Much information is spread across the web about the natural pest repellent qualities of cedar. Studies have shown that cedar in the form of shavings (chips), along with the essential oil of the wood does in fact repel some insects, bacteria and fungi. However, there are also many claims that are not backed by published evidence of studies conducted in a controlled environment.
Eastern Red Cedar
What is commonly called the eastern red cedar or Virginia red cedar is botanically known as Juniperus virginiana. This evergreen is not a true cedar, (a true cedar being a member of the pine family), but instead is a juniper, a member of the cypress family. This often pyramid-shaped species is widespread across the eastern United States and the wood and oil of this species has been shown to repel clothes moths. Thus, it is often the wood of "cedar chests" and its shavings in potpourri bags are added to clothing storage.
Two additional members of the Cypress family commonly called cedars are the American arborvitae, Thuja plicata, known as the eastern white cedar which grows predominantly in the northeastern U.S., and the giant arborvitae, Thuja plicata, called the western cedar, which flourishes in areas west of the Rocky Mountains. Both are important timber trees and both contain the essential oil thujone, known to repel clothes moths, Argentine ants, the odorous house ant, black carpet beetles, and some cockroaches and termites.
Cedar chips are used as mulch in flower gardens and around vegetable plants, to line small animal cages in an effort to reduce odors and ward off fleas, for bedding material for dogs kept outdoors, and in clothes containers to repel the common clothes moth (Tineola bisselliella). As aromatic wood chips will ward off some fleas, cedar shavings do have a limited capacity in that regard, but it does not work for all fleas and may be no more effective than another aromatic wood such as pine.
As a pest repellent, it has been shown the eastern red cedar can repel and destroy the Argentine ant. In a study published in the "Journal of Economic Entomology,” Heike E. Meissner and Jules Silverman of the Department of Entomology, North Carolina State University, showed that "Argentine ants suffered total or very high mortality at all aromatic cedar mulch periods of outdoor exposure." Additionally their findings suggest the effect may last up to four months before weathering reduces the cedar's potency.
Substantiated scientific experiments conducted with cedar have demonstrated the plant's significant repellent qualities against a limited number of pests. Eliminating the Argentine ant from an ecosystem can reap benefits for native species, plant and animal. As for unproven claims passed down via folklore and sometimes hyped in product promos, there is still much research to be done to be certain of the extent to which cedar chips can function as a pest repellent.