Of Flowers and Flies

Here's a nice shot from a windy, warm spring afternoon:

Toxomerus geminatus, Van Patten Woods FP, Lake Co, IL 5/9/2014

This is Toxomerus geminatus, a tiny hoverfly (family Syrphidae). They can be found over most of the warmer months, around here. Here's one from October a few years ago:

Toxomerus geminatus, Illinois Beach State Park, Lake Co, IL 10/22/2011

The adults are nectar feeders, which explains the other name for the group, the Flower Flies. The young, however, are primarily predators on aphids. (1) This has made them useful for agricultural pursuits, and they are sometimes encouraged by "insectaries", flower gardens meant to attract pollinators and predators of pest insects. (2)

Watching various nectar feeders at a flower, it's easy to remember that competition in nature can take all sorts of forms. Morse investigated this idea with various syrphid flies and bumblebees, and found some interesting things. While bumblebees displaced both Toxomerus and Melanostoma flies, Toxomerus immediately returned, and thus there wasn't much of an effect on them. Melanostoma took longer, and lost more time as a result. Melanostoma displaced Toxomerus, and since they stayed at each flower longer than bumblebees, they had more of an impact on Toxomerus. The authors suggested that bumblebee numbers might help regulate Toxomerus numbers by reducing Melanostoma populations through competition. (3)

This sort of cascade effect has been seen in other settings, but in this case, it gives us a better idea for how to manage those insectaries to encourage those species that are most effective at controlling pests. (2) An interesting thought when looking at these little guys with my knees in the grass.

(1) Reemer, M., & Rotheray, G. E. (2009). Pollen feeding larvae in the presumed predatory syrphine genus Toxomerus Macquart (Diptera, Syrphidae). Journal of Natural History, 43(15-16), 939-949.

(2) Ambrosino, M. D., Luna, J. M., Jepson, P. C., & Wratten, S. D. (2006). Relative frequencies of visits to selected insectary plants by predatory hoverflies (Diptera: Syrphidae), other beneficial insects, and herbivores. Environmental Entomology, 35(2), 394-400.

(3) Morse, D. H. (1981). Interactions among syrphid flies and bumblebees on flowers. Ecology, 81-88.