Great Barrier Island beekeeper Colin McLean details his growing concerns at chalkbrood in his beehives, and what he is doing to combat the fungal disease.
I have heard a number of beekeepers commenting on a more infectious strain of Chalkbrood over the past six years, or perhaps even more. I started seeing it through summer in the odd hive, something I had never seen before. I made the comment to an AFB inspector who said he would see chalkbrood in every hive!
Chalkbrood used to be a minor problem, appearing in a cold, wet spring and clearing up as the bees got stronger and the weather warmer. This strain is different and can be damaging. It can weaken a hive to the point of producing no honey. So, what do you do?
Adding brood from a strong hive can help them get on top of it, otherwise requeening with a resistant queen is your only option.
Disease resistance is usually focused on hygienic behaviour. There are two recessive genes, which means it needs to be with the queen and the drones she mated with to be effective. One gene stimulates the bees to uncap a diseased larvae and the other to remove it. With chalkbrood most die at a larval stage, before it is capped, and you usually see chalkbrood mummies in the brood nest. Chalkbrood spores seem to be abundant in beehives, just waiting for the right conditions to infect brood.
Beekeepers use different methods to select for hygienic behaviour and it relates to how well the bees uncap and remove dead larvae. Some use the pin method or liquid nitrogen and see how well the bees respond, some rely on observation. I knew a queen breeder who used to see how clean the bottom board was, which goes to show it doesn't matter what method you use as long as it works! I sometimes see subclinical infections where the bees will remove larvae at a very early stage before symptoms develop, which leaves a slightly patchy pattern in the broodnest.
The other area of disease resistance which never seems to be discussed is genetic resistance. Some bees are more resistant to an infection and beekeepers probably select for genetic resistance as part of selecting a breeder queen, without doing it specifically. I don't know how you select for genetic resistance except for a lot of observation and, ultimately, how the daughters perform.
Perhaps we have queen breeding all wrong and should only breed from hives with solid brood patterns – it sounds simple enough!
So, getting back to this infectious strain we are having to deal with, two years ago I had a five frame nuc showing a lot of chalkbrood in spring. I killed the queen and put in a cell. Within three weeks, much to my surprise, there were only three chalkbrood mummies left in the nuc. Everything else had been cleaned out. This nuc has a 16mm entrance hole 50mm up from the bottom, but it didn't stop the bees removing all the chalkbrood. I didn't see any more chalkbrood infection, so in this case the bees showed good hygienic behaviour, but the queen was susceptible to chalkbrood. The other 20 nucs in the yard were normal.
Last spring I had two yards with ongoing chalkbrood issues, and also high varroa. I'd seen several hives break down with chalkbrood in autumn, relating to high varroa numbers. I'd never seen this before. The hives wintered okay, but with ongoing chalkbrood in half the hives. Did I use a susceptible breeder, or did she mate with some suspect drones? Interestingly enough there were a few hives with no chalkbrood at all.
I decided to requeen the hives with caged queens from overwintered nucs. I also had the odd hive in several yards showing chalkbrood right through the broodnest. This is really unusual. These hives were moderately strong and conditions in mid-spring were good. I rechecked the hives every two weeks to see what was happening and there was variability between hives, but, for most, the levels of chalkbrood infection dropped dramatically, and this was before any new bees from the new queen started hatching. A few hives are still showing chalkbrood and I have added brood to some. It seems to take time to get on top of it.
Not only is selecting good breeder queens important, but also having good drones for her to mate with. These days I spend a lot of time just trying to control varroa and these challenges certainly test your ability and commitment as a beekeeper.
Colin McLean is a beekeeper of 40 years’ experience across Hawke’s Bay and now Great Barrier Island where he owns and manages 450 hives.
*Additional reporting on chalkbrood from 2024 includes Chalkbrood Explained by Dr Mark Goodwin and also this contribution from Frank Lindsay.
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