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Over the last 5 years, we've been clearing Dog-strangling vine (DSV) from the north slope of our pond. We cover a section with a tarpaulin for at least a year, then plant the bare slope with shrubs. But the bare soil has attracted tunnelling bees. How do we keep the slope free of DSV and minimize erosion without disturbing the bees? |
Andrenid bees and slope stabilizationChristine Hanrahan 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. 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. 2011This 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. When I heard that the northwest section of the slope, covered by a tarpaulin, was being eyed for shrub and tree planting (in order to try and control the DSV that grows so abundantly there), I noted that we should not do anything that would have a negative impact on the Andrenid bees, a useful pollinating species. In the end, I volunteered to work with Claudia Burns on the site, and come up with some plant species that might not deter the bees from nesting in the location, presently covered by the tarp. On May 6th, we took a look at the area and discovered, when we lifted part of the tarp away, that not only was DSV happily growing well, but so was burdock. We began digging up the DSV (and the burdock), surprised at how well it was doing under the tarp. However, when Sandy told me that the tarp had been down only since late fall 2010, it was not so surprising that the DSV was growing very well. For some reason, I thought the cover had been in place for several years. In light of this information, it made sense to me to cover the site back up and leave it for another year, in an attempt to "cook" the DSV, and begin planting in the spring of 2012. In the meantime, the Andrenid bees have also moved eastward, to the other side of the tarpaulin. There are a number of nest holes on both sides of the tarp, as well as west of the abovementioned track. On the west side of the tarpaulin, their "traditional" site, if I can call it that, as that is where I first saw them, we noted that various plants were growing: grasses mostly, but some mustards and others. I decided to leave that alone for 2011 and do nothing, but monitor it to see what else grows there and whether the bees continue using that location (unfortunately, the bees are only active for about a month or so). Andrenids prefer areas of bare soil with scattered vegetation, such as sparsely growing grass, and show a distinct preference for the tops of slopes. Reading I have done since last week indicates that they will nest under exposed tree roots, but they are much happier with bare soil. In our case, we know that if we leave the soil alone, once the tarp is removed, we'll have DSV back in no time (even if we manage to dig out the entire slope, and that is a big "if"!), hence the problem of finding suitable plants for the site. Plants that will not deter the bees from nesting and that may prevent DSV from taking root, at least solidly. Andrenid bee burrows are about 4-6 inches in length, as best as I can determine, so we don't want plants with deep spreading roots that might prevent them from burrowing. Some of the plants we have considered (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. This is as far as we've got thus far. Please see photos accompanying this note.
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