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Solitary ground bee and nest holes

In mid-April, I received this interesting e-mail.

We have bees swarming low all over our front yard. We have had them for the last 5 years. They show up about this time of year and stay for about three weeks. They make round holes in the ground about the diameter of a pencil. They are not aggressive. I can walk right thru them and they won’t sting me – I’m not even sure they have stingers. I think each bee has its own hole. Per the County Extension Service (Hanover County), I should be happy to have these bees because they aerate my yard. But I am not happy because the area the bees have taken over is getting bigger. Soon they will be everywhere…

Andrenid bee. Cheryl Moorehead, individual, Bugwood.org

Andrenid bee.
Cheryl Moorehead, Bugwood.org

Jane also sent some pictures along with her question.

These bees are solitary ground bees (Andrenid bees), sometimes called “mining bees” and it appears that they have decided that Jane’s lawn is the perfect place to dig their nests. The problem is that once they find a nesting site they like, many future generations will continue to nest in that same location unless the conditions change and become unfavorable. This can be annoying if it happens to be in the middle of your lawn as in Jane’s case.

But …

Andrenid bees are important pollinators and should not be killed unless they pose a real problem or danger. They are usually only around for about four to six weeks in the spring and unlike the social bees (e.g. yellow jackets and hornets), these solitary ground bees are not aggressive and generally will not bother or attack you. They are actually very docile critters as Jane says; in fact only the females have stingers (which are actually modified ovipositors) but they generally don’t sting unless they are really provoked or happen to get stuck in your clothing. The males lack a stinger all together.

A female andrenid bee sitting in the entrance hole of its nest.

A female andrenid bee in the entrance
hole of its nest. Nice photo Jane!

Solitary ground bees emerge from their underground nests in the early spring. Male bees usually emerge first and hover low over the ground waiting for the females to come out. Once the females emerge and mate, they begin digging their nest in the ground. The entrance hole of the nest is about 1/4″ in diameter (about the size of a pencil) and is often surrounded by a mound of loose soil. The nest consists of a main tunnel that goes straight down (up to a foot deep) with many small chambers about ½ inch long going off to the side.

You can see why they are such good pollinators! Susan Ellis, Bugwood.org

You can see why they are such good
pollinators! Susan Ellis, Bugwood.org

Once the nest is built, the female bee collects pollen and nectar from spring blooming flowers and brings it into one of the chambers. She forms the pollen and nectar into a ball about the size of a pea and then lays a single egg on it. Once the egg is laid, she seals off the chamber and repeats the process until all the chambers in the nest are filled with food and an egg.

When the eggs hatch, the larvae feed on the pollen/nectar ball. They molt several times before they pupate and eventually emerge as an adult bee. The adults chew their way out of the chamber during the warm days of early spring and leave the nest through the main tunnel. There may be more than one generation in a season but the last one will overwinter in the nest and emerge in the spring.

An aggregation of ground bee nests in Jane's lawn.

An aggregation of ground bee nests
in Jane’s lawn.

Solitary ground bees prefer to nest in dry, well-drained areas with sparse vegetation. If they nest in the lawn, it’s usually in an area where the grass is thin. Sometimes there can be large aggregations of nests in an area (as in Jane’s case) and this can be both bothersome and alarming to the homeowner. These bees are great pollinators so we recommend that you encourage them to move on rather that trying to kill them off.

Since they avoid damp areas, one way to discourage them from nesting in your lawn is to set up a sprinkler and drench the area with about an inch of water once per week during the nesting period. Start this when you first start seeing them hovering around in the spring. This is not a permanent solution, though and they may well return the following year.

Ground bee holes in bare soil.

Ground bee holes in bare soil.

The best way to send them packing permanently is to change the conditions that make your yard attractive to these ground nesting bees. A healthy, vigorous lawn makes a terrible nesting site for ground bees so keep your lawn adequately fertilized, properly watered, and mowed at the correct height to encourage the growth of dense, lush grass. A thick layer of mulch will usually prevent them from nesting in your gardens since they prefer to nest in bare ground.

Until next time – Happy Gardening!

Special thanks to Jane for sending me these photos of her bees!

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