Impacts of Insecticide Use on Monarchs
Most insecticides (and all other pesticides, including herbicides, fungicides, and nematocides) are used in agricultural applications. The widespread loss of milkweed in agricultural fields reduces the risk of immature monarchs (eggs, larvae, pupae) being killed by agricultural insecticide applications simply because, without milkweed, these stages no longer occur in high numbers within these fields. Adult monarchs traveling across agricultural fields in search of milkweed or nectar during times of insecticide application are at higher risk, however. Insecticide use in agriculture is a concern for other pollinators that forage for pollen and nectar in agricultural landscapes. Additionally, insecticide drifts into ditches, and field borders can affect monarchs. Insecticide use by commercial and government entities (to control herbivores and pests like mosquitoes and black flies), as well as in yards and gardens, often kills monarchs.
One group of insecticides that is raising concern is neonicotinoids, which are used on farms and around homes, schools, and city landscapes. While harm to humans and other mammals is minimal, these insecticides are extremely toxic to arthropods. They are systemic, meaning that when they are applied, plants absorb and distribute the compounds to all parts of the plant, making the leaves, nectar, pollen, and woody tissue toxic to insects and other arthropods that feed on them. A variety of application methods make neonicotinoids popular for use in pest control. Crop seeds can be treated before being planted, allowing uptake by the plant during growth and thus protection from plant pests for a period of time while the chemical remains in the plant tissues. Neonicotinoids can also be applied topically on plant foliage or as drenches to the ground.
Pollinators and other insects exposed to neonicotinoids* while foraging face lethal or sublethal effects. As treated crop seeds are planted, particles of neonicotinoid compounds are often carried with dust and settle onto nearby vegetation; this can cause direct mortality in bees. Additionally, pollinators can be directly exposed to these chemicals if they are foraging at the time when crops, garden plants, or natural areas are being sprayed with insecticide. A further concern with neonicotinoids arises because they persist in the soil and plants much longer than other compounds, making them dangerous to pollinators for a longer period of time after the initial application. Because they are systemic, nectar and pollen gathered from treated plants are contaminated. While this may not be a lethal dose for bees, sublethal effects such as the decreased ability to locate food sources or their hive may impact the productivity of the colony. Compounds that are not absorbed by the plants remain in the soil for extended periods of time and often leach into the groundwater or run off into natural water bodies.
Neonicotinoids and other insecticides, like organophosphates, carbamates, and insecticidal soaps that are often used in plant nurseries, can have a negative impact on pollinators. High plant density and a controlled temperature environment can foster insect pests that damage the plants being grown in a greenhouse or nursery. For this reason, nurseries (even those growing plants specifically for native plant gardens with the purpose of attracting and benefiting pollinators!) often resort to the use of insecticides to control unwanted insect pests. Because some insecticides persist in the plant tissues for months after the initial application in the greenhouse, nursery plants that have been treated with systemic insecticides pose an ongoing risk to pollinators. It is important to determine if plants have been treated before purchasing and planting them.
In urban and suburban areas, adult and larval mosquito populations are controlled vigorously and repetitively. Some species of milkweed grow in areas likely to be treated for mosquitoes, thus increasing the risk of monarch exposure to these chemicals. The University of Minnesota conducted research on how monarch larvae and adults were affected by exposure to insecticides commonly used in mosquito control (resmethrin and permethrin). These pyrethroids can be sprayed as ultra-low volume treatments or as barrier treatments. Ultra-low volume treatments are intended to affect insects as they are flying, whereas the barrier treatments remain on leaves, providing a barrier to mosquitoes that may not be out foraging during the day. Both the ultra-low resmethrin study and the barrier permethrin study showed negative impacts on monarch larvae and adults. Leaves from the barrier treatments resulted in higher mortality to monarch larvae than control leaves up to 3 weeks after the initial application. These insecticides do have harmful effects on monarchs if exposed, but population-level impacts will depend on the proportion of host plants treated in a given landscape or how much of the monarch population is directly exposed.
*Neonicotinoids include imidacloprid, clothianidin, thiamethoxam, acetamiprid, and dinotefuran. Look for these in any garden products that you purchase.
For more information:
Risks of Neonicotinoid Use to Pollinators, MJV Fact Sheet
Understanding Pesticides & Their Risks, Xerces Society
How Neonicotinoids Can Kill Bees, Xerces Society Report
Xerces Society IPI Database: Summaries of Research Articles on Pesticides & Invertebrates