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 

Codling Moth Control for the Small-Scale Fruit Grower

Of all of the fruit I have grown in Idaho, my favorite is the apple - not just any apple, a 'Criterion' apple picked fresh off the tree. Biting into one of these juicy, bursting with flavor apples is an orchardist’s delight.

Ironically, for any orchardist, commercial or small- scale operations in which this blog is dedicated to comes with a cost. And for many of those seasoned apple growers, significant cost of time, energy and resource is essential for the perfect marketable fruit. Ideal size (Goldilocks proportions), right color, superb taste and no blemishes. Now if we were living in the Garden of Eden, no problem, but we do not, so comes the consequences of a need for disease management, climate control, soil balance, branch and trunk care, and the dreaded pest control. 

For this article, I am only going to address pest control: one pest in particular that stands head and shoulders of all the other apple pests, the Codling Moth. This moth is a member of the Lepidopteran family Tortricidae. Interestingly, codling means “an unripe, half-grown apple”.  Now add a small worm to this scenario and you have the perfect storm of total destruction of a normal apple. And, just as one perfect apple can bring delight to the dedicated orchardist, it can also bring a grown man to tears when he discovers as he bites into that prospective delicious apple that a black headed cream-colored worm has riddled his sweet meal with several blackened distasteful tunnels…worthless to the core! 
For the untrained eye an outward examination of the suspect apple may puzzle the human consumer for even though many apples may bare the evidence of the unwanted worm that stings and eventually penetrates the skin of the apple many worms find it more convenient to enter through the “calyx end” of an apple. 
Because these worms have such a voracious appetite for apples, the small-scale fruit grower may become discouraged and find it not worth the effort to put up with these pests and so depend on the fruit stand or grocer to be their provider of these sweet worm-less treats. But for the fruit grower who holds to the tenants of religiosity, tenacity and creativity… and who has a plan in place (while these moths are in hibernation) there can be hope and there can be success! One educational source that has done extensive research on the control of the codling moth is Washington State University Research and Extension Services. 
  
 Several practical approaches starting with a better understanding of the life cycle of the codling moth can be helpful in winning the battle of the worm. WSU’s website has numerous charts ranging from life history of the moth to the pesticide spray schedule when the moth is most active throughout the year.  I have in the past used my own method of monitoring moth activity through the use of a molasses-based milk jug traps. I found at a local organic nursery  who recommends this method along with some other valuable advice using a more “organic approach”.
                                  
This is a key component of worm control in your apples. The idea is to know exactly or at least within a couple of days when the moths are on the move and are ready to lay their eggs on strategic apple tree locations to raise their offspring. 
What I like about this website is they take on more of a biological approach in controlling this moth by using pheromone traps and parasitoids I have had little success with these methods but have found a product named Spinosad to be fairly effective if used appropriately with the spray schedule for this area. I have just recently signed up online for the Pacific Northwest Pest Alert Bulletin. I highly recommend this site if you are really committed to being timely with your spray routine which could be several times throughout the life season of the moth.  As mentioned, Spinosad is my pesticide of choice and has some interesting history. In 1982, a distinctive soil dwelling bacterium called Spinosad was discovered on this Caribbean island. Spinosad has become a unique pesticide used in organic gardening.  This year I will be adding another product that meets OMRI standards, an all seasons horticulture oil with Spinosad. This product is considered an excellent fungicide, miticide and pesticide that can be used within 24 hours of harvest. 
            
The above links have some excellent additional advice on how best to control this nasty moth’s offspring appetite so we gardeners and orchardists can better reap the benefits of our fruit tree husbandry. 

Here is wishing you a juicy worm-free apple of your choice this summer. (even though Criterions are the best!)