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 indicates that herbicides may also have lethal and sublethal effects. 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 pesticides on monarchs in the U.S. Fish and Wildlife Service’s Supplemental Materials 1a for the Monarch (Danaus plexippus plexippus) Species Status Assessment Report.

Consider the following to minimize the effects of pesticides on monarchs:

• Implementing other management techniques as part of an Integrative Pest Management Plan minimizes the use of pesticides.

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

• Using targeted application methods and equipment and avoiding windy conditions and temperature inversions minimizes pesticide drift to non-target areas that monarchs may be using. 

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

• Choosing pesticides with less toxic active ingredients and shorter residual times reduces potential for exposure. 

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.

• Choosing less toxic pesticide formulations (e.g., granular, water-based liquids, and dry flowable formulations) reduces the potential for exposure.

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

• Applying pesticides when plants are not in bloom minimizes contamination to nectar resources. 

Plants are likely more responsive to treatment during young, non-flowering phases, which is also when insect pollinators will not be nectaring on them. Mowing blooming plants prior to pesticide application (taking care not to mow milkweed when monarchs might be present) minimizes contaminating nectar resources.

• Applying pesticides when monarchs (caterpillars or adults) are less likely to be present minimizes exposure. 

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.

• Creating buffers between areas of milkweed or nectar plants and areas where pesticides are applied minimizes drift to non-target areas that monarchs may be using. 

Habitat areas can be separated from areas receiving broadcast pesticide applications with a pesticide-free spatial buffer such as an evergreen vegetative buffer of 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.

The above conservation measures are adapted from the Arizona Department of Agriculture’s Arizona Management Plan for Pollinators, the Xerces Society’s Managing for Monarchs in the West and BMPs for Pollinators on Western Rangelands, the Pollinator Partnership’s Managing Public Lands for Pollinators, and Natural Resource Conservation Service’s Preventing or Mitigating Potential Negative Impacts of Pesticides on Pollinators Using Integrated Pest Management and Other Conservation Practices.

Mowing can kill larvae and adult monarchs when present on vegetation. In addition, mowing can negatively affect monarch habitat by reducing milkweed and flower availability or by spreading nonnative invasive plants. 

The Rights-of-Way as Habitat Working Group provides a comprehensive Resources Library for managing rights-of-way.

Consider the following to minimize the effects of mowing on monarchs:

• Timing mowing when monarchs are not likely to be present minimizes direct mortality and effects from habitat removal.

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. Arizona Department of Transportation has timing recommendations in the mowing chapter (p. 4) of their Roadside Management Guidelines, specific to the different biomes found in Arizona, that consider likelihood of monarch presence and the phenology of milkweeds and nectar plants.

• Timing mowing to allow native milkweed and nectar resources to flower and set seed promotes resources for monarchs. 

Arizona Department of Transportation has timing recommendations in the mowing chapter (p. 4) of their Roadside Management Guidelines, specific to the different biomes found in Arizona, that consider likelihood of monarch presence and the phenology of milkweeds and nectar plants.

• Mowing to create a mosaic of habitat types with areas of unmowed, natural vegetation, especially patches of milkweed and concentrations of nectar plants, leaves habitat available for monarchs to use. 

The Xerces Society’s Managing for Monarchs in the West cites several studies that have found benefits from this approach to pollinators.

• Mowing at higher heights protects younger milkweed and nectar plants.

• Mowing during mid-day when adult monarchs are more active increases the opportunity for them to fly out of the way. 

• Mowing at reduced speeds increases the opportunity for monarchs to fly out of the way. 

• Avoiding mowing when invasive weeds are in seed reduces their spread.

• Cleaning equipment when moving between sites minimize spread of invasive species.

The above conservation measures are adapted from the Xerces Society's Managing for Monarchs in the West and BMPs for Pollinators of Western Rangelands and Pollinator-Friendly Best Management Practices for Federal Lands. 

Prescribed fire is an important tool to accomplish a variety of land management objectives that can affect monarchs. In eastern prairie habitats, researchers have found that monarchs benefit from an increase in nectar resources following prescribed burning. However, there has been little research focused on fire and its effects on monarchs and milkweed in habitats in the western United States. Fire is a naturally occurring part of many western ecosystems. While it maintains natural habitats that monarchs and other pollinators use, it can also have negative effects on the species, especially in the short term. Fire can result in direct mortality of insect pollinators and changes in floral resources, breeding habitat (e.g., milkweed for monarchs), and nesting sites (e.g., substrate and cavities for bees). These effects vary with habitat type and fire intensity, timing, scale, location, and frequency.

Find more information about the effects of fire on pollinators, summarized from multiple studies, in Carbone et al. (2019) A global synthesis of fire effects on pollinators. Find more information about research pertaining to the effects of prescribed fire on monarchs and milkweeds, specifically in the west in The Xerces Society's Managing for Monarchs in the West.

Consider the following to minimize the effects of prescribed burning on monarchs:

• Retaining unburned patches, prioritizing areas with milkweed and concentrations of floral resources, can provide refugia for monarchs during and after a burn.

• Burning at a frequency according to the natural fire regimes of the vegetative community benefits the native vegetation that monarchs use.

• Timing burns in areas with milkweed and nectar resources during seasons when monarchs are less likely to be present minimizes direct mortality.

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.

• Burning in the fall after flowering may stimulate spring blooming.

• Burning during the warmer part of the day when adult monarchs are more active increases the opportunity for them to escape.

• Siting slash pile burning to avoid areas in and adjacent to milkweed and nectar resources and/or timing burns when monarchs are less likely to be present minimizes exposure.

• Including native nectar plants and milkweeds when reseeding areas after burning can facilitate restoring native vegetative communities used by monarchs. 

The above conservation measures are adapted from the Xerces Society's Managing for Monarchs in the West and BMPs for Pollinators on Western Rangelands and the Pollinator Partnership’s Managing Public Lands for Pollinators,