Threats to Monarchs from Pesticides

There are three types of pesticides. Within each category there are some that are natural and other that are synthetic, or chemical.

Insecticides kill insects by attacking their nervous systems, hormones, or protective coatings. Among the most harmful synthetic insecticides are the neonicotinoids such as imidacloprid (e.g., Merit). Some natural insecticides, such as neem oil, can be harmful to monarchs whereas others, like kaolin clay and insecticidal soap, are unlikely to cause harm.

Herbicides kill unwanted plants like weeds. They can also be very dangerous to insects and other animals (like humans). Examples of particularly dangerous chemical herbicides are DDT, 2,4-D (e.g., Weed-B-Gon) and glyphosate (e.g., Roundup). Some natural herbicides generally considered safe for pollinators are clove and citrus oil.

Fungicides prevent or treat disease caused by fungi like powdery mildew or rust. Examples of natural fungicides include sulphur and potassium carbonate. Examples of synthetic ones are myclobutanil (e.g., Immunox) and clorothalonil (e.g., Daconil).

A Brief History of Various Approaches to Controlling Pests

For thousands of years, human have used natural substances such as sulphur to kill insects attacking their crops.

All that changed in the 1940s, when chemical pesticides -- also called synthetic pesticides – were developed and became widely used on agricultural crops. Harmful to a wide array of plants and animals other than the target species, synthetic pesticides are also known for their persistence, often lingering in soil, water, and living organisms for decades.

Concerns about the harmful effects of synthetic pesticides quickly arose, and in the following decade scientists developed an approach called “supervised insect control,” which advocated supporting natural-enemy populations in an area and using synthetic pesticides only as a last resort.

Throughout the 1960s, this approach was further refined, resulting in Integrated Pest Management (IPM), an approach that is widely used in many agricultural and open spaces as well as in urban areas.

What is Integrated Pest Management (IPM)

The basic principles of IPM include taking steps to ensure that reliance on pesticides is a last resort, including:

  • modifying habitat to make it less suitable for pests

  • introducing pest-specific predators, parasites, herbivores, or diseases

  • introducing pest-resistant crop varieties in agricultural settings

While IPM is generally viewed as a positive approach, the sharp growth of the world population continues to pose a challenge to the competing goals of attaining housing and food security while managing the resulting loss of biodiversity and degradation of ecosystems resulting from habitat loss and pesticide use (Lamichhane, 2017).

How do federal, county, and city/town governments reduce the harmful effects of pesticides in Marin?

Areas managed by the federal government adhere to the IPM approach. IPM is cited as a governing principle with the within the Golden Gate National Recreation Area and Point Ryes National Seashore, for example. Herbicides are used as little as possible, and mostly for removing invasive non-native plants. Similarly, insecticides are rarely used and typically involve only targeted approaches such as wasp sprays in public access areas.  An exception to this approach, the controversial use of glyphosate in the ongoing Coastal Dunes Restoration Project was adopted only after years of attempting other methods for eliminating iceplant and beachgrass had failed.

Marin County has endorsed the IPM approach and gone further by banning the use of glyphosate and other toxic pesticides except in very special circumstances. The ban applies to county-owned and county-maintained properties, like county parks, open spaces, buildings, and roadsides.

Each city or town controls its own pest-management policies in its own buildings, parks, and roadside spaces. The city with the strongest anti-pesticide policies is Fairfax, which prohibits the use of all synthetic pesticides in parks, open space parcels, public rights-or-way, and town owned and maintained buildings. Others that have banned glyphosate include Novato, San Rafael, Larkspur, and Mill Valley.

 Use of Pesticides in Agriculture

The overwhelming majority of pesticides are used in agriculture. Not surprisingly, given that XXX% of the land in the US that is not open space, protected land, with housing and urban areas occupying the other XXX%

Non-agricultural uses, including domestic and industrial applications, represent only 10-15 percent of the global market by value.

In spite of these regulations, the effects of pesticides used in agricultural settings are felt in wild and urban spaces.

Pesticides can be absorbed into the soil where they may move below the surface into creeks, wetlands and riparian habitats.

They can also run off the soil surfaces, ending up in storm drains connected to urban areas and waterways in wild areas.

Some pesticides bind to soil particles and move downslope into wetlands.

And they can drift as dust particles or tiny droplets into neighboring habitats.

Look at the Marin Countywide Stormwater Pollution Prevention Program website for more information about pesticides in Marin’s waterways.

Does pesticide use from agricultural spaces affecting monarchs?

Even though Monarch butterflies do not typically congregate in agricultural spaces, they are nevertheless vulnerable to the poisonous effects of inappropriate use of pesticides in these areas. As noted earlier, pesticides applied in agricultural spaces can easily move into wild areas and home gardens. Recent research has demonstrated the presence of a variety of pesticides on milkweed in urban, wild, and agricultural spaces in Northern California.

For example, in a study conducted in the Central Valley of California researchers collected milkweed from 19 sites representing different land use types and tested for the presence of one or more pesticides on every plant. The sampling occurred during June, when monarch larvae were likely to be present on the milkweed. They found a variety of pesticides on every sample, even at sites with little or no direct pesticide use. Pesticides were detected on every land type, and at least some of the contaminants from the agricultural field had spread to the home gardens and wild areas. The authors conclude that their findings support the idea that pesticide exposure from agriculture could be a contributing factor to Western monarch declines, even though monarchs are rarely found on agricultural lands.

 

Neonicotinoids: An Example of a Particularly Harmful Insecticide

Neonicotinoids have been banned in the European Union since 2018. While the US federal government has not restricted their sale, a number of states have moved independently to do so for residential, non-agricultural purposes, including California, Colorado, Nevada, Maryland, Connecticut, Maine, Vermont, Massachusetts, New Jersey, Minnesota, Washington, New York, and Rhode Island. They continue to be used extensively in other states.

Some states also restrict agricultural use, such as Vermont, which banned neonicotinoid-treated seeds for certain crops. 

California banned over-the-counter sales of lawn and garden neonicotinoid pesticides on January 1, 2025, but their use is still permitted on land used for agriculture by licensed pesticide applicators.

A persistent risk to monarchs comes from neonicotinoids, or neonics, a class of insecticides widely used in agriculture and landscaping. Monarchs near agricultural land are often exposed to neonics when crops are sprayed with this substance. While initially marketed as less harmful than other insecticides, neonics are now known for their devastating impacts on pollinators and beneficial insects. Research on bees reveals that neonics affect the brain and reproductive system, deplete their energy, and prevent them from expelling parasites.

Use of Integrated Pest Management in home gardens and nurseries

Nurseries and home gardeners using IPM prioritize the following actions prior to considering chemical controls:

  • monitoring for pests

  • identifying pest thresholds before treating

  • use of biological controls such as predatory insects

  • use of “cultural” controls like spacing, sanitation, and resistant cultivars

  • use of mechanical controls (like washing and pruning)

Which nurseries in Marin are likely to follow IPM?

Adherence to IPM varies significantly from one nursery to another. Evidence suggests that the highest adherence can be found in local nurseries that are devoted exclusively to native plants and that have certifications or sustainability programs.

The following are some of the nurseries in Marin that focus on California native plants and neither use pesticides on their stock nor advocate its use for purchased plants.

Home Ground Habitats: 1875 Indian Valley Road, Novato

California Native Plant Nursery: 254 Shoreline Hwy, Mill Valley

O’Donnell’s Fairfax Nursery: 1700 Sir Francis Drake Blvd, Fairfax

Most other independent retail nurseries and conventional garden centers are likely to use parts of IPM but may not always have a firm policy of using chemical pesticides as a last resort.