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This is the time when parasitic wasps can produce another generation every week. During the summer months, when temperatures are higher than in spring and fall, these pesky little critters come out of the nest and search for new mason bee cocoons to parasitize. I don’t know how, but these tiny adult wasps can make a tiny pin hole in the tube and crawl out and parasitize more bees.
Every July, when most bee eggs have turned into adult bees inside their cocoons, I gather all nests, set them under a veranda- where the ambient temperature is still warm. I place two Highrises per net-bag (see wasp proof bags on my web site beediverse.com). They sit stacked against the wall of my home until the end of September when harvesting takes place.
I find that net bags are successful and keep out wasp parasites. Percent parasitism is usually not more than 5%. If you have had trouble with these parasites, you will be pleasantly surprised with the outcome.