life stages like egg hatch or adult flight.

Degree-days (DD) mean a measurement of heat units calculated from an average daily high and low temperatures.  DD tell us how many heat units useful for a specific insect’s development have accumulated. In the lab, researchers determine a base and an upper threshold temperature for each species. The insect will eat and mature during this range. According to Dr. Castrovilla, base thresholds are typically set from 40 to 50 degrees F.

Example of threshold list at http://uspest.org/wea/weafaq.html

You can easily calculate Degree-days using the following formula:

[(daily maximum temperature + daily minimum temperature)/2] – baseline temperature = # DD for that day.

Accurate calculation of degree-hours over many days requires a record of hourly temperatures and is complex for a number of reasons. 

The Integrated Plant Protection Center Phrenology Site Home Page

Make Your Own Science-Based Forecast:The public is welcome to use a free web site to forecast an insect or disease in a matter of seconds.  Hosted by the University of Oregon, The Integrated Plant Protection Center combines US weather and climate data (29,000+ locations) with 110 plant pest and disease models. 

Example of DDM results for codling moth at Nampa Zip Code

The above image is a table forecasting egg hatch dates every two weeks from May 17 to July 23. Dr. Castrovilla cautions that the DD Model is not 100% precise, but can predict a date within three to five days.

Accumulated DD help in timing crop scouting events like placing pheromone-monitoring traps. That helps predict the best timing of pest management. The DDM is a useful tool in predicting when a destructive pest is likely to occur. It is also helping to identify when beneficial insects are at dangerously low levels.

To answer our original questions, it is a fact bugs will develop earlier if spring temperatures warm up earlier than normal. Few insects develop in Idaho when the temperature is below 41 degrees F. Used in combination with scouting and pheromone traps, the DDM helps determine treatment timing for fields and back yard gardens alike. That can protect crops, save money, and minimize negative effects on the environment. To be notified, Sign up for PNW Pest Alerts  

Other related resources:

"Degree day Models." How to use various models to calculate Degree-days. Washington State University. Link: http://jenny.tfrec.wsu.edu/opm/displaySpecies.php?pn=-50 

"IPM Methods:  Determining Treatment Timing Using Degree Days and Insect Phenology Models." The focus is on orchards. University of Utah. Link: http://www.intermountainfruit.org/ipm-methods/treatment-timing

"Research and IPM Models: Insects, Mites, Diseases, Plants, and Beneficials." A long list of insect thresholds. University of California. Link:http://ipm.ucanr.edu/MODELS/index.html