What does data show regarding risks to monarch butterflies from Bt crops?

SouravNovember 8, 2024

Answer

The data regarding the risks to monarch butterflies from Bt (Bacillus thuringiensis) crops, particularly Bt corn, presents a complex picture. Here’s a summary of key findings from various studies:

Low Risk from Bt Corn Pollen

1. Limited Exposure: Research indicates that the risk posed by Bt corn pollen to monarch butterfly larvae is generally low. Studies have shown that the concentrations of Bt corn pollen that would need to be present on milkweed leaves (the exclusive food source for monarch caterpillars) to cause harm are rarely found in natural settings. Specifically, levels exceeding 1,000 grains of pollen per square centimeter are needed to have any significant negative impact, which occurs less than 1% of the time in the environment.
2. Pollen Dynamics: The mobility of corn pollen is limited; it tends to drop quickly after release due to its adaptation for wind pollination. This means that milkweed plants located some distance from cornfields are less likely to be contaminated with harmful levels of Bt pollen. Additionally, studies have shown that the timing of pollen release often does not overlap significantly with the timing of monarch egg-laying, further reducing potential exposure.

Laboratory Findings vs. Field Conditions

3. Laboratory Studies: Initial laboratory studies suggested that when monarch caterpillars were fed milkweed leaves dusted with high concentrations of Bt corn pollen, they exhibited reduced growth rates and increased mortality compared to those fed non-transgenic pollen or pollen-free leaves. However, these conditions were artificial and did not accurately reflect typical environmental exposures.
4. Field Studies: Follow-up field studies have demonstrated that under natural conditions, the impact of Bt corn on monarch larvae is negligible. For instance, research has shown no significant effects on black swallowtail larvae feeding near Bt corn fields, suggesting that similar outcomes may apply to monarchs.

Broader Environmental Context

5. Habitat Loss and Pesticide Use: The decline in monarch populations is more significantly attributed to habitat loss and widespread pesticide use rather than exposure to Bt corn. Monarchs face threats from reduced milkweed availability due to herbicide-tolerant crops and other agricultural practices that eliminate milkweed in fields. The overall impact of Bt crops appears minimal compared to these larger environmental challenges.
6. Conservation Considerations: While some studies indicate potential risks from specific strains of Bt corn, the consensus among researchers is that these risks are minor when viewed in the context of other factors affecting monarch populations, such as habitat destruction and climate change