Gall Mite Emergence Model

Here we are, hard on the heels of the chill model, with another MODEL. It keeps the mathematicians happy.

Gall mite is another pest that develops in a somewhat predictable manner based on spring temperatures. The goal with spring sulphur applications is to have the sulphur coating the outside of the galls by the time a few mites have started to emerge, so that over the next few weeks when the rest of their compatriots stumble out from their overwintering bud into the light of day, they track through the sulphur and die. We are aiming to prevent those microscopic pests reaching the haven of a new bud, and starting their pestiferous cycle anew.

The model we use was developed in England, and we have adapted it to use in NZ. Traditionally we have started to count spring warmth on 15th August, and we find the mites are just starting to emerge at about 120 "growing degree days (GDD)" after August 15th. A GDD is a way to show, mathematically, that bugs and plants develop faster when it's warmer. For gall mite, we figure that over 4 degrees Celcius, they're developing. The model counts the number of hours above 4 degrees, and counts faster when temperatures are higher.

Typically, we're expecting the gall mite emergence to start between 5-15 September in Canterbury.  

Our experience of cold winters (2024, 2017) is that the gall mites come out earlier than the model predicts.  This year, given that it's been cold, I checked our gall mite plot early.

The mites are just barely poking their noses out of the galls in Lincoln, so I think they might be a little early to emerge, but not remarkably so.  

Time to get ready for the sulphur spray.  

Given that we don't really have much green tissue yet (phytotoxicity not a big concern), I would put a wetter with the sulphur to help the product stick to the outside of the galls even through rain events and get under the plant hairs.  Not oil (and not a penetrant!), but a non-ionic wetter/spreader.  

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