Life on a Dogwood

—This article by Dick Fisher first appeared in our WildforaRI, Spring 2021

We are by now aware of the importance of pollinators in our ecosystem to promote diversity, fertilize native and food crop plants, and take their place in the food web. Pollinator gardens are the rage and for good reason. But the story does go deeper, so let’s start with a nice summer day last August.

Passing by an actively growing red-twig dogwood, aka red osier dogwood (Swida sericea), it was obvious some leaves were missing and stems were wiggling. Indeed the leaves of several small branches had been stripped and were being devoured by a large number of gray, striped, clamoring caterpillars. Although the shrub was small, less than one percent of it seemed affected. The caterpillars remained bunched together and confined themselves to the several adjacent twigs. Over the next few days their numbers decreased, and then they were gone; the red-twig dogwood did fine.

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This was a red-humped caterpillar (Schizura concinna) and is just one of 113 species listed in the National Wildlife Federation database that feed on the dogwoods in our area.1 Several species are limited to dogwoods only but most, like Schizura, feed on a multitude of native trees and shrubs.2

Lifecycle

The red-humped caterpillar is the larval stage of a small brown inconspicuous moth. The adult moth emerges from the pupa in the spring or early summer and lives only seven or so days during which the female deposits small yellow eggs on the underside of the host plant’s leaves. The eggs hatch in about two weeks to begin the larval stage as caterpillars and begin eating, perhaps increasing their mass one thousandfold over one and a half months until they are mature.2

As they grow they molt, shedding the outer cuticle and forming a new covering. Between molts they are known as “instars.” There are multiple instar stages as the caterpillar grows. The first instar is the larval form that hatches from the egg, and the final instar forms the pupation cell. Most butterflies spin a cocoon in which the pupa develops while moths usually develop an unprotected pupa cell. The red- humped caterpillar’s final instar drops into leaf litter and soft ground beneath the host plant and spends the winter in a pre-pupa stage. Pupation begins after the winter and ends with the emergence of the adult moth. In our area there is usually just one or perhaps two cycles per year.2

The red-humped caterpillars were not alone last summer. The dogwoods here hosted at least three other dependent species including the rusty tussock moth (Orgyia antiqua), the dogwood sawfly (Macremphytus tarsatus), and the yellow sawfly (Macremphytus lovetti). Sawfly larvae look and behave much like caterpillars but differ in some fundamental anatomical features such as the number of appendages or eyespots.3 True caterpillars belong just to butterflies and moths, but the life cycles of both the true caterpillars and sawfly larvae mentioned in this article are similar as is their dependence on the dogwood.

Diversity

We see these caterpillars and larvae for just a short time, but it is the time when they are most vulnerable. Schizura concinna, like the others, is a food source for something at every stage of its life cycle. The eggs are attractive to other insects, the adult moth is not a beautiful butterfly but is food for birds, and the pre-pupa and pupa stages are prey for soil organisms. The larvae, being highly visible, are at high risk, so much so that they have developed several defensive strategies. When disturbed these particular caterpillars show a collective warning display by thrashing about in unison, and they excrete an offensive fluid containing formic acid.3 Despite that behavior, they slowly disappeared over the next week or so, and I am not sure where they went. Perhaps some dropped into the leaf litter to begin their next stage, but many were still small and I suspect became part of the food chain for local birds.

In addition to our red-twig dogwood, two other dogwood species grow here: alternate-leaved dogwood (Swida alternifolia) and gray dogwood (S. racemosa). They flower and require pollination to produce seed. If not the red-humped moth then maybe another adult stage from the 113 species they host will help out. But they also have many non-lepidoptera pollinators including bees, wasps, and, yes, sawflies. The sawfly adults look like a cross between a fly and a wasp, they do not sting, and they can pollinate many crop and flowering plants.4

As the dogwood seeds ripen in mid and late summer, we can watch the birds clear them from the branches and disperse the seeds to begin the dogwood life cycle anew. The dogwood and its predators require interaction with the soil microbiome and the vast ecosystem it supports. It is all a fragile system, which we should take care to respect and support.

References
1  https://www.nwf.org/nativeplantfinder
2  Wagner, David L. Caterpillars of Eastern North America. Princeton University Press, Princeton/Oxford. 2005.
3  https://breedingbutterflies.com
4. https://www.whatsthatbug.com