Which Bees are Attracted to Douglas Fir Sap?
Ask the Expert
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Rebecca Herron asks —
We live in western Oregon and have a large acreage of 20-year-old Douglas firs. They were planted years ago for Christmas trees. This time of year, and when there’s drought, the trees produce sap on the needles. There are bees everywhere right now! They are interested only in getting the sap. They aren’t aggressive. They’re small in size, and they are very active. Could they be mason bees? Definitely not yellow jackets or honey bees. However, in years past, the yellow jackets move in around September, and the smaller bees leave.
Rusty Burlew replies:
Many different types of bees collect sap from trees, bushes, and herbaceous plants. In fact, there is a whole group of bees closely related to the mason bees that are called resin bees. Honey bees, too, collect sap that they scrape from plants and chew to form propolis, an all-purpose material used for building, gluing, and coating their homes.
Sap and similar resins are made from a variety of phytochemicals that are designed to protect the plant from invasion by pathogens and predators. Plants use it for wound dressing and bees use resins for much the same reason. Honey bees smear propolis inside the hive to reduce the pathogen load, and resin bees often seal the entrance to their nesting tubes for the same reason.
I frequently see honey bees on the bark of Douglas firs and western red cedar. Smaller bees often collect it from leaf buds, needles, stems, and twigs. The resin bees bite it with their mandibles and fly home with it the same way mason bees carry mud. Honey bees carry it in their corbiculae (pollen sacs). You will see a lot of resin collection on these hot days when the resins are most pliable. In cold weather, the sap gets brittle, hard, and difficult for bees to handle.
Many places, including Oregon, have Dianthidium bees. Some of those actually collect tiny rocks and build “stone” houses which they glue to walls or tree limbs all cemented together with resin. Since so many species build with resin, I can’t tell you which kind you have, but if you can get a decent close-up photo, I can take a shot at it.