Monarch larvae sensitivity to Bacillus thuringiensis- purified proteins and pollen

Area of Science:

• Environmental Toxicology
• Entomology
• Genetically Modified Organisms

Background:

• Concerns exist regarding the potential impact of Bacillus thuringiensis (Bt) crops on non-target organisms.
• Monarch butterflies are a key non-target species for assessing Bt corn pollen toxicity.

Purpose of the Study:

• To determine the relative toxicity of specific Bacillus thuringiensis (Bt) toxins and Bt corn pollen to monarch butterfly larvae.
• To evaluate the influence of pollen processing contaminants on toxicity assays.

Main Methods:

• Laboratory bioassays using purified Bt toxins (Cry1Ab, Cry1Ac, Cry9C, Cry1F) incorporated into artificial diets.
• Pollen bioassays with Bt corn pollen applied to milkweed leaf discs.
• Assessment of larval survival and weight gain across different treatments.

Main Results:

• Purified Bt toxins Cry9C and Cry1F showed low toxicity to monarch larvae, while Cry1Ab and Cry1Ac were toxic to early instars.
• Older monarch instars exhibited significantly lower susceptibility to Cry1Ab toxin.
• Pollen contaminants from processing artifacts caused significant negative impacts on larval survival and weight, potentially yielding spurious results.
• Only pollen from Cry1Ab event 176 Bt corn hybrids consistently affected monarch larvae; this event represents a small percentage of current corn cultivation.

Conclusions:

• Pollen from most current Bt corn events (Cry1Ab events Bt11 and Mon810, Cry1F, and experimental Cry9C) is unlikely to cause acute adverse effects on monarch larvae in field conditions.
• Pollen processing methods can introduce confounding factors into toxicity assessments.
• The specific Bt event and potential contaminants are critical factors in evaluating Bt corn's impact on monarch butterflies.