Monarch butterflies are in urgent need of conservation, which is why the U.S. Fish and Wildlife Service recently proposed to list the monarch as a threatened species under the Endangered Species Act (ESA). This designation could provide vital resources and protections needed to support their recovery. However, even with ESA listing, many challenges remain — in particular, the pervasive threat of pesticides. While we may associate pesticides with agricultural landscapes, their impact in homes, parks and urban areas shouldn’t be overlooked. 

A mysterious dieoff strikes the Pacific Grove Monarch Sanctuary

On January 25, 2024, volunteers stumbled upon a devastating scene: scores of dead and dying monarch butterflies scattered across the lawn of a private property adjacent to the Pacific Grove Monarch Sanctuary overwintering grove in California. While volunteers periodically encounter both live and the occasional dead monarch on the ground near the grove, several details about this incident struck them as unusual. 

Most notably, the dying butterflies were spasming, a symptom commonly observed in response to pesticide poisoning. The number of butterflies involved was also alarming, as approximately 200 out of the nearly 2,000 monarch butterflies overwintering in the sanctuary at the time were affected. While some of the monarchs were scattered across the lawn, the majority were grouped in several piles parallel to the edge of a nearby building. These grouped butterflies showed no signs of predation or rodent caching, suggesting that something else was responsible for their unusual positioning.

Xerces’ partners at the Pacific Grove Museum of Natural History immediately reached out to inform us about the incident, and in coordination with the California Department of Fish and Wildlife (CDFA), the United States Geological Service (USGS) and Pacific Grove city staff, we decided that a sample of the deceased butterflies should be collected for pesticide residue testing.

The analysis by USGS revealed that the monarchs had been exposed to a variety of pesticides including multiple insecticides, fungicides, and herbicides. On average, each butterfly contained residues of 7 different pesticides in their bodies. Three extremely toxic pyrethroid insecticides were each found at or near lethal levels in the tested butterflies. These findings confirmed our suspicions that pesticide poisoning was likely the cause of the butterflies' deaths.

With these results in hand, Xerces staff and partners filed reports to the U.S. Environmental Protection Agency (EPA), CDFA, city officials in Pacific Grove, and the Monterey County agricultural commissioner. In April 2024, the County Agricultural Commissioner launched an investigation into the source of the pesticides; however, they were unable to identify any reported pesticide applications that could have triggered the incident.

Given the timing, location, and profile of pesticides detected, it is likely that the monarch’s deaths were caused by an unreported or untraceable pesticide application by a local resident or business.
 

Many urban pesticide applications go unreported

Although California has one of the most stringent and detailed pesticide reporting systems in the world, there are still a variety of pesticide applications that go unreported. Homeowners and other unlicensed pesticide users do not have to report their pesticide use, and applications made by licensed commercial pesticide users, such as landscape maintenance or structural pest control, are only reported at the county level. Without detailed data on these applications, it is difficult to trace incidents like the one in Pacific Grove, or to understand the full picture of urban pesticide risks to monarchs. 
 

Raising awareness to prevent future monarch pesticide deaths

At Xerces, we are committed to preventing future incidents like this from occurring. We are collaborating with overwintering site managers to educate local residents about the risks urban pesticide use poses to monarchs. We are also taking steps to ensure that policymakers, regulators, and other key interest groups understand these risks and address them in conservation and recovery plans for monarchs and other imperiled species.

To raise awareness of this incident among the research community, we have submitted a peer-reviewed scientific manuscript, which is currently under review. We have also petitioned the EPA to strengthen their pesticide risk assessment framework by incorporating tests for bumble bees, solitary bees, butterflies, and moths. Additionally, we are collaborating with the Western Association of Fish and Wildlife Agencies (WAFWA) to update their Western Monarch Conservation Plan. The updated plan will detail strategies for protecting monarchs from pesticides across a variety of landscapes, including urban spaces.

USFWS is currently seeking public comments on the proposed listing for the monarch butterfly. You can help advocate for monarch protections by signing on to Xerces’ letter to USFWS, or by submitting your own comment to USFWS.
 

How to protect monarchs from pesticides at home

While it is especially important for residents living near monarch overwintering sites to be cautious with pesticide use, it is also crucial for residents across North America to take steps to protect this iconic and imperiled species from harmful pesticides.

Here are some actions you can take at home to protect monarchs and other pollinators from pesticide exposure:

• Buy native nectar and milkweed plants that are grown without long-lived, persistent insecticides (such as neonicotinoids), and do not apply pesticides to these plants.
• Reconsider whether pesticides are truly necessary. Accept that some ‘pest’ activity is a normal part of a healthy ecosystem.    
• Avoid preventative pesticide treatments, foster habitat for natural enemies, and use nonchemical prevention methods whenever possible.
• Do not apply pesticides to open flowers or when monarchs and other pollinators are active.  
• Keep in mind that organic pesticides are not necessarily safe for monarchs and other pollinators. For example, the organic pesticide, Bacillus thuringiensis (Bt), is highly toxic to monarchs and other butterflies.
• Remember that pesticides applied to impervious surfaces (such as driveways, walkways, patios, and building exteriors) can wash into vegetated areas and contaminate plants and soil. 
• Be aware that pesticides used to control “household” pests such as termites, ants, cockroaches, and spiders can also be deadly to pollinators and other beneficial insects.
• If pesticide use is necessary, always choose the least toxic chemical and most targeted application method possible to reduce exposure risks. For example, if you have an ant infestation, try using ant bait stations rather than a spray.