In my last post I showed this photograph of a tiny, stalked egg I found on the surface of a bolete, and suggested it might belong to a eucoiline figitid wasp.
Matt Buffington, a specialist in figitid wasps, wrote to inform me that this is unlikely because eucoilines lay their eggs inside the fly larvae that they parasitize. He said that the Ganaspis I photographed, which appeared to be laying eggs, was busily probing with her ovipositor to find a suitable host. Ovipositing takes time, and these wasps have a special “clip” on their ovipositors to prevent the fly larvae from retreating deeper into the mushroom during egg laying.* He suggested the possibility that it might be a neuropteran egg.
This egg does resemble a neuropteran egg in shape, but I do not believe that it is one. Green lacewings (Chrysopidae) and the uncommon beaded lacewings (Berothidae) lay much larger eggs, a millimeter or more long, which are on stalks that are 5 mm to 1 cm or more long. Mantidflies (Mantispidae) lay smaller eggs on much shorter stalks, but they are still at least twice the size of the egg on the mushroom–easy for me to see without magnification–and are laid in large masses. The smaller neuropterans, like dustywings (Coniopterygidae), do not have stalked eggs. Also, neuropteran eggs have a micropylar knob at the end, which should be conspicuous at this magnification.
I don’t have another idea of what this egg might be at this point, and if I ever find another one I will certainly collect it and watch it closely. I do want to point out that not all endoparasitoid wasps insert their eggs into their hosts. Trigonalid wasps insert their eggs in leaves, where they remain until caterpillars or sawfly larvae swallow them while eating the leaves. The egg hatches inside the herbivorous larva, and if the larva is parasitized by an ichneumon wasp or a tachinid fly, the trigonalid hatchling is able to parasitize the parasitoid and complete its life cycle. It could also end up parasitizing a vespid wasp larva whose parent (a paper wasp, yellowjacket, or hornet) has fed the caterpillar to it. Perilampus hyalinus and other perilampids parasitize parasitoids of caterpillars using the same strategy, except they lay their eggs on the surface of a leaf rather than inserting them, and the larvae actively burrow into the caterpillar rather than waiting to be eaten. P. chrysopae does the same, except it targets green lacewings by ovipositing on aphid-infested foliage (since lacewing larvae prey on aphids). Eucharitid wasps deposit masses of eggs in or on vegetation, and the hatchlings ride ants back to the nests where they burrow into their larvae. So given that these convoluted strategies exist, it didn’t seem too outlandish to suppose that some kind of wasp might lay its eggs a short distance from the fly larvae its offspring will parasitize.
* Buffington, Matthew L. 2007. The occurrence and phylogenetic implications of the ovipositor clip within the Figitidae (Insecta: Hymenoptera: Cynipoidea). Journal of Natural History 41(33–36): 2267–2282.