Wednesday, June 29, 2011

Black Raspberry Bloom, 2011 Part 2

In this post I'll address these questions:

-Does O. aglaia forage from black raspberry?
-What other bees forage from black raspberry?
-What other flowers does O. aglaia visit, and what else was available to them on the Sturm farm?

Notice that I won't say what's pollinating the raspberry leading to mature fruit. In many cases when bees visit a flower, they pollinate it, but not always. The only way to be sure would be to carefully control the visits that a flower gets by particular bee species, and then to follow the flower through fruit development.

When the sun poked tentatively through the clouds on the morning of June 16, the first foraging O. aglaia that we saw were visiting white clover and dandelion. These were the closest flowers to the webcam shelter. The O. aglaia seemed to be taking nectar but not pollen.

Sorry about the blurry image. It documents that O. aglaia foraged from dandelion. (Actually, on second thought this looks like it might be Agapostemon or Augochlora! - July 20)

When the afternoon turned sunny, we saw O. aglaia on the black raspberry flowers! They move rapidly, so they are hard to photograph on the flowers, but I managed to get a few photos of the foraging females. The first female is visiting a bud that is just starting to open.

I'm pretty sure that's a nectar visit. According to McGregor (Insect Pollination of Cultivated Crop Plants, Agriculture Handbook No. 496, 1976): "When the flower opens, the anthers are immature, with the filaments bent over the immature styles. Subsequently, the outer stamens bend back toward the petals and their anthers dehisce. As dehiscence progresses toward the center of the flower, the receptacle expands, the styles grow, and the receptive stigmas appear at their tips; later, the anthers nearest the stigmas dehisce, and, if cross-pollination has not already been brought about by insects, some selfing may result. The degree of such selfing seems to vary with species and cultivar, but most of them are largely self-sterile."

It's hard to be sure from this photo, but it is possible that there is a little pollen on the underside of the abdomen of this bee. We looked hard to try to determine if there was pollen in the scopae of bees returning to the nests, but did not see any either on foraging bees or on bees returning to the nests. I suspect that the O. aglaia were just beginning to search for nesting sites at this time.

A variety of other weeds were available in the area around our bee shelters, but none as abundant as white clover and dandelion. In addition, there were two crop species near the webcam shelter. South of the shelter was a small strawberry patch with blooms as well as immature and ripe berries. Just east of the strawberries were some blueberry plants in bloom. See the earlier post about bee introduction in May. On June 17, a sunny day, I saw few bees visiting the blueberry: especially compared with raspberry visitation. There were a few bumble bees. I saw some O. aglaia sunning on the ground in the blueberry patch, and I'm pretty sure that a couple of them flew up to the blueberry flowers, but I didn't get a good look at them.

Here's a photo from the west side of the Sturm fields. The white shapes on the left are honey bee hives.

Don doesn't pay anything to get honeybees in his berry fields. Rather, he works with a beekeeper who is happy to bring hives to Don's field because of the absence of insecticide sprays. On June 16 I counted 19 hives with a total of 50 supers in this row near the west end of the fields. Then on June 17, the beekeeper added 24 more supers in 10 hives closer to the marion berries where our second Osmia shelter is located. I hope that's all that were brought to the field. The honey bees are likely to have an impact on the growth of the O. aglaia population.

Bombus vosnesenskii is one of the species of bumblebees foraging on the black raspberry.
She is apparently only taking nectar, because there does not appear to be any pollen on the corbicula of her hind leg.

In addition to the honey bees and bumble bees, we observed a few ground nesting bees foraging on the black raspberry on June 17. The morning was sunny, though the temperature was only 55oF. These included some small Lasioglossum (Dialictus), and possibly an Andrena.
Visitors to the black raspberry seemed to change with the weather. The morning of June 16 there seemed to be equal numbers of honey bees and bumblebees in the cool, overcast weather. That afternoon when the sun came out the bumble bees became much less abundant.
We decided to quantify the numbers. At 5:45pm John and I each counted bee visits to 6 inflorescences of black raspberry close to the webcam shelter for 15 minutes. The temperature was 65oF, and the sky was overcast. John counted 10 honey bee visits and one visit by a Lassioglossum or Andrena (on 11 young open flowers, 41 total flowers in 6 inflorescences). On a different patch of 6 inflorescences I counted 19 honey bee visits (on 10 young open flowers, 73 total flowers in 6 inflorescences).
We repeated the counts on the morning of June 17. The temperature was 55oF, the sky was sunny. We started our 15 minute counts at 10:17am. John counted 23 honey bee visits, 2 O. aglaia visits, 2 Lasioglossum and 1 fly visit (on 10 young open flowers, 49 total flowers in 6 inflorescences). I counted 27 honey bee visits, 12 O. aglaia visits, and 6 Lasioglossum visits (on 17 young open flowers, 38 total flowers in 6 inflorescences). . We saw a few bumble bees, but bumble bees were much less abundant than they had been on the cold morning, and they did not show up in our counts.

One mystery that we never solved: What were the Osmia lignaria visiting? They were active at the shelter, even in the cold June 16 morning. They seemed to fly toward the blueberries, but I did not see them foraging on blueberries or raspberries. The nearby maple trees had developing fruit, so they were not attracting bees. So, it's not clear if the blue orchard bee was contributing to pollination of any berries.

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