Commentary on the gall mite model

It has been said that “all models are wrong, but some models are useful.”

The messy reality of biology is that it’s hard to capture in a mathematical model, and sometimes we get anomalies that buck the trends.  This is what we’re seeing for the gall mite emergence model in 2024.

Mite emergence matters because when they’re inside a bud they are safe; we can only control them with sprays like sulphur once they have emerged.

Geoff has been running the model, developed in the UK, for more than 15 years, since 2008.  The model says that gall mites start to emerge in spring once they have felt a certain amount of heat, specifically 122 growing degree days (base 4), counting from 1^st August.  By 200 GDD, 5% of the mites have emerged.  By 315 GDD, 50% of the mites have emerged.

Of course, as sceptical scientists we always check the buds under the microscope to see if the biology is behaving according to what the model predicted.

In 2011, based on observations, Geoff moved the growing degree day start date to 15^th August, because that fit better with the field observations.  Through 2023 that has been a reasonable fit.

This spring is the anomaly, as indicated by the first emergence date being very early.  We first looked at the galls on 26^th August (55 GDD from 15^th August), and mites were very much emerging, probably already at that 5% emergence.

We might speculate on the influence of winter chill.  Perhaps the mites also judge the progression towards spring by how much cold they have felt—they have the same need as plants to not emerge too soon and get frosted.

Previous to 2024, the 2017 winter had the most chill accumulation. Winter chill in 2017 was actually very similar to 2024.

And, when we look back at the 2017 data, what do you know!?  Going back to the gall mite communications from 2017, first emergence was also early. We first checked 28^th August, and emergence was already started, just like in 2024. At that date the GDD count from 15^th August was only at 40, so again, much earlier than the model predicted.

It looks like a cold winter leads to an early gall mite start. Especially a cold winter followed by a warm period in August, which is what we have had.

A question to ponder is how long the mites continue to emerge from the galls.  Past observations have led us to believe that mites could still be dribbling out of galls late in the summer, even into the autumn.  Somehow they manage to be in the terminal buds which really aren’t set until February when the shoots stop elongating.  Normally this is not very important, especially if the number of galls in the block is low, but for curiosity’s sake we plan to keep an eye on our gall mite plot this season.

Any grower who wishes to mail or drop of galled buds for us to check emergence is most welcome.

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