Question
I live in Central Wisconsin.I have noticed greenish colored fly on my siding. After watching I noticed them going into holes in the grooves in my siding. There is a greenish colored substance where the holes are located. There are places I noticed where woodpecker must have tried to get at them. What could this be? Should I call an exterminator,or is there something I could spray on my siding to stop this?

Answer
Jean,

These are probably solitary bees rather than flies. There are many small bees that nest in holes created in the ground or in wood. They are not considered to be structural pests but can cause minor damage. They are actually beneficial in that they contribute to pollination. The green mass is plant material that the bee has packed into the hole to feed its young. The holes can be caulked and painted over but don't need to be treated with insecticide.

Jack DeAngelis, PhD
Extension Entomologist (ret.)
My website about home and garden pests: http://www.livingwithbugs.com