As more and more people inhabit Earth, demanding more and more space and resources from a limited supply of both, the other creatures with which we share the world are going extinct.

People are generally familiar with the plights of the larger and better-known animals, such as mammals and birds, and the reasons that are most commonly at the root of the problem, such as habitat loss and introductions of exotics.

But generally, people are not aware of the large numbers of small, less-conspicuous creatures that are going extinct, if they are not already. In most cases, these smaller creatures are at risk of extinction because of similar threats to their larger distant cousins, but there is one situation where this is not so: co-extinction.

American chestnut (Castanea dentata).
Foliage and immature burrs of American chestnut. Image: Daderot © Public domain

Co-extinction is the loss of a species because the species upon which it depends for survival has vanished.

Co-extinction is poorly understood and there are only a handful of well-documented cases.

Yet, despite this lack of knowledge, the tight associations between the huge numbers of parasites and their hosts, and plant-feeding insect species and the plant species they feed on, may render co-extinction one of the greatest threats to current biodiversity.

Not only does it affect these highly diverse plant-feeding insects and parasites, but the effects of co-extinction are thought likely to cascade down through food webs, resulting in species loss among many unrelated, but successively dependent, organisms.

This is a very serious situation deserving of far more attention than it currently receives.

Two men stand among trees.
Huge American chestnut trees in North Carolina, U.S.A., in 1910. These huge trees have been called the redwoods of the east. Image: Courtesy of the Forest History Society, Durham, NC.

Let’s look at the case of Castanea dentata, the American chestnut. At the turn of the 20th century, Castanea dentata was one of the most important trees in eastern North American forests. Mature trees reached 30 metres in height and over 3 m in diameter; they were colloquially referred to as the redwoods of the east.

However, in 1904 the fungal pathogen Cryphonectria parasitica, or chestnut blight, was introduced to the New York Zoological Park (now the Bronx Zoo) in New York City. The blight quickly spread across the eastern United States and in the course of a few decades, it effectively killed almost all of the chestnut trees in the east.

While new shoots often sprout from the roots remaining after the main trunk has died, blight will continue to infect these new shoots and kill them before they reach any significant size or a mature reproductive state.

A few large trees still survive within the native range, perhaps because of isolation or partial blight resistance, but the reproductive history of these trees is not well known.

Leaves, burrs and nuts arranged on a table.
Some foliage and chestnuts of the American chestnut. Image: Timothy Van Vliet © Timothy Van Vliet (CC BY-SA 3.0)

Like almost all tree species, American chestnut has a number of host-specific insects that feed on it and little, if anything, else. Among these insects are a number of species of moths and at least two species of weevils, the greater and lesser chestnut weevils, Curculio caryatrypes and Curculio sayi, respectively.

According to the International Union for the Conservation of Nature, the reproductive extinction of the American chestnut resulted in the concurrent loss of at least two of these species of moths, Ectodemia castanea and Ectodemia phleophaga, and likely at least five others, some of which were last seen in 1936.

The case of the weevils has not been addressed until now. The lesser chestnut weevil, Curculio sayi, is known to still exist, reproducing in the nuts of other species of Castanea such as introduced Chinese chestnut Castanea mollissima and the native chinkapin Castanea pumila and its southern relatives.

Composite: Two weevils, each with a 2 mm scale bar.
Male (left) and female (right) Curculio caryatrypes. These weevils can be recognized by their large size for the genus Curculio, and by the second segment of the antennal funicle’s (the part beyond the elbow) being obviously longer than the first segment. Females use their much longer snout to excavate small, deep holes in chestnuts into which they lay their eggs. Images: François Génier © Canadian Museum of Nature

However, the most recent study of the taxonomy of the genus Curculio, which brought together thousands of museum specimens of the genus, could find no specimens of Curculio caryatrypes collected any later than 1956. Other efforts at sampling insects living on surviving American chestnut trees have similarly not collected this species.

Spurred by this knowledge (or lack thereof), I recently polled a variety of insect collections throughout the native range of American chestnut to see if they have since added any specimens of this large, distinctive weevil species to their collections.

Sadly, other than two specimens reared in spring 1987 by now-deceased lepidopterist Eugene Munroe and his wife Isobel of Ottawa, Canada (from nuts collected from a large, now-dead American chestnut tree in Prince George County, Maryland, U.S.A.), no other post-1950s specimens have been found.

There are still many reports of weevils in introduced chestnuts and native chinkapins, but where examined, these are all the lesser chestnut weevil, Curculio sayi. Despite some comments in the old agricultural literature to the contrary, it is likely that the greater chestnut weevil was associated only with Castanea dentata. With the demise of that plant, so went the weevil.

Several weevils on a burr.
Male and female Curculio caryatrypes weevils on an American chestnut burr. You can see why the females need such a long rostrum to reach in among the spines to excavate the oviposition hole. Image: © Brooks and Cotton 1929, U.S. Department of Agriculture

It’s not a happy day when we can declare another species as extinct, but today, along with the two species of already red-listed, extinct chestnut moths, I believe that we can now add the greater chestnut weevil (Curculio caryatrypes) to the ever-growing list of extinct organisms.

Perhaps we should now turn our attention to the co-extinction threat being imposed on ash trees and their insect associates by the introduced emerald ash borer (Agrilus planipennis) before that fauna is lost as well.