negative impact, if any, on the population densities of monarch butterflies is yet unknown, they have focused attention on the question of whether widespread dispersal of insecticidal pollen can occur if entire regions are planted with wind-pollinated Bt plants (such as corn, poplar, and pine). It should be noted, however, that some Bt corn cultivars apparently do not produce any Bt in their pollen (EPA 1998b). In those cases, Bt toxin would not be a potential hazard to nontargets even if pollen dispersed great distances.

One recent field test indicates that at least 500 Bt pollen grains per square centimeter is necessary to sicken monarch caterpillars and that milkweed plants growing adjacent to corn fields had only an average of 78 grains per square centimeter (Kendall 1999). Eighty-eight percent of milkweed within one meter fell below the level of toxicity to caterpillars. On the other hand, laboratory tests by Hansen and Obrycki (1999b) demonstrated that when monarch caterpillars consumed milkweed leaves experimentally dusted with 135 grains per square centimeter of pollen (comparable to pollen concentrations they found in the field), there was 46% mortality if the Bt pollen source was insertion event Bt11 and 65% if the source was event 176. Moreover, research suggests that wind direction, rainfall, and other factors can significantly affect pollen concentration (Weiss 1999). Further field-based research is needed to determine whether dispersed Bt pollen could have detectable effects on the population dynamics of nontarget organisms.

2.6.3 Indirect Effects

Studies in which predators were fed insects that developed on pest-protected cultivars have often produced adverse effects on the predator. For example, the compound DIMBOA is found in lepidopteran-resistant grain crops, including corn. Ladybugs that consumed aphids that had fed on diets containing DIMBOA had slower development (Martos et al. 1992). The tobacco hornworm parasitoid Cotesia congregata is adversely affected when the hornworm feeds on tobacco or artificial diets with high nicotine concentrations (Thorpe and Barbosa 1986). Cucumbers that are resistant to spider mites typically have higher concentrations of the triterpenoid cucurbitacin-C than nonresistant cultivars (DaCosta and Jones 1971; Gould 1978). Spotted cucumber beetles that feed on the resistant cultivars sequester cucurbitacin-C in their bodies and eggs (Ferguson and Mecalf 1985). The cucurbitacin-C in their eggs keeps the eggs from being killed by the entomopathogenic fungus Metarhizium anisopliae (Tallamy et al. 1998).

Although a pest-protected plant could have adverse effects on beneficial insects in the agroecosystem, the value of the cultivar is generally



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