Wasp Sept 7

The tree in our front yard had fewer caterpillars this summer, so the yard had fewer thread-waisted wasps. Instead of a daily swarm of wasps under our tree, I saw one or two a week.

Wasp Sept 7

The wasps dug burrows as usual, but I didn’t see any of them return to their burrows with prey.

Wasp Sept 11

Twice I waited over an hour as wasps searched through leaf litter on the ground and branches overhead. Both times the wasps were still hunting when appointments called me away.

Wasp Sept 11

My wasp failures were disappointing, but it’s always worthwhile to spend a few hours sitting quietly in the yard. Last year as I waited on the wasps, I found a wolf spider carrying her army of spiderlings. This year I found flies.

Fly Sept 11

The flies caught my attention because they seemed as interested in the wasps’ activity as I was. They watched as intently as I did.

Fly Sept 11

As the wasps dug, three or four flies positioned themselves within a few inches of the developing burrows. Each time a wasp carried a pinch of excavated dirt away, the flies zoomed in and flew quick figure-eight patterns over the burrow. When the wasps returned and resumed digging, the flies lit nearby and watched until the wasps left again.

Fly Sept 11

The longer I watched, the more convinced I became that the flies were kleptoparasites. They were waiting to deposit their larvae in the wasps’ larder, alongside the wasps’ hungry larvae.

Fly Sept 11

The behavior is well-documented. It’s one of those complicated, clever twists of nature that fills me with questions. How do the flies learn to follow the wasps? Generation after generation, flies see a thread-waisted wasp and something whispers deep within their experience. Follow it. And they obey. Why?

Thread-Waisted Wasp Video

Another thread-waisted wasp. This time I got a few video clips as it dug and sealed its nesting burrow.

Thread-Waisted Wasp

Some years ago, the tree beside our mailbox became infested with yellownecked caterpillars. (I believe the following moth is the adult form of these caterpillars. Please comment if you can confirm or correct my identifications!)

That first summer, the tree’s lower branches were stripped of leaves by fall. The next summer, thread-waisted wasps arrived in the yard.

Dozens of these wasps dug burrows in the loose soil under the tree. The following year, we saw very few caterpillars and even fewer wasps. The tree kept most of its leaves. Another year later, the caterpillars surged again. More wasps, as well.

They continue on in this pattern. Every other summer, we have caterpillars and wasps, with the between years bringing decreased populations of both.

The wasps are very efficient. A burrow takes only fifteen or twenty minutes to complete. They dig with their front legs and jaws, vibrating their wings as if to loosen the soil faster, and carry the excavated dirt several feet away. Each trip clears a pea-sized lump.

When the burrow is deep enough, they fly into the tree, sting a caterpillar, and let the stunned victim fall. They find the caterpillar on the ground, grasp it in their impressive jaws, and drag it into the burrow. A few minutes for egg laying, and the job is done. (I missed this part of today’s activity because my camera batteries died. 😦  Maybe I’ll get another chance tomorrow.) When finished, the female stuffs clumps of dirt and small stones into the burrow’s entrance. Then she moves to a different part of the yard and starts all over again.

The sandy parts of our yard, where grass grows poorly, are peppered with burrows right now. Next year, I expect the tree will keep its leaves all summer.

As a final note today, this might be the same mockingbird that I photographed yesterday. It certainly had the same sneeze…