With anecdotes of more chalkbrood presenting in beehives this season, we check in with leading bee scientist Dr Mark Goodwin to find out what causes chalkbrood infections, what beekeepers can do to prevent the disease and, should it strike, what can be done to help a colony recover.
New Zealand’s Journal of Agriculture published an article offering beekeepers advice on managing chalkbrood infections in 1957, however Goodwin says the fungal disease of honey bee brood first displayed widely in the 1990s.
“It was a major problem for a couple of years,” Goodwin says.
“When it first came into New Zealand for about two years almost every hive I looked in, anywhere in New Zealand, had quite bad chalkbrood. Three or five years after that, it was very rare to see it and now it is usually a symptom of something going wrong with the colony, rather than being there all the time.”
When varroa entered the country around the year 2000 the parasite killed off many of the feral bee populations, which in turn greatly assisted in reducing chalkbrood infections. The ‘British Black Bee’, which most feral colonies were descended from, were particularly susceptible to chalkbrood infection. Once they were largely removed from the breeding population, and queen breeding programs advanced genetics which were more inclined to resist chalkbrood, the disease’s prevalence reduced greatly.
“I think the combination of varroa, plus beekeepers being careful what they breed from, took us from almost every hive having chalkbrood, to it becoming reasonably rare,” Goodwin says.
That is the clinical signs of chalkbrood being rare though, as the former long-time Plant and Food Research scientist points out that most colonies probably have the fungus (Ascosphaera apis) present to some level.
“There is probably a whole lot more chalkbrood these days than what we know about, but most colonies have pretty good hygienic behavior and as soon as the larvae is looking sick, they will remove it.”
Therefore, preventing chalkbrood from significantly impacting hive health and productivity is a matter of maintaining colony nutrition, health and strength, and making sure climatic conditions are not supportive of the fungal disease. Therefore, sunny, well-ventilated and dry hive sites are better than shaded and damp.
“If a colony has good hygienic behavior, and lots of nurse bees, they can remove infected brood very quickly. It's only when something's going wrong that chalkbrood usually presents, such as conditions for foraging are really bad, or because of really cold temperatures and they are not able to keep the brood up to the temperatures they would like to,” Goodwin explains.
Chalkbrood displays as hard, shrunken chalk-like mummies in the brood and in and around the entrance to the hive. The mummies will be white to grey-black in colour. Infected hives also show a scattered brood pattern or appearance. The cell caps of dead larvae may contain small holes, appear slightly flattened or have been chewed away by the honey bees. Worker bees will usually uncap the cells of dead larvae, making mummies clearly visible, before sometimes removing the mummified larvae and depositing them on the hive floor or at the entrance to the hive. In heavily infected colonies the worker bees will not be able to uncap all of the affected cells. Mummies still in uncapped cells may fall from the comb when it is inspected.
Once infected, larva and fungus swells until it fills the cell it is contained in, then after a few days the white fungal growth hardens, adopting the cell’s hexagon shape, to form a white, chalk-like ‘mummy’ which gives the disease its name. The mummified larva will transition from a white to grey-black colour, which shows the completion of the fungal life cycle and the creation of new spores capable of infecting a new larval host. Therefore, it is only when the black or grey larval ‘mummies’ are present in a hive that the disease is capable of spreading.
Drift of bees between colonies, or transfer of infected equipment between hives aids the spread. Therefore, cleaning off baseboards of infected larvae and culling diseased frames is an important preventative.
“It's probably a good idea to cull combs that don’t look healthy in any case,” Goodwin points out.
“Also, anything you can do to boost the colony’s strength, such as adding shakes of healthy bees, will increase the chance of tidying the infection up.”
Most of the time there is one other management practice which Goodwin recommends to be undertaken to clear up chalkbrood infections, which should be used in conjunction with ensuring hive health, nutrition and good siting of hives.
“Usually, most beekeepers can get chalkbrood to disappear again by requeening the hive. That is on the condition that they check where the queens are coming from, and I'm assuming most queen producers are not going to graft from a colony that looks sick in any way. Then that should make chalkbrood go away.”
