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Time to Clean Your Mason Bee Cocoons

By Rich Little | November 7, 2016


For those of you who took your Mason Bee tubes and blocks down in early June, now (October-November) is time to clean your cocoons.

Basic steps for cleaning cocoons

In October or November, gently open the nesting tubes or separate the laminated blocks. Using something flat, like a narrow nail file or flathead screwdriver, remove the cocoons to a shallow dish, taking care not to damage them.
You will see the gray-brown cocoons, which are often covered with small, dark-colored pellets. This is frass, the larva’s excrement. You will see the mud partitions between cocoons, possibly a few remaining yellow sticky pollen balls, or yellow or brownish sawdust-like mites. Cocoons that are dimpled like a raisin or open and crunchy are dead. Discard these.


For pollen mites on the cocoons (using a 10X hand lens) or evidence of chalk brood, a bleach treatment is recommended.
Place the cocoons in a bowl. Using a spoon, swirl the cocoons in a 0.05 percent bleach solution (1-2  teaspoons of household bleach into 1 gallon of water). After 1 to 2 minutes, rinse for 3-4 min in cool water to get the bleach off. Again closely examine the cocoons and discard those that look damaged or diseased.  If there are still high levels of mites, wash the cocoons again in a fresh bleach solution followed by rinsing in cool water. (Tap water or water the temperature of refrigerated water is fine. Do not use icy cold water or hot water). Remember the cocoons are made of silk, so a longer wash will eat holes in the cocoon.  Watch the time and do not go over 2 minutes.
Once cocoons are cleaned of mites, leave them to air-dry on a screen or paper towels for at least an hour.
Another way to clean cocoons that are heavily infested with mites is to “wash” them with sand. We do not recommend the sand wash method because any contamination from chalk brood will likely be spread throughout your cocoons as you “wash” them. If you do use the sand method, do not reuse the sand.


Storing cocoons for the winter

Put overwintering cocoons in a refrigerator at 37-39°F and 60 to 70 percent humidity (you can use a household refrigerator). This is to ensure that the bees remain dormant, are protected from predation by birds, and are not subjected to extreme temperature fluctuations. Cold storage may also help increase survival of the bees that emerge in the spring. Even though the bees in the cocoons are dormant, they are alive and respiring, which uses energy. Dormant bees stored at a cold (37-39°F), steady temperature use their stored energy at a slower rate than those overwintering at higher temperatures.
Place the cocoons in a ventilated container with some moisture to keep them from drying out (a moist—not dripping wet—paper towel in a small dish placed within the container is adequate).
Check the paper towel every week and moisten as necessary. If the cocoons develop surface mold, re-wash them with the bleach method for a minute and air-dry them for at least one hour on a dry paper towel or screen before returning them to the vegetable bin section of your refrigerator.

If you have questions about this article, please contact Rich Little. Join Benton SWCD on Tuesday, December 6, 2016 from 6:30 – 8:00 pm at the Corvallis Library for a special presentation: “How You Can Help Native Pollinators. During this fun and interactive talk, BSWCD Director Jerry Paul will share tips on how to encourage and care for native pollinators. Find out if you are maintaining your mason bee box properly.


This article was originally published in BEE NOTES: Pollinator Happenings in the Willamette Valley from the Linn County Master Gardeners.
Bee Notes provides timely and helpful information to encourage more pollinators in your garden. You can subscribe to Bee Notes at this link.

Linn County Master Gardeners will be present at the Native Plant Sale on February 25, 2017, to answer your questions about bees, including proper bee box maintenance.



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