Monday, August 22, 2011

Wasp scores caterpillar

While on a field excursion last Saturday in Adams County, I noticed this thread-waisted wasp carefully searching the leaf litter of the forest floor. My initial hunch was that it was one of the spider-hunting species, and by the way it was behaving I figured it was on the trail of a victim. Even though such dramas play out countless times each day, anywhere that decent habitat exists, one doesn't often get to witness a hunt such as this. So, we quietly settled in to watch, and I made a series of photos.

Later, we determined the species of the wasp: Eremnophila aureonotata. I haven't found a species specific common name, other than thread-waisted wasp, and there are many of those.

Our hunch was correct, although it was a caterpillar and not a spider that was in the wasp's sights. After a few minutes of circling and seemingly homing in on a certain spot, the wasp suddenly pounced and in the blink of an eye seized this luckless caterpillar and began tugging it from a niche in the leaves. The wasp undoubtedly stung it, but that happened so quickly that we couldn't see it. I've seen spider-hunting wasps capture spiders a few times, and their paralyzing sting also takes place with such rapidity that you can't even tell it happened.

It didn't take long and the wasp had completely extricated the inert but living caterpillar. We later determined that the victim was a variable oakleaf caterpillar, Lochmaeus manteo. This species of caterpillar reportedly can expel a noxious acid in an attempt to repel predators, which may account for these wasps' ability to sting and disable the caterpillar with such speed.

Most caterpillars hide during the day, largely to avoid fates such as this. The majority of their predators are diurnal (day active), hence it behooves caterpillars to emerge and feed at night, when most birds, wasps, flies and other enemies are mostly inactive. Hide as they may, there will be still be hunters that ferret them out.

In a herculean display of strength and endurance, the wasp began trundling off with a load that must have outweighed it by many times. Over sticks, branches, and leaves it went, and we had front row seats to the spectacle. By now, we were guessing that the wasp had a burrow somewhere close at hand, and it was hauling the prey to its crypt. There, the wasp would entomb the doomed caterpillar after laying an egg on it. When the wasp grub hatches, it has fresh meat to feast upon.

As with all photos on this blog, you can click the picture to enlarge it. Do that on the one above, and you'll have a pretty good look at the long powerful mandibles that this wasp sports. It grabs the caterpillar with these tongs, straddles it, and scampers along with impressive speed.

We were quite surprised when the wasp reached the trunk of a Virginia pine - this was about ten feet from where it caught the caterpillar - and headed right up the tree with no hesitation. Now this strongman feat took on a whole new dimension. Climbing vertically while carrying what, to a human, would be the equivalent of holding a sleeping bag filled with 500 pounds of coins in your mouth while free-climbing a wall. It was even more impressive when the wasp dropped the caterpillar at the five foot level, went back to the ground and retrieved the victim, and started all over.

By now, we're confused as to the outcome. This must either be a wasp that builds an aerial mud adobe nest, or uses tree cavities, we think. Or, it is climbing to gain altitude, and will then launch and flutter-drop out to where its ground burrow is located. In this photo, the wasp and its caterpillar are near the end of that broken branch some 15 feet off the forest floor and right at the edge of my camera's ability to focus with its macro lens.

The red arrow points to the wasp. To our surprise the wasp began to eat the caterpillar, starting with the head or so it appeared from our lowly perch. I know some wasps will consume caterpillars in addition to using them to provision their nests, so I guess that's what was going on. By this point, we really had to go as the wasp hunting diversion had made us late for other activities. When we left, the insect was still munching away. Later research revealed that Eremnophila aureonotata does indeed raise its offspring on caterpillars that are entombed in ground burrows, but apparently the adults eat them too.

No human possesses anything even close to the strength and endurance to do what this wasp did, on a comparable scale. In human terms, it would probably be akin to a 200 lb. man ascending the side of the Dubai Tower bare-handed while carrying three sacks of cement - in three minutes. It makes me wonder if anyone has ever studied the physiology that allows an insect such as this to carry out such remarkable feats.

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