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Daily News Blog

23
Apr

Monarch Butterfly Larvae Adversely Affected by Pesticide Drift from Contiguous Soybean and Maize Crop Fields

(Beyond Pesticides, April 23, 2020) Pesticide spray drift from adjacent farmlands expose butterfly larvae to lethal pesticide concentrations, according to research published in Environmental Toxicology and Chemistry by Iowa State University (ISU). Lack of previous experimental pesticide toxicity data makes it unclear as to what degree insecticides impact monarch butterfly (Danaus plexippus) productivity in milkweed (Asclepias spp.) habitats near pesticide-treated pasture. This study adds weight to the idea that pesticides are playing a role in the ongoing decline of this iconic butterfly, as researchers find insecticide drift from adjacent fields to be strongly associated with larval mortality. Future monarch butterfly conservation efforts should consider risks stemming from pesticide exposure when developing butterfly rehabilitation efforts, according to advocates. As co-author Niranjana Krishnan (ISU graduate student) states, “In order to make the best decisions about how and where to plant milkweed, we first need to find basic toxicity and exposure data.” 

ISU researchers established monarch butterfly colonies by collecting larvae from roadside milkweeds, which they then reared in the laboratory for incubation. To analyze the relative toxicity of various insecticides on monarch butterflies, researchers applied normal field-application rates of each pesticide at different larval development stages. Scientists used a bioassay to measure the cellular concentration of foliar insecticides (insecticides applied directed to plant leaves) exposed to monarchs by cuticular (dermal) or dietary means. They chose five common active chemical ingredients in foliar insecticides used on soybean and corn crops: beta‐cyfluthrin (pyrethroid), chlorantraniliprole (anthranilic diamide), chlorpyrifos (organophosphate), and imidacloprid and thiamethoxam (neonicotinoids). Researchers compared the bioassay’s concentration-response curves to AgDRIFT computer-modeled pesticide spray data to determine in-situ larval mortality rates downwind from treated fields.

Results of the study found that dermal and dietary exposure to beta-cyfluthrin and chlorantraniliprole was most toxic to monarchs, and resulted in high levels of larvae stasis and mortality. Notably, neonicotinoid exposure uniquely halted monarch ecdysis (molting) and pupation from caterpillar to butterfly. ISU researchers estimated the greatest larval mortality to occur 0 to 15 meters (m) downwind of pesticide-treated soybean/maize fields. Aerial pesticide applications extended larval mortality range to 60m downwind of treated fields compared to boom pesticide spray applications. This study demonstrates that data and field-scale mortality estimates will help scientists elucidate the impacts of pesticides on monarchs and establish sustainable habitats.

The monarch butterfly is fighting an uphill battle against the harmful effects of pesticides. Sharp declines in the butterfly population present a poor prognosis for monarchs, as the U.S. Fish and Wildlife Service indicate an extinction risk of 86% within 50 years. Pesticide exposure and lodging availability are key to survival during larval development. Previous studies indicate threats to the survival of the milkweed plant monarchs rely on for lodging and larval development. Genetically engineered (GE) soybean and corn crops tolerate repeated exposure to herbicides, and researchers attribute spray drift from weed killers like glyphosate to milkweed extermination in many agricultural areas of the country. 

This study indicates that pesticide exposure indirectly affects monarch populations by interrupting larval development and success rate. The reproductive success of monarch butterflies depends on milkweed availability as monarchs require milkweed to complete its lifecycle. Milkweed acts as an obligate host for monarchs to exclusively lay their developing larvae on leaves and stems. Researchers suggest planting an additional 1.3 to 1.6 billion milkweed stalks will improve monarch butterfly resilience. Milkweed grows most successfully on agricultural land, yet the study finds that pesticide drift from adjacent maize and soybean pastures threaten the obligate host’s survival.  

Monarch butterflies need improvements in protection policies to safeguard reproductive success and decrease pesticide drift exposure. The data in this study has implications for a multitude of butterfly species and their exposure to pesticides upon the establishment of additional milkweed conservation habitats. Organic land management curtails the need for toxic agricultural pesticides. Learn more about the science and resources behind pesticides’ pollinator impact and take action against the use of pesticides. To find out more about what you can do to protect butterflies, and other pollinators, check out information on pollinator-friendly landscapespollinator-friendly seeds, and organic agriculture.

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: Iowa State University

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