Codling Moth Ecology as Important as Biology

We've noticed in recent years that codling moth flight curves don't always necessarily align with model predictions. Generally, when flight curves are "off", they parallel what the models predict, but occur later.

It's all too easy to blame the models as being faulty somehow, and it's a common refrain to hear that the "model is off". In reality, codling moth phenological development is absolutely correlated to temperatures - that's been proven repeatedly by entomologists who are a lot smarter than me.

So why, then, do we observe flight curves much later than model predictions? Is mating disruption resulting in delayed trap catch? Almost certainly in some cases. Are we selecting out the early part of the generation with our spring control programs not only for codling moth, but also for other pests such as obliquebanded leafroller and western flower thrips? More than likely - in many cases we hammer the early spring pest populations and are taking out those early first emergers of the generation, pushing the observed flight curves later.

Aside from these effects, though, is there maybe something else going on? Codling moth biology is about as absolute as you can get in the world of entomology - development is correlated to temperature. Codling moth behavior, on the other hand, is tied not only to temperature, but is also influenced by other factors like wind and rain. As long as temperatures are sufficient, codling moth phenological development will continue at the temperature-mediated rate, regardless of any other external factors (except perhaps effects from insect growth regulators or sub-lethal doses of larvicides that could delay development).

Codling moth behavior, though, such as flight and mating, is influenced by more than just temperature. So, you could have fully developed and mature moths ready to mate, but environmental factors can delay that behavior. In other words, biology and ecology are two different things, and both will influence flight, mating, and observed trap catch.

One of the other possibilities that's been debated is the difference between in-orchard and extra-orchard temperatures. Most of our weather stations sit outside of our orchards and are not affected by things like canopy, cooling effects of grass drive middles, irrigation, overhead cooling, shade cloth, etc. Could that be a reason for observed differences between trap catch, flight, and model predictions?

We looked at that this year by installing two ATMOS weather stations at a commercial orchard near Othello, Washington. One station was sited according to standard protocols established by WSU-AWN for Tier 3 stations. The second was only about 50 yards away from the first station, but was located in a tree row under the canopy of a mature, full-size Granny Smith block that was overhead cooled in the heat of the summer.

We ran development models using extra-orchard and in-orchard temperatures for codling moth, obliquebanded leafroller, western cherry fruit fly, and San Jose Scale, along with some horticultural and disease models. I was surprised to see that there was really very little difference between model predictions based on extra-orchard vs. in-orchard temperatures. Over the course of the entire season, there was never more than about 2 day's difference. Now, this was an abnormally HOT summer, and we had some nights where the low temperatures never even got down to the upper developmental threshold for codling moth or OBLR. So, it's just one year of data, and an unusual year at that, so I'm not reading too much into this until we see if the results are repeatable over more than a single season.

However, one of the things this allowed us to do was to go back and look at mating opportunities of the overwintering generation early in the season. When we looked at temperatures at dusk, there were 8 evenings in the month of May with temperatures warm enough for successful flight, female location, and mating to occur. I don't know what is "normal", but based on temperature alone, that seemed to provide ample opportunity.

However, looking at other factors that can affect codling moth behavior, those opportunities were reduced by 50%. There was no rain in May, so there that was not a factor in flight. However, when we looked at wind speed during those dusk hours, there was a huge impact on flight. With wind factored in, there were really only 4 opportunities for successful mating to occur. Those happened to be on 4 consecutive evenings the 2nd week of May (there was one additional opportunity on one evening the end of May, but by then mating would have been delayed for so long that success rates were likely very low - similar to the effects of mating disruption).

The reduced mating opportunities and their timing would show as a delayed flight curve, leading to the conclusion that "the models are off", when they really aren't.

We'll continue to look at these things for codling moth as well as other pests, but the early take-home message is when looking at differences between actual flight curves and model predictions, don't focus so strongly on codling moth biology that we forget about codling moth ecology.