Andrenid bees at the FWG

by Christine Hanrahan

Originally posted in May 2011

About 5 years ago, I discovered an aggregation of Andrenid bees nesting on the north slope of the Amphibian Pond. At that time they were in one location only. The next year I went back to the pond in early spring, hoping to find them. It wasn’t until late April that they appeared, in the same location, at the time a nearby willow was in flower. Eventually, I discovered that they were Andrena dunningi, a species known to time its emergence to coincide with the flowering of willow trees.

Andrenid bee. These bees were very active in early afternoon under a warm, sunny sky. I would guesstimate about 30-40 were seen, fewer than the number of nest holes.

Over the next few years, I found them mostly in the initial location, but in 2007, I also found some nesting in another section of the north slope, where an abundance of non-native plants, mostly mustards and Chenopodium species, were growing (for that one year). The nesting, of course, took place before these plants had grown to full height.

This year, 2011, the bees were first noticed in late April, and, interestingly, I saw they had spread from their original site to the upper section of the track around the pond (not the Bill Holland Trail, but the “informal” track that was created when the bridge was closed last year), and to the west side of this track. I counted about 30 entrance holes at that point.

Constant foot traffic on the track meant that the burrows were continually being closed over, although new holes appeared regularly.

Andrenids prefer areas of bare soil with scattered vegetation, such as sparsely growing grass, and show a distinct preference for the tops of slopes. Their burrows are about 4-6 inches in length, as best as I can determine, so we don’t want to plant species with deep spreading roots that might prevent them from burrowing. Reading I have done since last week indicates that they will nest under exposed tree roots, but they are much happier with bare soil.

Some of the plants we have considered for this location (and we are not restricting the list to native only) include clover, grasses and sedges that form clumps (such as poverty oat grass and peduncled sedge), and possibly vines that will grow down the slope such as wild grape or virginia creeper.

Freshly dug nest site: you can see the mound of soil around the hole.

Andrenid nest entrance. Freshly dug holes often have a mound of soil around them (see photo above), but rain washes this away. Many of the nest holes are not apparent at first, especially if not surrounded by a mound of soil. They are also very small, approx. 1/4 inch or less.

NOTE: As of May 2017, we have found at least 7 species of Andrenid bees at the FWG: Andrena cressonii, A. dunningi, A. miserabilis, A. nasonii, A rufosignata, A. vicina, and A. wilkella (the last one is the only non-native). In addition to this location next to the pond, we’ve also seen them in the Old Woodlot.

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