Assessing the impact of Cry1Ab-expressing corn pollen on monarch butterfly larvae in field studies

Abstract

Survival and growth of monarch larvae, Danaus plexippus (L.), after exposure to either Cry1Ab-expressing pollen from three Bacillus thuringiensis (Bt) corn (Zea mays L.) events differing in toxin expression or to the insecticide, lambda-cyhalothrin, were examined in field studies. First instars exposed to low doses ( approximately 22 grains per cm(2)) of event-176 pollen gained 18% less weight than those exposed to Bt11 or Mon810 pollen after a 5-day exposure period. Larvae exposed to 67 pollen grains per cm(2) on milkweed leaves from within an event-176 field exhibited 60% lower survivorship and 42% less weight gain compared with those exposed to leaves from outside the field. In contrast, Bt11 pollen had no effect on growth to adulthood or survival of first or third instars exposed for 5 days to approximately 55 and 97 pollen grains per cm(2), respectively. Similarly, no differences in larval survivorship were observed after a 4-day exposure period to leaves with 504-586 (within fields) or 18-22 (outside the field) pollen grains per cm(2) collected from Bt11 and non-Bt sweet-corn fields. However, survivorship and weight gain were drastically reduced in non-Bt fields treated with lambda-cyhalothrin. The effects of Bt11 and Mon810 pollen on the survivorship of larvae feeding 14 to 22 days on milkweeds in fields were negligible. Further studies should examine the lifetime and reproductive impact of Bt11 and Mon810 pollen on monarchs after long-term exposure to naturally deposited pollen.

Figure 1

Survival of first-instar monarch larvae feeding on milkweeds placed at 1, 3, 5, and 10 m inside (negative values) and outside the field edge of event-176 field corn in Maryland. Means ± SE are based on separate bioassays conducted on days 3, 6, 9, and 14 of anthesis. Within each graph, columns with the same letters are not significantly different (P < 0.05; Tukey's test).

Figure 2

Weight gain of first-instar monarch larvae feeding on milkweeds placed at 1, 3, 5, and 10 m inside (negative value) and outside the field edge of event-176 field corn in Maryland. Means ± SE are based on separate bioassays conducted on days 3, 6, 9, and 14 of anthesis. Within each graph, columns with the same letters are not significantly different (P < 0.05; Tukey's test).

Figure 3

Survival and weight gain of first-instar monarch larvae feeding on milkweeds placed at 3 m inside and outside the edge of plots consisting of Bt11, non-Bt (untreated), and non-Bt (λ-cyhalothrin-treated) sweet corn in Maryland. Data are based on separate bioassays conducted at 3, 6, and 9 days of anthesis. Within each graph, columns with the same lowercase letters are not significantly different for the interaction effect; pairs of columns with the same uppercase letters are not significantly different for the treatment effect (P < 0.05; Tukey's test).

Figure 4

Survival curves for monarch larvae placed in and near Bt and non-Bt corn fields. (a) Iowa. (b) New York.