Maryland Oaks Would Rather “See No Weevil”

Contact: Cathy Stragar, MDA |

Photo courtesy of Chris Joll © 2017

ANNAPOLIS, MD (August 01, 2017) – A minor player on the invasive team's bench of heavy hitters, the Asiatic oak weevil is still an invader to watch, especially for forest managers and invasion ecologists. Indigenous to Japan, the Asiatic oak weevil was first detected in North America in July of 1933 in Montclair, New Jersey. By the 1950s it was established in most of the eastern United States. Currently this weevil is found throughout eastern North America, west to east Texas and north to Indiana and into Canada. Because adult weevils emerge in July and their feeding damage is visible in August, the Maryland Invasive Species Council has selected Asiatic oak weevil as the August Invader of the Month.

Initially found in Maryland in 1940, the Asiatic oak weevil, Cyrtepistomus castaneus), became an economic pest of chestnut nurseries and orchards in the state. Somewhat of a generalist feeder, with a taste for a range of host plants in the beech family (Fagaceae), this weevil prefers oaks and chestnuts, including the native chinquapin (Castanea pumila). The exact impact of feeding damage on native forests remains understudied and mostly unknown, but researchers believe the Asiatic oak weevil warrants further study and certainly monitoring during chestnut restoration efforts.

This introduced and well established weevil is often one of the most abundant leaf-chewing insects found on oaks in eastern North America. The weevil is parthenogenetic — it breeds without males, and lays unfertilized eggs that contain only female larvae. Developing larvae feed on root hairs in the soil from December through spring and sometimes into June. Pupation of weevils is thought to occur between May and June, with adults emerging in July. Adult weevils feed on leaves, whereas larvae feed on fine root hairs and sometimes on the emerging radicals of acorns. Adult weevils are small, about 1/4" long, reddish brown to black with tiny green scales, but the damage from their feeding pattern is BIG and distinctive. Adults chew through all but the mid-vein of the leaf in chunky rough blocks. In Maryland damage from adult weevils feeding on foliage becomes visible in early August and increases in severity until mid-September.

In response to the chestnut blight, which functionally eliminated the American chestnut, various state, federal, and private entities have initiated efforts to breed and establish blight resistant chestnut trees. Current efforts use a back-crossed breeding approach to confer blight resistance from the Chinese chestnut to the American chestnut. Data from two sites in the southern Appalachian mountains suggest that site placement for these back-crossed chestnut nurseries may be important to keep feeding damage from C. castaneus to a minimum. Sites near oak forests had greater feeding damage than open sites or sites near non-oak forests.

Adult Asiatic oak weevil feeding damage. Photo courtesy of Bud Mayfield, USFS

In Missouri, the Asiatic oak weevil has yielded some interesting new observations on the ecology of invasions. When an introduced pest, especially an herbivore, arrives, there are multiple factors that determine the success of the invader's establishment. Is its host plant available? Will a lack of predators or parasites that have evolved with the invader in its country of origin leave its population unchecked and better able to exploit its new habitat? The list of interactions within this new community are numerous and it seems that invaders such as the Asiatic oak weevil can take advantage of multiple factors to become abundant, including exploiting the work of certain insect ecosystem engineers.

Researchers have known that leaf-tying caterpillars have a positive effect on the abundance and diversity of native arthropod communities in white oak trees, effectively acting as ecosystem engineers. The rolled leaves they create become refugia from predators, parasites, and environmental factors like desiccation for a number of arthropod species. The revelation was to recognize that this same interaction increased local populations of Asiatic oak weevil, an introduced insect, on trees with leaf-ties. Host quality and an enemy-free space are both strong factors in weevil abundance but researchers in Missouri have presented evidence that leaf-tying has a strong positive effect on weevil abundance, even in less preferred host trees such as sassafras. This positive effect on an invasive species' population abundance from activities of a native ecosystem engineer is probably not an isolated incident, but just an overlooked interaction in the current study of invasion ecology.

For more information about other Invasive Species of Concern in Maryland, visit the Maryland Invasive Species Council or call the Maryland Department of Agriculture at 410-841-5920.

For more information, see:

Unexpected Pest of Chestnut Trees

Baer, C. B., and R. J. Marquis. 2014. Native leaf-tying caterpillars influence host plant use by the invasive Asiatic oak weevil through ecosystem engineering. Ecology 95(6):1472-1478.

Case, A.E., A. E. Mayfield, III,S.L. Clark, S. E. Schlarbaum and B.C. Reynolds. 2016. Abundance and frequency of the Asiatic oak weevil (Coleoptera:Curculionidae) and defoliation on American, Chinese, and hybrid chestnut (Castanea). Journal of Insect Science 16(1):29;1-8.

Evans, R.1959. Notes on the biology of the Asiatic oak weevil in Maryland. Journal of Economic Entomology 52(1):177.

Frederick, K.H., and J.C. Gering. 2006. A field study of host tree associations of an exotic species, the Asiatic oak weevil [<em>Crytepisomus castaneus</em>&nbsp;(Roelofs 1873), Coleoptera:Curculionidae]. American Midland Naturalist 155:11-18.

Johnson, W.T.1956. The Asiatic oak weevil and other insects causing damage to chestnut foliage in Maryland. Journal of Economic Entomology 49: 717-718.