Linda Butler
Professor of Entomology
Division of Plant and Soil Sciences
West Virginia University
1998
In an effort to slow the spread of gypsy moth and to protect timber resources, various insecticides including lead arsenate, DDT, and carbaryl (Sevin), have been sprayed over the years. In the 1970s, the biological insecticide Bacillus thuringiensis (Bt) and the insect growth regulator diflubenzuron (Dimilin) were seen as being more environmentally safe than previous materials.
In recent years, state and federal agencies and private landowners have used Bt or Dimilin to suppress gypsy moth. In West Virginia, the first gypsy moth treatment was in 1979 with Dimilin. The intended target of the spray application is the gypsy moth caterpillar. Any other organism adversely affected is referred to as a "nontarget." Nontarget effects may be direct or indirect. Direct effects result in death or reproductive failure of unintended insects in forest canopies, litter, soil, or water. Indirect effects are seen up the food chain as invertebrate parasites (e.g., parasitic flies and wasps), predators (e.g., ground beetles), and vertebrate predators such as birds and bats suffer from a reduced food supply.
How do Bt and Dimilin affect arthropods?
The Bt variety used against gypsy moth (Bt kurstaki) is toxic primarily to caterpillars, larvae of Lepidoptera. The alkaline nature of the caterpillar digestive system dissolves crystalline toxins and activates spores of Bt, releasing proteins that disrupt the gut. Dimilin is characterized as an insect growth regulator; specifically, it interferes with synthesis of chitin, an essential component of the arthropod exoskeleton. This insecticide has a broader action against immature insects and their relatives during molting periods when chitin degradation and synthesis normally occur. Generally, both Bt and Dimilin need to be ingested by insects as they feed.
Are Bt and Dimilin equally detrimental to nontarget arthropods?
No. Although the various Bt toxins have been shown to be lethal to a wide variety of insects, the Btk variety used in gypsy moth suppression is lethal primarily to lepidopteran larvae (Boberschmidt et al. 1989). In Oregon, Miller (1990) observed reduction in species richness (number of species) and in abundance of caterpillars following three applications of Btk. within a single year for eradication of gypsy moth. The treatment block was 2,000 hectares. Larval abundance recovered within three years after treatment, but species richness remained reduced. Typical spray block size in West Virginia is considerably smaller. In studies on 20-hectare plots in this state, Sample et al. (1996) and Butler et al. (1995) found Btk to significantly reduce nontarget caterpillar abundance and richness for several weeks after treatment. Btk typically does not remain active on treated foliage past 10 days. The effect did not carry beyond the treatment year.
Dimilin is a more broad-spectrum insecticide affecting several types of arthropods, not just caterpillars. Various nontarget aquatic arthropods are affected by Dimilin (Bradt and Williams 1990; Hansen and Garton 1982; Swift et al. 1988). Dimilin persists on treated foliage until leaf drop in the fall and then moves into the litter layer and forest streams (Wimmer 1995). Martinat et al. (1988) found Dimilin impact in the forest canopy on chewing herbivores including macrolepidopterous and microlepidopterous caterpillars and sawfly larvae. In a related study, Martinat et al. (1993) found a Dimilin effect in the litter layer on spiders and orthopteroids (crickets, etc.). In a recent longer study, Butler et al. (1997) found that Dimilin produced a reduction in overall arthropod family diversity and abundance, and nontarget caterpillar diversity and abundance. These reductions persisted until the end of the study, 27 months after Dimilin application. Declines also were seen on treated plots for ground beetles, camel crickets, bark lice, predatory thrips, and some sapfeeders.
Do these insecticides affect humans or other vertebrates?
When applied against gypsy moth, Bt and Dimilin have been found to have no direct effects on vertebrates with the exception of possible skin and eye irritation in some humans from inert materials in the spray formulations. No direct effects to songbirds, fish, or other wildlife have been noted. Both Bt and Dimilin produce indirect effects on some wildlife, however, through reduction in arthropods that serve as their food supply. Rodenhouse and Holmes (1992) found that Bt reduced the biomass of forest caterpillars in the spring which resulted in reduced nesting attempts by songbirds. A dietary shift and increased emigration from Bt-treated areas was noted for masked shrew (Bellooq et al. 1992), a species that feeds heavily on caterpillars. Songbirds on Dimilin-treated blocks needed increased forage time and food search area, and consumed fewer Lepidoptera larvae (Cooper et al. 1990); showed dietary shifts (Sample et al. 1993a); and had reduced fat reserves (Whitmore et al. 1993). Sample (1991) and Sample et al. (1993b) found that Dimilin reduced the number of moths available for the endangered Virginia big-eared bat. Seidel and Whitmore (1995) noted that the white-footed mouse showed dietary shift and altered adult/juvenile ratios in treated areas.
Because Dimilin suppression of nontarget arthropods persists throughout the season and over multiple years (Butler et al. 1997), indirect effects to vertebrate diets should be greater than that for Bt.
Are Bt and Dimilin equally effective against gypsy moth?
A recent article by Liebhold et al. (1996) presents excellent information on this question. The researchers used Geographic Information System (GIS) data from the Gypsy Moth Appalachian Integrated Pest Management (AIPM) project in Virginia and West Virginia to analyze the relative effectiveness of the two insecticides. They concluded that Dimilin treatments resulted in greater foliage protection and population reduction only when pretreatment egg mass counts were greater than 1,000 per hectare. Neither treatment provided foliage protection in the year following treatment, especially when treatment blocks were small or close to defoliating populations. The authors further noted that Dimilin is toxic to a wide spectrum of arthropods and has a long residual persistence in canopy and leaf litter. Presumably, these undesirable effects would be exacerbated when Dimilin is applied over large blocks (Liebhold et al. 1996).
Because of concern over nontarget effects, Canada and several states including New Jersey and Pennsylvania have stopped using Dimilin and rely solely on Bt for gypsy moth suppression (USDA 1996). In 1996, 60 percent of the acreage treated under the cooperative suppression program in Maryland was with Bt; 29 percent in Virginia, 88 percent in Delaware, and 5 percent in West Virginia (USDA 1996).
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Transfer Handb.
FHM-NC-05-95.
