This is one of the 4 watermelon fields we've planted wildflowers in this summer, and this weekend I was seeing an explosion in bumble bees showing up on the wildflowers compared to the other days in June I've been in this field. All four species of bumblebees common in our area were present, both males and females, mostly foraging on Cosmos sulphureus, some on Zinnia elegans. Before this day, I was seeing a huge number of Dieunomia heteropoda, seen below, and a variety of predatory and parasitic wasps and flies. Dieunomia heteropoda is one of the pollinator species in my study that we've only ever collected and seen on the wildflowers in our fields, never on watermelon, so it exclusively visits the Asteraceae flowers in our wildflower strips. Fifty-six percent of the pollinator species that were collected or seen foraging on the watermelon flowers in 2016 and 2017 were also collected or seen on the wildflowers, so they are many others that will forage on a wider diversity of floral resources.
The watermelon flowers in this field, however, are still not being visited by many pollinators. It is very strange to see that, at peak flowering. We have seen the two spotted long-horned bee on watermelon here, as well as some metallic green bees in the Augochlorini tribe (very difficult to ID in field), and a small number of Lasioglossum sp and a bumble bee here and there, but overall visitation has been very low, especially for a field with thousands of flowers at this point.
One thing that has always intrigued me is that we see male bumble bees foraging on the wildflowers and the females too, but we've never seen the male bumble bees on watermelon flowers whereas we often see females. There are some other species where we see both males and females, such as leafcutter bees (Megachile spp.), metallic green bees (Agapostemon spp.), Halictus poeyi and Lasioglossum spp. Often the males have different coloration patterns and are readily distinguishable from the females, as in the case of bumble bees where the males will look very different in terms of coloring, which is the main way we distinguish these species in the field. For example, the American bumble bee male has golden hairs that extend all the way down its body while the females have a black butt and gold hairs only about halfway down the abdomen. The males are only drinking nectar as a water and carbohydrate source (and vitamins and minerals) and eating pollen (amino acid source) for themselves, and aren't storing them on their bodies as the females do, since they don't help in provisioning the offspring. Why the male bumble bees stay off the watermelon but are often seen on the wildflowers in my study may have something to do with nectar availability on the wildflowers vs the watermelon.
I also saw this gorgeous Palamedes swallowtail butterfly (seen below), and this robber fly, a bumble bee-mimicking predatory fly, one of my favorite insects I've encountered. I've seen it in my wildflower patches all 3 years now, usually with an unsuspecting bumble bee in its grasp.
It started raining as I was leaving, and I looked back behind me and saw that the rain didn't chase away many of the bees, as they continued to do their work despite the rain.