There too shall I bee

Here's two leaves off the Cercis occidentalis (Western redbud) tree in my back yard. I picked these leaves because they show evidence of nibbling. There's perhaps tens of similar leaves among many unmolested leaves. I suspect that the nibbling came from leafcutter bees that nest in my yard. Leafcutter bees use the segments to make nesting cells in pre-existing holes. In the wild they'll use reeds and the like. However, in my yard they use purpose-drilled holes that I've made in blocks of scrap wood and hung on my fence. I have holes of 1/8", 3/16", and 1/4" inch diameter and I've seen nests in all three. I ought to add 5/16" and 3/8" options. The smaller holes are used by a mason bee or wasp (they use mud to make protective larval cells). The 1/4" holes are favored by the leafcutter bees who use leaves in the same fashion.

There's three false starts to the nibbling process: Two on the right hand side leaf at about 1:30 and 7:00, and one on the LHS leaf at about 8:45. Additionally, the leaf segments appear to group in three sizes: small, medium, and large. I wonder if that's due to different species of bees, different uses for the leaves (cell walls vs. larval food production, perhaps), or some other reason. If I have time I'll make some measurements and report. The investigation is slightly muddied because the leaves were nibbled in early summer, so they may have grown or scarred back, thereby changing the size distribution (uniformly, one would hope).

The year prior to planting the Cercis, I noticed similar nibbles on my tomato plant leaves along with the nesting activity. This year I also grew tomatoes, but the preference was definitely for Cercis over tomatoes, despite the closer proximity of the tomatoes to the nest sites (far side of yard versus near side). Chalk one up for the native plants, again. Overall nesting activity seemed to be down last summer, perhaps due to low drought conditions.

By the way, solitary bees (which may or may not be native, but which are generally regarded as benign if not beneficial) are not aggressive. Since they have no hive or queen to defend, they don't often sting. They are also better pollinators than honey bees.