Caterpillars overwinter outside

One fact from Doug Tallamy‘s The Nature of Oaks that surprised me especially: Some caterpillars in colder climates overwinter outside!

Bernd [Heinrich] decided to dissect golden-crowned kinglets that had been killed during the winter by window strikes in Maine. To his surprise and my amazement, their tiny crops were jam-packed with caterpillars — caterpillars the birds had eaten the same freezing cold day that they had smashed into a window and died!

It turns out that many species of moths, particularly inchworms like the common lytrosis (Lytrosis unitaria), spend the winter in the caterpillar stage… When it drops below freezing, they rely on glycerin, the same chemical that we use to make antifreeze, to keep their cells from bursting. The branches of trees that in winter look so bare are actually overwintering sites for caterpillars that sustain insectivorous birds like kinglets and brown creepers…

Doug Tallamy, “The Nature of Oaks”, p. 33.

In the paragraph leading into this section, Tallamy mentions a former graduate student of his had found caterpillar heads in owl pellets in winter. He concludes by noting that even in winter, insects, and so the plants that support them, are important to birds and the rest of the ecosystem.

I’ve personally seen inchworms (geometrids) in Iowa, and from observations in iNaturalist, it looks like golden-crowned kinglets are present there in winter, brown creepers too. So, it seems likely birds were surviving partly based on overwintering caterpillars around me when I was growing up, and I just didn’t know.

Tallamy cites a 1995 paper from Heinrich and Bell. Here’s a quote describing their findings:

Out of a total of 483 identified prey items in the gizzards of the 16 Golden-crowned Kinglets in winter, 287 (or 59%) were lepidopterous caterpillars (of which 60 were parasitized by Tachinid fly larvae), and of all these caterpillars, 95% were geometrids… Small (5-10 mm) lepidopterous larvae were the only consistent diet items found in all 16 birds, with a mean of 18 per bird (range = 6-39). Additionally, remains of numerous other … kinds of arthropod food items were retrieved, but all appeared only sporadically among the different individuals…
… although the birds include some species of Collembola in their winter diet, they appear to subsist primarily on geometrid caterpillars. Our estimate of primary reliance on lepidopterous caterpillars… is a conservative one because we tabulated only numbers of items, and the volume of individual caterpillars was far greater than the volume of most of the other prey items such as aphids, arthropod eggs, seeds, etc. that we tabulated…

Heinrich, Bernd, and Ross Bell. “Winter Food of a Small Insectivorous Bird, the Golden-Crowned Kinglet.” The Wilson Bulletin 107, no. 3 (1995): 558–61. http://www.jstor.org/stable/4163582

Looking at Bernd Heinrich’s Wikipedia page, I see he is the author of Why We Run (?!). I bought this book at a library sale years ago; I think I’ll finally take a look today.

Google Scholar led me to this 2013 article on overwintering caterpillars, with photos, freely available: “Caterpillars in Winter” [PDF] by Sam Jaffee.

And finally, a new word to me: “diapause”. From “Insect diapause: from a rich history to an exciting future”: “Diapause, a stage-specific developmental arrest, is widely exploited by insects to bridge unfavorable seasons.”