The Impacts of Neonicotinoid Exposure in Monarch Butterflies

Monarch butterflies have captivated the public and scientists for years. Their lengthy annual migrations across states and countries have been a focal point for research. Exclusively feeding on milkweed plants, monarchs evolved to sequester cardenolide toxins from the plants that are converted into a predator defense mechanism. However, because of their long distance travels and diets, scientists now question whether neonicotinoid exposure is the cause for population decline. 

Neonicotinoids are among the most widely used North American insecticides. Despite their widespread use, these pesticides do not discriminate between targeted and beneficial insects   such as bees. Unfortunately, the chemical compounds that make up neonicotinoids are known to linger in the environment for months, and they can have half-lives that last hundreds of days without UV exposure. Although used to target specific insect pests, approximately 5% of the applied neonicotinoids are taken up by the plant, leaving the remainder of the application to leech into the environment and pose serious environmental problems. Since milkweed plants are common in agricultural fields, neonicotinoid use has been suggested to negatively impact monarch health and population density. 

In a new two-part study led by CEID researcher Sonia Altizer, monarch development, flight, and survival were measured to assess the impacts of two types of neonicotinoid exposure. Altizer and her colleagues also examined these effects across varying levels of cardenolide exposure from three different milkweed species: A. incarnata, A. syriaca, and A. curassavica.  

Captive monarch caterpillars were fed low levels of neonicotinoids, and no significant impacts on pupal development, survival, and adult flight from exposure were observed. Secondly,  monarch larvae fed milkweed leaves painted with significantly higher neonicotinoid levels were raised on low (A. incarnata), moderate(A. syriaca), and high (A. curassavica) levels of cardenolides. 

Monarchs exposed to the highest levels of neonicotinoids were found to have on average smaller body size, deformities, lower levels of survival, and weakened strength. However, these effects were most evident on monarchs that were fed the lowest level cardenolide milkweed plants while monarchs reared on the higher level plants showed no significant effects from the neonicotinoids on any variable tested. This is the first study to the researcher’s knowledge that illustrates how the species of host plants can potentially minimize the effects of neonicotinoid exposure. 

From this study, researchers were able to determine that neonicotinoids applied at common application levels do not significantly affect the breeding populations of monarch butterflies, and neonicotinoids may be less relevant to migration patterns than previously thought. For more information about this study please click here.