This season, growers faced an unexpected challenge. An unusually warm May set the stage for heavy codling moth pressure in 2023 compared to previous years.
Let's explore what occurred when in Washington, and discuss strategies to approach this type of situation in the future.
Impact of Weather on Codling Moth Biology
First, we need to discuss a bit of codling moth biology.
Codling moth, like all insects, don't generate their own body heat. This is important because it means that their development rate (e.g. the amount of time it takes for a codling moth egg to become an adult), and the ability of adult codling moth to fly, mate, and lay eggs is governed by local weather conditions.
If it's too cold the codling moth can’t develop, fly, and most importantly mate and lay eggs. Keep this in mind as we explore the conditions for codling moth pressure in 2023
Comparing 2023 Codling Moth Pressure with Recent Years
Codling moth development rates can be measured and tracked using degree days.
The monthly codling moth degree day accumulations from the last 8 years are shown in Figure 1. This graph shows the mean cumulative monthly degree days from 6 farms across central Washington, those approximate locations are shown on the map below.
Figure 1. Boxplot of cumulative monthly degree days from 6 farms across Washington State, 2018 to 2023.
As you can see, April 2023 was cool compared to the average but nothing too unusual when considering the previous 8 years.
However, May 2023 was definitely an outlier. The average degree day accumulation in May 2023 was almost 500 degree days Fahrenheit (DDF), more than double what we saw in 2022 (ie. ~220DDF). The last time we saw a May like this was in 2018.
The Impact on Codling Moth Life Cycle
It's interesting to note that the degree day accumulations in May 2023 were more typical of a cool June.
Let's put this in the context of the codling moth life cycle. It takes approximately 150DDF for codling moth eggs to hatch. Therefore codling moth eggs laid in May 2022 would have taken 20 days to hatch, whereas in 2023 it was around 9 days.
In a practical sense, this means codling moth has to start a lot earlier in 2023 than in 2022, and this makes the timing of insecticide-based management even more important.
Orchard Temperatures and Codling Moth Behavior
Building on that, let's take a look at orchard temperatures during May, and in particular, let’s compare 2023 and 2022.
From the academic literature, we know that codling moths are unlikely to fly and engage in mating behaviors at air temperatures less than 59°F (15°C).
Figure 2 shows a temperature series from a Semios weather station for the month of May in both 2022, and 2023. We have colored the temperature seasons according to the time of day. The period of time during dusk and dawn when codling moths are behaviorally active is blue, and we have marked off the 59°F (15°C).
Figure 2. Temperatures for the month of May from 2022, and 2023, from a single Semios in-canopy weather station located in a Washington pome fruit orchard. Blue is the period of time between dusk and dawn, when codling moths are most active. The black horizontal line is 59F (15C), the temperature below which codling moths are unlikely to fly and mate.
If you look closely at these temperature series you will clearly see many days and multiple days in a row in May 2023 when the temperature at dusk exceeds the 59°F (15°C) threshold. Contrast that with 2022, where the temperature at dusk exceeds 59°F (15°C) on a few specific days.
This data suggests that the weather in May 2023 was comparatively more favorable than the weather in May 2022 for codling moth behavior. Keep in mind that any individual codling moth only lives for a matter of days, therefore in 2023 a moth emerging in May, could have experienced multiple consecutive days of weather conducive to their reproductive behavior.
Comparative Analysis Across Washington
Figure 3 below illustrates the cumulative night-time hours greater than or equal to 59°F (15°C) for the months of May and June from six farms across Washington. This graph shows a reasonable estimate of the maximum amount of time codling moths could have performed mate-seeking behaviors in these orchards.
Unfortunately, at this time we have no way of saying how many hours are needed for a codling moth to cause damage, but it's clear from this comparison that May 2022 and May 2023 played out very differently.
Figure 3. Cumulative night-time hours greater than or equal to 59°F (15°C), for the months of May and June from six farms across Washington. Cumulative hours during 2023 is indicated by the green lines, and 2022 is indicated by the blue lines.
Final Thoughts
May witnessed a surge in codling moth pest pressure in 2023 with:
- Higher than average degree day accumulations
- Long periods of time (consecutive days, and hours total) with temperatures conducive to codling moth mating behavior
In brief, the weather conditions in May 2023 were optimal for codling moth reproduction and development compared to recent years.
Furthermore, early cohorts of codling moths that emerged in May 2023 experienced temperature conditions more typical of June weather.
If your codling moth management was similar to 2022, there's a possibility that the larger flight in May could have had a late spray or a missed spray application. This can have compounding effects on the codling moth population for the entire season.
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