Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

The Digger Wasp and the Caterpillar

Friday, November 1, 2013

A short time ago, my wife and I took a little trip. We wanted to see some of the best wildlife that our state has to offer. So we decided to spend a week visiting the Audubon sanctuaries in Massachusetts. These sanctuaries are known for their preservation of natural places and their diversity of wildlife. However, the most amazing wildlife action we encountered took place on our return home, in our own backyard. It never ceases to amaze me the wildlife we encounter in our own neighborhood when we take the time to look.

What I found as I was touring the back yard was a wasp, a thread-wasted wasp, Eremnophila aureonotata. The wasp, about 1 inch in length, was straddling a caterpillar larger than itself. It had grasped the caterpillar just behind the head and was dragging the caterpillar along the ground. The wasp obviously knew where it was going, for it walked in a straight line for about 25 feet. Traveling at a fast pace, it passed over a pile of mulch, a grassy area and headed straight for a bare patch of ground. Once on the bare ground, it dropped the caterpillar and started to wander in a circle about 1 foot wide. Having found what it was looking for, it returned to the caterpillar and dragged it to the found spot. Looking at the spot, I could not see anything special about the area, but it was obviously familiar ground to the wasp.
Once again, it dropped the caterpillar and then began picking up and moving a pile of smallnest hole pebbles with its mandibles. In a short period of time, it had uncovered the entrance to a hole in the ground. Back it went to the caterpillar, grabbed it by the neck and carried it down the hole.

The wasp I had found is a type of solitary wasp. Unlike the paper wasp, the solitary wasp is, as its name implies, solitary. Once mated, the queen takes off by herself. She will build no communal nest. She has no worker wasps to help her raise a family. Once she has mated and laid her eggs, she will have nothing to do with any other wasp of her kind, either adult or young.

Any bare patch of ground will serve for her nest, which consists of a half inch hole about 1-2 inches deep. Once the hole is dug, she will cover the entrance with small pebbles. It would not do for some other animal to take up residence in the hole while she is away. And away she must be, for she now needs to search for food to nourish her young. In the case of my wasp, the food she will provision the nest with is a caterpillar. Other species of thread-wasted wasps may choose a bug, cricket, cicada or spider. Regardless of the food supply, it must be fresh. Dead rotting food will not do for the young wasp. So the trick is to supply her young with food that is incapacitated but not dead. This the wasp accomplishes with a very well placed sting, or series of stings - in just the right location to paralyze the food but not kill it.

I first learned about hunting wasps from the writings of Jean Henri Fabre. Fabre was a French teacher who lived from 1823 to 1915. His true love was insects, and he spent the last 30 years of his life studying and writing about them. In his book on hunting wasps, Fabre describes how he discovered their method of paralyzing the prey.
Fabre observed a wasp paralyze a weevil by stinging it two or three times in the joint between the first and second pair of legs. The result of these stings was that the weevil instantly became immobile, yet remained alive for a couple of weeks. Theorizing that to produce the instant immobilization, the weevil must be stung in a nerve center, Fabre recreated this effect by using a needle tipped in ammonia to stab a fresh weevil in the nerve centers found between the leg joints. Instantly, the weevil was paralyzed. Fabre went on to produce the same effect with other insects that the hunting wasps prey upon, stabbing each one in their respective nerve centers. In the case of the large caterpillars that my thread-waisted wasp hunts, a sting in the joint between each of the body segments is necessary.

egg on caterpillarOnce the caterpillar has been dragged into the hole, the wasp lays an egg and glues it to the side of the caterpillar. Next, the wasp fills in the hole, first dropping in a few pebbles, then kicking in some dirt. More pebbles, more dirt, and the process is repeated until the hole is completely covered. Now there is nothing to give away the presence of the wasp nest below.
Soon the egg will hatch and the young larva will have at its feet enough food for its complete development to an adult wasp, when the whole process will be repeated.

To see a video of the digger wasp provisioning her nest, check out the video at: