Eggshells

On Monday I spotted these little eggshells on a tall blue lettuce (Lactuca biennis) plant growing under the black cherry (Prunus serotina) tree in my yard:

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There aren’t many insect eggs I would identify to species with confidence, but these brown-rimmed hockey pucks were without a doubt deposited by a polyphemus moth (Antheraea polyphemus). This makes four giant silk moth (Saturniidae) species I have now found in my yard. Our first winter here (January 2014), I noted an old parasitized Callosamia promethea cocoon within ten feet of the spot where I found a caterpillar last week; one night in June 2018, an Actias luna adult came fluttering to the bathroom window; and this past winter I found a Hyalophora cecropia cocoon on a trellis in the middle of our front yard (I had previously found a caterpillar munching on birch leaves in the woods behind our house).

All of these are exciting to find in the yard, not just because they are big, fancy moths, but because giant silk moths have been in decline for decades. A major reason for this decline seems to be the parasitoid fly Compsilura concinnata (Tachinidae), a European species that was introduced to North America in 1906 to control gypsy moths (Lymantria dispar). Like many early biological control efforts, this was not well thought out; C. concinnata is an extreme generalist and is known to attack over 200 other insect species (it also isn’t particularly effective against gypsy moths).

On a related note, on Tuesday I found this group of eggshells while Julia and I were harvesting some dill in the garden:

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These belong to another import, the multicolored Asian lady beetle (Coccinellidae: Harmonia axyridis). I know this not because the eggs are so distinctive but because the little hatchlings were crawling all over the leaves within a few inches of the eggshells. Here’s one of them chomping an aphid, as it was brought here to do:

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Unfortunately, this is another generalist that eats not only all sorts of aphids, scale insects, thrips, and mites, but also eggs of butterflies and moths. In addition to annoying people by invading their houses in large numbers in the winter, this species has also largely displaced our native ladybugs. When this one landed on the lawnmower yesterday…

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…I assumed it was one of those native species, but darn it, it turns out to be another European import, the fourteen-spotted lady beetle (Propylea quatuordecimpunctata). When I first saw it it was just beginning to munch on an aphid, but by the time I had run inside, switched lenses, replaced the flash batteries, and run back out with the camera, the aphid had been reduced to a little ball of mush with an antenna sticking out.

I found one other eggshell in my yard this week, and it was not a welcome sight: When I went to check on the promethea caterpillar on the cherry tree, it was looking a little sickly:

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See that little white egg?

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It’s a tachinid fly egg, which means there is a fly larva—likely Compsilura concinnata—feeding inside the caterpillar. Oh well; at least I know a few giant silk moths are somehow surviving to adulthood to produce the eggs and caterpillars I’m finding in my yard.

(A huge “thank you” to Sam Jaffe of The Caterpillar Lab, who made this post possible by generously lending me his macro lens while mine are being repaired!)

…Added the next morning: I just woke up thinking, “Wait a minute—Compsilura concinnata larviposits!” Females of that species insert larvae directly into their hosts, so this is the egg of some other (presumably native) tachinid. Although it doesn’t change the fact that this caterpillar is likely doomed, this makes me less disappointed to have seen the egg. I’d even be excited if it turned out to be a fly that specializes in giant silk moths, but I just checked the Host-Parasite Catalog of North American Tachinidae (Arnaud 1978), and the half dozen species recorded from Callosamia promethea are all generalists that could just as easily have settled for some run-of-the-mill caterpillar.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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8 Responses to Eggshells

  1. Judy says:

    So, are you ever tempted to squish the invaders or pink off the larva attacking a caterpillar? Just wondering.

  2. marianwhit says:

    Awesome post! It clarifies what a biological stew we have created by random and intentional mixing of species. There are several possible results here…a hastening of evolution in making new hybrid possibilities, and/or a reduction in species and unique diverse ecosystems. I wish I could see any of these possibilities as a good thing for people. But we forge ahead with false confidence, looking for easy and economically efficient ways to solve our problems, eliminating any creature that poses the least inconvenience to us. We have not been doing our homework…we studiously avoid the most obvious reasons for declines because addressing the issues might inconvenience us. Light pollution.

  3. Virginia S Dyson says:

    I am sometimes spiteful…..

  4. Steve says:

    How certain are you about the “egg”on the Promethea Moth caterpillar? Looks like a parasitic braconid wasp cocoon. Rarely do I see the egg of a parasitic fly or wasp, it is usually inserted into the host. Also looks too large and not the right shape for a fly egg.

    • I am 100% certain it is a tachinid egg. It is less than 1mm long, much too small for a braconid cocoon. Many tachinids glue their eggs to the surface of their hosts, and this is a typical appearance for those species with “macrotype” eggs.

  5. Catherine Klatt says:

    Thank you for solving an ID puzzle of mine! Last year I had an identical egg, laid on an ornamental weeping cherry leaf – now I know it’s a polyphemus moth egg.

  6. jezziebezzie says:

    The Asian Ladybugs they brought in to get rid of aphids that “bugged” us a few years ago caused such devastating effects. The idea that kids can’t lie in the grass & have a sweet Ladybug crawl on them anymore to begin to introduce them to insects in a positive way is so sad. These critters are mean! Not to mention all the actual harm they do & ugh, the swarming in cold weather. Always something fascinating here, Charlie. Thanks.

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