Pick-up Sticks

American entomologist Karl V. Krombein conducted a magisterial study of wasps and bees across a full decade from 1953 to 1964, collecting and examining some 3,400 trap nests. The results of this study were published in 1967 by the Smithsonian Press as Trap-nesting Wasps and Bees: Life Histories, Nests, and Associates, or, as I like to refer to it, the trap-nesting bible. With the aid of this book and the information it contains, I’ve conducted a limited amount of trap-nesting myself and have enjoyed it immensely.

Last year I added a bundle of hollow stems (all snipped to about five inches in length) at the base of our front yard bee block, to provide some additional nesting opportunities for the wasps and bees. The hollow stems, which are easily split open, work very well as natural trap nests. Seeing that a number of the stems had been occupied by midsummer, I placed one of them in a container so that I could examine the adults when they emerged from the trap. Usually, I photograph the wasps that emerge and then release them. In this instance, the wasps all emerged while we were away on vacation and all died before I could release them. Five Keyhole Wasps (genus Trypoxylon) emerged from the stem, two males and three females.

Over the winter, birds continuously ransacked this bundle of stems, eventually scattering them all to the ground where they were covered by snow and lay until spring. Today I played pick-up sticks, replacing them into their holder on the bee block. One of the fallen stems that I picked up was still plugged at the end. A closer look showed the stem to be cracked lengthwise so I decided to open it and have a look at the wasps inside. I was in for a surprise.

Upon opening the nest, what at first I took to be pupae turned out to be masses of parasitic wasp larvae that had devoured the overwintering Trypoxylon pupae. At some point last fall or this spring the nest had been compromised. The cracks in the stem had allowed the tiny parasitoid wasps to gain access to the nest cells. I remembered reading about such infestations by Melittobia chalybii in the Krombein book. Here is Krombein’s description: “This eulophid parasite was a very serious pest in trap nests. It not only parasitized a number of nests in the field at several localities but also it caused serious secondary infestations in other nests in the laboratory…. Melittobia has limited powers of dispersal, thus accounting for its presence in traps placed in these particular situations [i.e. dead trees or structural lumber], rather than traps suspended from branches of living trees. Although Melittobia females are winged, they apparently do not fly at all but merely hop a few inches or walk about on the substrate.”

More recent studies have shown that these parasitoids have a complicated life history involving some curious dimorphism: short-winged and long-winged females as well as blind, flightless males that don’t resemble the females in the least. I’ve kept the larvae with the hope that they will pupate and emerge as adults and I’ll be able to have a look at some of these morphological differences. As can be seen from the photographs of the one dead female wasp found in the nest, their body shape is remarkably flat, no doubt helpful in prying their way into host nests, the head being especially odd in having the shape of a flat disc.

Posted on 07 de abril de 2017, 02:52 AM by scottking