The other day Fred and I were supervising dusk when I thought I saw a yellowjacket swoop down on a line of fairly large, black ants, nab an ant, and fly away with it. Fred didn't see it so I figured I'd made a mistake, for who has ever heard of an insect preying on formic- acid filled ants? I went away without more ado, but then a bit later Fred called me back saying he'd seen a yellowjacket land on an ant with wings, break off the wings, sting the ant, and fly off with it. Then we sat and watched as several times a yellowjacket came buzzing the ant-lines, being very persnickety about which ants were chosen, and occasionally carrying off an ant. You can review what a yellowjacket looks like at http://bugguide.net/node/view/14144.
I'm still astonished to find that yellowjackets prey on ants but, now that I'm convinced they do, I figure that this is the very best time of year for it. That's because right now yellowjacket nests are filled with larvae needing to be fed, and yellowjacket larvae are carnivorous. Adult yellowjackets eat food rich in sugars and other carbohydrates, such as flower nectar, tree sap and fruit juice (that's why they buzz sodas and watermelon slices), but the fast-growing, immobile larvae crave protein. Therefore adult yellowjackets hunt a wide variety of insects, spiders and the like, chew them up, and then feed the nest-bound larvae with the chewed stuff. In return the larvae secrete a sweet substance relished by the adults. The technical name for this regurgitation-based, "community-stomach" form of food exchange is "trophallaxis."
On my backpacking hike this week I saw lots of yellowjackets buzzing around spider webs and combing the forest floor, so clearly nowadays yellowjackets are after a lot more than ants.
The yellowjacket life cycle is something to think about. You start out in spring with a single female who has been impregnated the previous fall, and who has spent the winter in a protected spot -- anything from beneath a piece of loose bark to the inner walls of a house. In early spring this lone female builds a paper nest, which may be outside in a protected place, or in a cavity in the ground, depending on the species, for there are several kinds of yellowjacket. This nest has 30-50 brood cells and the mother yellowjacket hustles to provide for them all. When the larvae pupate and emerge as adults they are all infertile females, known as workers.
At this point the mother yellowjacket's life changes drastically. The new workers start taking care of all her needs while she concentrates on producing more eggs, and the new workers do all the nursery duty. By late summer and early fall a yellowjacket nest can have up to 15,000 cells producing new yellowjackets. You can see a five-foot-long nest built by invasive German Yellowjackets in a Nebraska porch ceiling at http://entomology.unl.edu/entpeopl/yelljack.htm.
As cold weather approaches, the workers start creating cells that produce fertile females and males. The resulting adults mate, the females wander off and choose a place in which to ride out the winter... and all the poor workers and the males simply die.
I have often thought that one direction of evolution is to get rid of males, who tend to be unpleasantly war- mongering, status-hungry and politically extreme. When I see the family structures of such recently evolved, very sophisticated beings as yellowjackets, where males are restricted to a very limited timeframe and a single well defined, simple task, after which they die, I think I may have glimpsed the shape of things to come.
You can read much more about yellowjackets at http://ohioline.osu.edu/hyg-fact/2000/2075.html.
By the way, the yellowjackets' ant prey were "Pavement Ants," members of the Myrmicinae, probably TETRAMORIUM CAESPITUM, as identified with the picture-based Ant Identification Key at the University of California at http://www.ipm.ucdavis.edu/TOOLS/ANTKEY/index.html.