When I took the cover off my picnic table's sun umbrella, I found a number of these structures running down one of the folds (see top photo). They look like they are made of dried mud and inside each one is a vivid yellow powder and a single thin, curled-up larva. Each structure was about 5 centimetres long. What did it, how did it do it, how long did it take - and should I get rid of them? This is a nest created by the female red mason bee, Osmia rufa, for her young. These are solitary bees and the female works alone from March to June building the nest one cell at a time. In each cell she lays an egg and stores a supply of pollen. The larva will hatch, eat the pollen and overwinter in a cocoon before hatching into an adult.
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