It’s widely believed that high varroa levels and the associated viruses (deformed wing virus and bee paralysis virus) are the main causes of winter colony loss. Some of the authorised chemical treatments for varroa are themselves harmful to bees and potentially to the beekeeper too.
Sugar dusting has for sometime been recommended as part of integrated varroa management. Beebase recommends sugar dusting when used together with other biotechnical methods (for example, drone culling, shook swarm, queen trapping) with chemical treatments used as a last resort.
This page is a summary of information on sugar dusting gathered scientifically by Randy Oliver. If you wish to read the original detailed information, you can find it here:
Randy Oliver, scientific beekeeping .com
Randy found that dusting removed between 30% to 50% of phoretic mites (ie: those clinging to adult bees.) sugar dusting has no effect on mites contained within brood cells. Randy also found that 80% of the mites that fell dropped off the bees within one hour of dusting and that powered sugar dusting gave a more accurate measure of varroa levels than natural mite fall or washing bees in alcohol.
Effect of regular dusting on a varroa mite population
According to Randy’s data, a weekly dusting would result in a decrease in the mite population and both monthly and twice monthly dustings are beneficial.
Why does sugar dusting work?
The powered sugar causes the mites to lose their grip. It also encourages grooming behaviour between the bees which causes more mites to drop off.
How to apply the sugar
Dust the icing sugar over the top bars using a sugar shaker or sieve. Use 125g (1/4lb) icing sugar per full brood box. You don’t need to dust the supers. Then brush any sugar on the top bars into the seams. Check out Randy Oliver’s website (link above) which has some good pictures of this. There is no need to take out each frame to dust them individually. Research has shown dusting across the top is just as effective and doesn’t disturb your bees. Also, you don’t really want sugar entering the brood cells.
When to apply
Sugar dusting can be carried out at any time of the year including during a nectar flow and in late autumn. You can even do it in winter on days where the temperature is above 8c and the bees are flying.
Sugar dusting is most effective during a brood break or when brood-rearing is at a low. That’s because sugar dusting only affects the phoretic mites which increase in numbers when brood-rearing is low. It’s also very useful to treat a swarm or artificial swarm with icing sugar as there will only be phoretic mites present and they are at their most vulnerable to sugar dusting at this time.
Dusting in October is also very effective. Four to six treatments at five to seven day intervals should bring your varroa count down into single figures. This would mean there is no need for chemicals at all. A dusting session in March, if needed, knocks the mites back again and sets colonies off to a good start.
Why not sugar dust at the end of your first Spring inspection? Before you do, put in a clean monitoring board and come back after an hour to see how many mites are knocked off. Check again after 24 hours.
Dusting in the spring and autumn is advantageous for another reason. You won’t have any/many supers on at this time and so the dusting is much easier to manage. Randy Oliver recommends two or three dusts in the spring and two or three in the autumn to keep on top of mites. His research shows that dusting when brood is present in quantity, only has a small and temporary affect on mite numbers but dusting when brood levels are low is highly effective.
There are no health hazards to bees. Research has shown sugar dust does not enter their trachea or spiracles. There are no health hazards for humans either.
Type of sugar
Use pure icing sugar with no additives such as dicalcium phosphate, silicon dioxide or glycerine as these may be toxic to bees. Avoid fondant icing and make sure the sugar is dry. Icing sugar cost about £1.55 for 500g; much cheaper than chemical treatments. The application of sugar takes about two minutes per hive.
If you have applied sufficient sugar, it will come through your mesh floor onto the monitoring board. The debris on the monitoring board needs to be washed off well away from your bees. Avoid dusting on windy days or wet days as the powder will clog. If you don’t clean the monitoring board promptly it can attract ants and wasps and promote robbing during a nectar dearth. Try not to spill sugar near your hive.
Disturbance to the bees
Sugar dusting causes minimal disturbance to the bees when dusting over the frames. It is also very effective when you only have a queen cell or virgin in your colony as most of the mites will be phoretic. If you’re only opening the hive to dust, do it as soon as you’ve lifted the crown board. Have your smoker ready just in case but you shouldn’t need it as the sugar settles the bees. Have your sieve or shaker ready to dust as soon as the crown board is removed.
Always keep your top bars clear of burr comb. It makes brushing surplus sugar off the top bars easier. Brace comb between frames also reduces effectiveness, so make sure all the frames are clear of it.
There’s no need to dust supers, only boxes with brood in. If you’re running brood and a half or double brood, dust each box separately as the sugar is unlikely to make it all the way down. Dust the top box first, wait a couple of minutes then lift it off; the sugar should be enough to settle the bees.
In autumn/winter dust at the warmest time on a calm sunny day. Flying bees may congregate at the entrance, grooming themselves before going inside. You wouldn’t want them flying out when it’s cold as they may chill and be unable to return to the hive.