Best Management Practices

Monarchs can be exposed to pesticides during and after pesticide applications. During application, monarch larvae and adults may be exposed through direct spray or drift. After application, pesticides can remain as residue on the surfaces of leaves and flowers, and systemic pesticides can become incorporated into plant tissues, such as leaves and nectar. Larvae may be exposed when they feed on contaminated leaves and adults when they nectar on contaminated flowers. Most studies about effects of pesticides to lepidopterans (the order of insects that include butterflies) have focused on insecticides, as they target insects; however, some research suggests that herbicides may also have lethal and sublethal effects on lepidopterans. In addition to exposure to chemicals, herbicide use can affect monarchs by reducing milkweed and nectar plant availability. 

Find more information about the effects of pesticide on monarchs in the U.S. Fish and Wildlife Service’s Supplemental Materials 1a for the Monarch (Danaus plexippus plexippus) Species Status Assessment Report.

Best Management Practices to minimize the effects of pesticides on monarchs:

• Implement other management techniques as part of an Integrative Pest Management plan to minimize the use of pesticides, especially insecticides.

Learn about IPM at the University of Arizona’s Arizona Pest Management Center.

• Minimize drift by using targeted application methods and equipment and by avoiding windy conditions and temperature inversions. 

Find more details in the “Drift reduction” section in NRCS’s Preventing of Mitigating Potential Negative Impacts of Pesticides on Pollinators Using Integrated Pest Management and Other Conservation Practices.

• Choose pesticides with less toxic active ingredients and shorter residual times. 

The University of California’s Statewide Integrated Pest Management Bee precaution pesticide ratings tool rates pesticides based on their potential toxicity to honeybees, which may be used as guidance for other insects such as monarchs.

• Choose less toxic pesticide formulations (e.g., granular, water-based liquids, and dry flowable formulations).

Find more details in the “Choice of formulation” section in NRCS’s Preventing of Mitigating Potential Negative Impacts of Pesticides on Pollinators Using Integrated Pest Management and Other Conservation Practices.

• Avoid applying pesticides in or adjacent to areas with blooming flowers. 

Apply herbicides during young plant phases, when plants are more responsive to treatment, and when monarchs and other pollinators are less likely to be nectaring on the plants. If you must apply during blooming, mow the plants prior to application (take care not to mow milkweed when monarchs might be present).

• Avoid applying pesticides when monarchs (caterpillars or adults) are likely to be present. 

Refer to the Southwest Monarch Study's peak fall migration dates and the Xerces Society's recommended management timing for monarch breeding habitat to plan around active times in Arizona. Find more timing considerations in the “Application timing” section in NRCS’s Preventing of Mitigating Potential Negative Impacts of Pesticides on Pollinators Using Integrated Pest Management and Other Conservation Practices.

• Create buffers between areas of milkweed or nectar plants and areas where pesticides are applied. 

Separate habitat areas from areas receiving broadcast pesticide applications with a pesticide-free spatial buffer and/or evergreen vegetative buffer of coniferous, non-flowering trees to capture chemical drift. The appropriate monarch and pollinator habitat spatial buffer size is contingent upon several factors, including weather and wind conditions, but at a minimum, the habitat should be at least 40 feet from ground-based pesticide applications, 60 feet from air-blast sprayers, and 125 feet from any systemic insecticide applications or seed-treated plants. Find more details in the “Create drift barriers” section in NRCS’s Preventing of Mitigating Potential Negative Impacts of Pesticides on Pollinators Using Integrated Pest Management and Other Conservation Practices.

Prescribed fire is an important tool for land managers and can help maintain open landscapes. Fires can increase the abundance and diversity of plants available for pollinator species. A large amount of research has looked at using prescribed fire as a tool to improve habitat for butterflies. In the east, scientists have found monarchs to be positively correlated with the nectar resources that followed a prescribed burn. However, there has been little research focused on the topic of fire and the impacts it has on monarchs and milkweed in the western United States. Prescribed fire has the potential to improve habitat for monarchs by increasing plant abundance and diversity after a burn. Nevertheless, if land managers are not careful with the fire intensity, timing, scale, and location, prescribed fire can be detrimental to monarchs and other pollinators as it can take two decades for the insect communities to recover from a burn.

Find more information about research pertaining to the effects of prescribed fire on monarchs in The Xerces Society's Managing for Monarchs in the West.

Best Management Practices to minimize the effects of prescribed burning on monarchs:

• Manage the location and scale of fires to increase habitat heterogeneity within and between sites.

Leave at least 1/3 of the area unburned until burned areas green up, and avoid burning small, isolated habitat fragments. To protect other pollinators, plan unburned areas to create refugia within dispersal distance of pollinator species of concern.

• Allow 5 years between burns in areas of documented pollinator habitat and do not burn any area twice in the same year.

• Avoid burning when monarchs are likely to be present.

Refer to the Southwest Monarch Study's peak fall migration dates and the Xerces Society's recommended management timing for monarch breeding habitat to plan around active times in Arizona.

• If you must burn while monarchs or other pollinators are in your area, consider burning during the warmer part of the day when adult butterflies and other pollinators are more active and more likely to escape.

• Time and site pile burning to avoid the active times and areas for monarchs and other pollinator species of concern.

• Include native nectar plants and milkweeds when restoring burned areas.  

The above BMPs come from the Xerces Society's Managing for Monarchs in the West and Pollinator Partnership's Managing Public Lands for Pollinators.