Gall Mite Biology

Since the gall mite emergence this spring has been abnormal, I’ve been curious to learn a bit more about how these creatures make their living.  Possibly I’m the only one who is this curious, but at least humour me by reading a little summary.

A few basic biology facts about gall mites (Cecidophyopsis ribis), with credit to many UK and Polish researchers who have studied this pest over the past century.  Literally.  There are many references back in the 1950s and 60s, because as a vector of reversion virus, gall mite has been highly consequential for growers for a long time.  We’re lucky we don’t see the reversion virus in NZ.

The timing of when the mites start to come out of their overwintering galls in search of new buds varies by cultivar.  The timing doesn’t correlate well to a single plant growth stage, meaning it can start at flowering in some cultivars and before flowering in others.

A single gall can have almost unfathomable numbers of mites, like 35,000.  Roughly 60% of them emerge in spring, while the others get stuck in the dying bud and never make it out.  They mainly emerge over a 4 week period, which for us in NZ usually peaks in September.  A few mites can continue to dribble out of the buds until early December, especially from the “cabbage buds” which loosen and don’t dry out.

Once out of the gall, only 1% of them make it back into a newly formed bud before they desiccate and die.  Those that do tuck themselves into new buds will stay there feeding for their whole life.  Their saliva causes the plant cells to grow nutrient-rich plant tissue just for them, inducing the gall growth.

In the bud they reproduce by eggs and have multiple generations.  They have a system a little like honeybees in that unfertilized eggs produce males, and fertilized eggs produce females.  By the time spring rolls around most of the gall inhabitants are females.  One female reaching a developing bud is all it takes to make a colony and, later, a gall.

If you’re not as fascinated by microscopic mite biology as I am, then this bit should at least be of interest.  Sulphur doesn’t get inside the bud, so mites only encounter the toxin when they wander out.  This year we started to wonder how long the sulphur sprays we apply to the exterior of the bud last and kill emerging individuals.

We put some treated buds on a petri dish over the weekend to see what would happen to emerging mites.  We found that on sulphur-treated blocks emerging mites had died as they reached the exterior of the gall, but on our untreated galls they lived and wandered out over the dish looking for a new home.

Our observation was on 30 Sept, from galls that were treated 23^rd of September, so a week post spray.   By 11 days post spray, a few mites that were still crawling out of the galls were surviving. From that observations, we might infer that the sulphur residue is working for more than a week but less than two weeks.

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