Tuesday, March 13, 2012

Warmer Weather Bringing Out Some Pests, by Mike Waldvogel

Ground-nesting bees. Warm temperatures and the recent rains are promoting a burst of insect activity. We received a call a couple of weeks ago from a school system about "swarms" of bees showing up on the school grounds. These are most likely the solitary “digger” bees, particularly andrenid bees. Colletid bees will follow suit within a few weeks. People often mistake andrenid bees for honey bees because they are similar in appearance, although typically have blackish abdomens and light-yellow hairs.

Andrenid bee (Photo by Tony DiTerlizzi)
The bees begin foraging for food and seeking out new nesting sites. This activity will continue for the next 2 months or so depending on the area of the state and the species of bee. The bees dig a vertical tunnel in the soil, particularly in areas where the soil drains well and vegetation may be sparse. They make small side chambers off the main tunnel and provision them with pollen for their offspring. These bees are "solitary" which means there is no true colony which is supported by worker bees. Mating takes place at this time, so a lot of the "swarming" is just males and females trying to pair up. The bees frequently make small mounds in the soil, and small clusters of these nests may be seen together, but each mound is all made by an individual female bee. At one school in Raleigh, we’ve had reports of 5 or more nests in just 1 square foot of lawn. The damage can be unsightly with large numbers of mounds in the turf, but it is mostly a cosmetic issue. The bigger problem is usually that people walking by panic because they assume that these are swarms of honey bees (or a similar bee) that will likely attack them if they venture too close to "the nest." Turf-nesting bees can sting but rarely do, so unless you accidentally step or sit on one (which kids might do while playing), they are not likely to sting. Since they are not "social," there is not a large number of worker bees trying to protect a nest. So, there is no "mass attack" as might occasionally occur during a close encounter of the yellowjacket kind (and it's still early in the year for us to see any yellowjacket colonies out there).

These ground-nesting bees are beneficial and should be left alone if possible. Any pesticide that you typically use outdoors for such sites could be applied, but success is likely to be marginal because of the duration of activity. The bees often try to dig into the sand in play areas at schools and childcare facilities, which is a concern particularly for parents and teachers with children that are severely allergic to bee stings. Any chemical treatment in sandy play areas is strongly discouraged because of the contact kids have with the soil (and which they might get in their mouths as well). If the bees try to nest in a sandbox, a simple solution is to cover it during the day but it will take a few weeks for you to deter most of the bees that show up over time. While this isn't always successful, you could saturate the area with soapy water which will bring the bees out and probably kill some in the process since soap does work as an insecticide. Since the bees prefer well-drained areas, water-staturated soils will deter the bees but again we're looking at activity that can take place over a few weeks.

Information, including pictures, of these bees and the "damage" that they cause can be found at:

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