Strange Behavior Explained (Sort of)

The other day (Monday, June 6 to be exact) I was exploring Adams Open Space behind the public library in Fountain, Colorado with my wife. I happened to notice a small ichneumon wasp on the underside of a leaf and snapped a couple of images. This was the best one, and I was shocked to see a cluster of eggs beneath the wasp's abdomen. What was going on?

It had been my previous understanding that female ichneumon wasps deposited their eggs on or in their host, as they are parasitic on other kinds of insects (and spiders in some cases). Why was this one laying her eggs in a mass on the surface of foliage?

I posted the picture and posed that question to a Facebook group of world authorities on Hymenoptera, the order of insects to which ichneumons belong. The next morning I got an answer, courtesy of one Sasha Varga, a doctoral student in the Ukraine. First, he offered the genus identification of Polyblastus. That was surprise enough because the overwhelming majority of ichneumon wasps can barely be identified to subfamily from images alone. Thank you, Sasha!

Varga went on to explain that "as I understand, egg remains on the ovipositor after unsuccessful oviposition, but only one and why Polyblastus accumulates several eggs I really don't know." I was simply impressed with Sasha's command of the English language at this point. Ok, so now I at least have some clarified information I can run with.

Wait, Jitte Groothuis added a link to an article that might shed a little additional light. Jitte is likewise a PhD student, at Wageningen University in the Netherlands. Wow, this little observation of mine is circling the globe....

The article cited is entitled "Eggs and Egg Loads of Field-collected Ctenoplematinae (Hymenoptera: Ichneumonidae): Evidence for Phylogenetic Constraints and Life-History Trade-Offs." It is authored by Heather M. Cummins, Robert A. Wharton, and Aubrey M. Colvin, all of Texas A & M University. It was published in 2011 in the Annals of the Entomological Society of America, vol. 104, no. 3, pages 465-475. This sounds like it is over my head, too, so let's investigate together.

Apparently, an understanding of this paper hinges on an understanding of the differences in life histories between various groups of ichneumon wasps. Some ichneumons are "idiobionts," which means that further development of the host insect is arrested at the time it is parasitized by the idiobiont. Typically, a pupa stage is targeted, but sometimes a larva host, and the ichneumon wasp larva is invariably an external parasite. Idiobiont ichneumon wasps are also much more likely to be "generalists" with a wider range of host insects they can expolit.

Other ichneumon wasps are "koinobionts," meaning that the host organism continues its development at least until the wasp larva completes its development, at which time the host often (usually?) dies. Koinobionts can be either external or internal parasites, though the wasp larva is usually attached to the host, never straying to another host or otherwise moving freely away from the host. Koinobionts are also much more likely to be specialized on a narrow range of host species, in contrast to the generalist idiobionts.

Our Polyblastus here is a koinobiont. In its larval stage it is an ectoparasite (external parasite) of sawfly larvae. The adult wasp emerges from the cocoon spun by the host sawfly larva. This makes sense. We even found a female Elm Sawfly, Cimbex americana at the same location on the same day, so we know potential hosts are present.

According to the abstract of the article, the egg loads of female koinobionts like our Polyblastus are "significantly larger" than the egg loads of idiobiont ichneumons, though the eggs themselves are smaller in koinobionts. Unfortunately, an abstract is all I could find online. I will amend this post should I secure a copy of the entire paper. Maybe having so many eggs, our female wasp is forced to carry some of them externally? How does she keep from losing some simply in the course of searching for a host? How does she manage to attach her eggs to the host one at a time (assuming she deposits ova singly)?

Once again, an innocent observation turns into a convoluted and puzzling mystery. That is exactly what I love about entomology, though. It is a science that keeps you in a constant state of awe, forces you to think imaginatively, and challenges your research skills to the maximum.

Sources: Carlson, Bob. 2010. "Tribe Tryphonini," Bugguide.net.
Cummins, Heather M., Robert A. Wharton, and Aubrey M. Colvin. 2011. "Eggs and Egg Loads of Field-collected Ctenoplematinae (Hymenoptera: Ichneumonidae): Evidence for Phylogenetic Constraints and Life-History Trade-Offs," Ann. Entomol. Soc. Am. 104(3): 465-475.
Goulet, Henri and John T. Huber, eds. 1993. Hymenoptera of the World: An identification guide to families. Ottawa, Ontario: Agriculture Canada. 668 pp.

Wasp Wednesday: Netelia

Notice: I am no longer allowing comments on this post. They have become redundant and mostly an exercise in venting. Enough. Thank you for understanding.

Surprisingly, you may find some wasps at your porch light after dark. Among the more common nocturnal hymenopterans are ichneumon wasps in the subfamily Ophioninae. They are large, gangly wasps, usually uniformly orange in color with long antennae and large ocelli (“simple eyes” arranged in a triangle at the crown of the head, between the compound eyes). The ovipositor is very short, if it is even evident at all.

If you see a slightly smaller orange ichneumon with a longer ovipositor, it is likely to be a wasp in the genus Netelia, in the subfamily Tryphoninae. There are currently 73 species in six subgenera in North America north of Mexico (Carlson, 2009).

Unlike many ichneumon wasps, the females of Netelia can sting painfully if handled carelessly. The sting is mostly used to temporarily paralyze the large caterpillar hosts of these parasites. The female then lays an egg on the stunned victim, puncturing the body wall of the caterpillar in doing so. The egg is stalked, and in newly deposited eggs the coil of the stalk is elastic. It later becomes rigid. The egg holds firmly to the flexible exoskeleton of the host larva by means of a plug or anchor.

The larval wasp that hatches from the egg remains attached to it via specialized bristles on its posterior end. The wasp larva feeds on the caterpillar as an external parasite. Parasites that do not arrest the development of their host, but allow it to grow normally instead, are called “koinobionts.” Netelia ichneumons are placed in this category.

Below is an example of an ichneumon in the subfamily Ophioninae for comparison to Netelia.

Note in particular the absence of an obvious ovipositor at the end of the abdomen. More on the Ophioninae in a later edition of “Wasp Wednesday.” Look for both species coming to outdoor lighting near you!

Sources: Carlson, Robert W. 2009. "Database of Hymenoptera in America North of Mexico: Superfamily Ichneumonoidea, Family Ichneumonidae," Discover Life
Goulet, Henri and John T. Huber, eds. 1993. Hymenoptera of the World: An identification guide to families. Ottawa, Ontario: Agriculture Canada. 668 pp.
Kasparyan, D. R. 1989. Fauna of the USSR: Hymenoptera vol. III. Brill Archive. 414 pp.