This letter is in response to a recent letter to the editor about Fairfax County’s Fall Cankerworm Insecticide Spraying Program.
I am a professional entomologist and I have been in charge of the U. S. National Insect Collection at the Smithsonian Institution, a Fairfax County resident for 21 years, as well as a biologist experienced in conservation. I have been following the issues regarding the program in Fairfax County to spray Bacillus thuringiensis var. kurstaki (Btk), a kind of biological insecticide, to control a native moth, the Fall Cankerworm (scientifically, Lepidoptera: Geometridae or Inchworms).
Bt was first used to control Lepidoptera (moth) pests of agricultural crops; however, this naturally-occurring bacterium was found to be deadly to all moths and butterflies (Lepidoptera). Since this discovery there have been many types of Bt developed, but those used against moths are still deadly to non-target species, including native species. However, the areas in Fairfax County being sprayed are not crops or timberlands for harvest, but rather forests, parklands, neighborhood gardens. These are places to preserve wildlife and nature, and to protect them for the benefit of not only Fairfax County residents, but also for all Virginians and visitors. Without scientific monitoring or surveys, we do not know how many or even which native species live in any particular area being sprayed. Therefore, to inadvertently spray this deadly microbe, even if the intention is to target certain areas, is not prudent.
The letter writer does acknowledge that spraying Btk can kill other kinds of caterpillars, “… but only if they are up and eating the first baby leaves.” However, it is well known that plant chemicals that often prevent insects eating certain plants only are present in older leaves (for example, tannins in Oak leaves); therefore, many species of caterpillars are only able to eat, and even prefer, what he calls “baby leaves.” He also claims that research shows that spraying increases the diversity of other species of butterflies and moths. However this research is apparently about the Gypsy Moth, a non-native (invasive) species from Asia. Reducing Gypsy Moth numbers could indirectly benefit other, native insect species. However, spraying to kill the native Fall Cankerworm is likely to severely harm many (as yet undetermined) native species of butterflies and moths and may even increase diversity of other non-native pest species potentially harming the fauna and flora of Fairfax County. Getting rid of the Fall Cankerworm will eliminate a potentially vital food source for many migratory and resident birds, as well as reptiles, amphibians, predatory insects, and other wildlife that rely on caterpillars for their survival. His assertion that “birds can safely eat the dead caterpillars” is misleading because in spring (when the spraying actually takes place), many insectivorous birds need to find enough live caterpillars to feed their nestlings or fuel their northward migration and it is not clear if they would even eat dead caterpillars.
Episodic population outbreaks (“infestations”) of native species such as the Fall Cankerworm are part of the natural ecological rhythm of our forests and woodlands, so why disrupt it? Based on previous research I have done (for example with Elms) and observations of the natural history in the forests of the northeastern USA, I do not believe that occasional and/or partial or even complete defoliation of most native eastern forest tree species will cause tree death. The severe defoliation from by Gypsy Moth in the 1970s demonstrated that only very severe and repeated defoliation over many years truly killed native trees; our local Fall Cankerworm outbreaks are natural and nowhere as severe and will not become so.
The far wiser approach to this problem would be for Fairfax County to sponsor some scientific surveys, to determine which species of butterflies and moths are present in the intended treatment areas, i.e. the biological diversity. Also advice to homeowners about how to monitor and/or treat their gardens would be helpful.
I have discussed this Fall Cankerworm issue with several experts on butterflies and moths at the Smithsonian and the USDA and they all agree with my statements above.
In conclusion I feel strongly that Fairfax County should cease spraying against Fall Cankerworm, because such spraying does more harm than good to the ecosystems and neighborhoods of Fairfax County.
David G. Furth, Ph.D.
Department of Entomology
Smithsonian Institution