On page 50 of Tracks & Sign of Insects… there is a photo of a mottled mass of alderfly eggs, with a mix of pale, brownish eggs and dark, blackish ones. I was able to say in the caption with confidence that the dark ones were parasitized because I had collected that egg mass, and four days later I had come home to find a bunch of impossibly tiny wasps walking around on the leaf, having chewed their way out of the blackened eggs. I took a couple of pictures of these wasps, but my 105 mm Nikon “micro” lens was no match for them. So a few weeks ago when I found an identical egg mass (on the same day I saw the blister beetle triangulin and leaf-rolling weevil), I took the opportunity to collect an identical egg mass and try again, now that I had a macro lens with at least a fighting chance against the minuscule creatures that would emerge. The leaf with the new egg mass happened to be overhanging the same pond and was probably on the same glossy buckthorn shrub as that one I’d photographed three years earlier.
Two days after I collected the eggs, I checked them and saw some little specks moving around. On closer inspection, they were not wasps but alderfly larvae that had hatched from the unparasitized eggs. The gills were not evident, but they had the narrow “tail” (shown here on an older larva) that distinguishes alderfly larvae from their relatives, dobsonflies and fishflies.
More than a week later, I found some even smaller specks moving in the bag containing the egg mass. Many of them were wingless wasps like the ones I had seen three years earlier, but some had wings. I assume the wingless ones were females and those with wings were males, as is generally the case with insects that have a wingless sex*. These wasps are in the genus Trichogramma (Trichogrammatidae), of which three species are known to parasitize alderfly eggs according to the Universal Chalcidoidea Database.
* Added 3/17/2012: Apparently this is not the case with trichogrammatids. John D. Pinto informs me that the winged individual pictured below is also a female.
I was puzzled about what these wasps were going to do with themselves between now and next June when more alderfly eggs are available for them to parasitize–until I searched the Universal Chalcidoidea Database for known hosts of each of the three Trichogramma species, and found that they are not at all host-specific as most parasitoid wasps are. 132 hosts are listed for T. semblidis, 919 for T. minutum, and 943 for T. evanescens–flies, beetles, wasps, moths, and more. According to the 1979 Catalog of Hymenoptera, “any species of this genus can be reared in the laboratory or insectary on the eggs of almost any convenient host. . . The true host specificity for Trichogramma in nature seems to be to appropriate insect eggs that are deposited in particular environmental niches rather than simply to the eggs of particular host species.” T. evanescens, introduced from Europe, parasitizes “insect eggs deposited on herbaceous vegetation, primarily in cultivated areas”; T. minutum parasitizes “insect eggs deposited in arboreal habitats,” and T. semblidis parasitizes “insect eggs deposited in aquatic or semi-aquatic habitats,” making this last one the most likely suspect. The Catalog notes that alderflies, fishflies, deer flies, and horse flies are typical hosts for T. semblidis in nature.**
Most alderfly egg masses I have found have had more parasitized eggs than not. However, on occasion they are able to escape parasitism entirely. Yesterday Conrad Vispo, whom I met at the Berkshire BioBlitz the day after my alderfly hatchlings emerged, sent me this photo of a dense mass of hatchlings clinging to the eggs from which they recently emerged (I believe they are alderflies, although I’m not 100% certain they’re not fishflies):
Quite a sight! Since alderflies aren’t insects that are familiar to everyone, I’ll end with a photo of an adult. This is an example of a species that lays its eggs standing on end rather than lying flat.
** Added 3/17/2012: John Pinto has kindly provided me some more up-to-date information:
Unfortunately the host list for minutum in the on-line chalcidoid catalog stems from a literature rife with misidentifications. In the early days (before 1970 or so) there were only a few species of Trichogramma recognized in North America; we now recognize about 70. In fact at one time some entomologists believed that T. minutum was the ONLY species in U.S. agriculture. Consequently, a huge, almost entirely fictitious, host list developed for this species. We will never know the true identity of the Trichogramma called minutum in these publications. Most were something else but unfortunately the data seem forever tied to the name minutum. This is what happens when a group’s popularity, in this case for biological control, precedes taxonomic work. Harold Compere (in litt.) characterized the published record in Trichogramma as “one of the most beautiful examples of chaos in the entomological literature”. I tried to explain this problem in some detail in my revision of the genus***. To summarize, the literature prior to 1978 cannot be trusted regarding the species names used; and one has to be careful after that date as well.
One other thing. It is not at all clear that Trichogramma evanescens was ever successfully introduced in the US, reports in the literature notwithstanding. A closely related species T. brassicae Bezdenko apparently has been however. This involves another story of taxonomic “chaos”.
You are largely correct in your statements about host non-specificity in Trichogramma. I also believe that microhabitat is more important than host. However, preferences do occur and some eggs are not acceptable to some species.
*** Pinto, J. D. 1999. The Systematics of the North American Species of Trichogramma (Hymenoptera: Trichogrammatidae). Memoirs of the Entomological Society of Washington. No. 22 (1998), 287 pp.