Temperature is a key driver of insect development. Degree days (DD) is a measure of temperature over time and it is used to predict the timing of life stages of certain insect pests.
A degree day model counts the total time that temperature is above the minimum required for the pest to develop (lower developmental threshold).
There are established DD models for codling moth (CM), light brown apple moth (LBAM) and oriental fruit moth (OFM). These are best at predicting the first generation of activity and typically become less reliable with subsequent generations as the season progresses.
For this reason, some insecticide labels for these key pests include DD recommendations for timing of the first sprays.
Knowing how to calculate DD will help the grower to time their first sprays effectively.
What do you need?
To use DD for your first spray timing, you will need:
• pheromone traps to determine biofix
• thermometer (max-min or weather station)
• calculator or spreadsheet
What is biofix?
Biofix is the date of the first sustained flight of adult moths recorded in pheromone traps.
It is used as a starting point for the accumulation of degree days and to guide the timing of the first spray.
How to determine the biofix
Deploy traps at a density of about one per hectare, ensuring coverage of the warmest part of the orchard and any known hotspots where damage occurred in the previous season(s).
Establish traps at least a week before bloom for codling moth and oriental fruit moth, but budbreak for light brown apple moth.
The aim is to record at least a couple of weeks with zero moths in the traps before the first flights begin. This will increase your confidence in determining the biofix when moths do emerge from their overwintering pupation sites and fly into the canopy.
Checking traps daily until the first sustained moth flight is recorded will increase the accuracy of the biofix date that you set.
Calculating and accumulating DD from biofix
A simple formula for calculating DD using daily maximum and minimum temperatures and the lower developmental threshold for your pest is shown in Equation 1.
The lower developmental thresholds for CM, LBAM and OFM are listed in Table 1.
DDs are calculated daily from biofix and added together to give Cumulative Degree Days (CDD). If using a max-min thermometer, this is best housed in a Stevenson screen (Figure 1) to ensure accurate measurement of ambient temperature, which can be recorded in a spreadsheet or on paper.
Some weather stations with inbuilt models will track DD accumulation and predict first spray timing.
Table 2 provides an example of how to accumulate degree days in a spreadsheet format.
Label recommendations for spray timing
Most insecticides for CM, LBAM and OFM target the start of egg hatch (i.e. larval stages).
The active ingredient fenoxycarb (e.g. Insegar®) is a notable exception that only controls newly laid eggs.
Codling moth and oriental fruit moth egg hatch occur on average at about 110 CDD from biofix, whilst LBAM egg hatch occurs around 140 DD.
Refer to product labels for recommendations on timing applications based on cumulative degree days.
See this article, tables and equation in Tree Fruit Nov 2020
Download the Plant Protection Guide 2020-21
