When to Mow Fields for Butterflies and Beneficial Insects

Recently a design colleague wrote inquiring as to the best time to mow her client’s fields as she was concerned about disrupting the breeding cycle of the Monarch butterfly. I am often asked this question and it is well worth considering, not only for the sake of the Monarchs, but for the survival of the myriad species of butterflies, bees, and other pollinating and beneficial insects that find food and shelter in untilled fields.

Newly Emerged Monarch Butterfly

Dear Laurel,

I generally advise my readers and design clients that own similar untilled fields to alternately mow in stages–half a field at a time. The Monarch is a large, charismatic butterfly with an easily observed life cycle.  The typical field comprised of native (and introduced) wildflowers and grasses creates a rich biodiversity, supporting innumerable species of butterflies and beneficial insects. It is hard to know when exactly to mow for each different species and when to mow for even one single species because, from year to year, depending on many variables including temperature and air currents, the insects breeding times are somewhat variable. For example, this year I have had three broods each of both Monarchs and Black Swallowtails, when in a more typical year I may only have two broods.
Monarch Butterfly Caterpillars Attached to Milkweed (Asclepias syriaca)
I think of not too long ago when we were primarily an agrarian society. Farmers then would have mowed different fields at different times during the growing season. A woman in our community, whose field is rife with common milkweed, always mows in late June or early July. Initially I thought that this was perhaps not good practice for the Monarchs, but the thing with Common Milkweed (Asclepias syriaca) is that when mown to the ground in early summer, it shoots right back up again. By the time the late July, early August Monarchs have arrived and are breeding in our region, her milkweed has re-sprouted, grown at least a foot, is lush and green, and flowering.
That your client is interested in caring for the flora and fauna that abounds in her fields is wonderful! We want weeds (wildflowers) growing in our fields–they provide food and shelter for benefiel insects and wildlife and also help to retain moisture in the soil.
The single greatest threat to the Monarch butterfly is the use of Monsanto’s Roundup Ready genetically modified corn and soy bean seed, which are designed to tolerate potent does of Monsanto’s herbicide Roundup, however, Roundup kills all other surrounding plants and all beneficials insects and their larvae. Additional threats include the extreme weather condiitons caused by climate change, overdevelopment in the US, which has led to loss of habitat, and the unrelenting poverty in rural Mexican villages, which is leading to the deforestation of the butterflies habitat in Michoacán
Monarch Butterfly Chrysalids
At a lecture I attended presented by Doug Tallamy, Professor and Chair of Entomology and Wildlife Ecology at the University of Delaware, he also suggested mowing fields in alternate stages, so we are in good company!
An additional suggestion is to give a corner of your field or garden over to a patch of milkweed, which would then insure a steady supply for the Monarchs. We grow both Common Milkweed and Mash Milkweed (Asclepias incarnata) side-by-side. The marsh flowers slightly earlier, attracting earlier Monarchs. The males intently patrol the milkweed patch and after mating, the females flit from flower to leaf, alternatively depositing eggs on both species of milkweeds, while taking sips of nectar from whichever milkweed is in flower.
Kind regards,
Note form Laurel:
Hello Kim,
I have a new client who has both cultivated ornamental gardens, which I am caring for, and open meadows, which she has mown once or twice per season. There is lots of milkweed in the field, with some seedlings sprouting in the garden. We pulled the seedlings and brought them home to later place in a field near where I live. We had them in jars in water and were surprised and delighted when a few days later we discovered 7 tiny Monarch larvae. We have raised them to the butterfly stage and released them. Fascinating! During the process I did some reading about the Monarch and learned that there are 4 or 5 generations each season, with the final butterflies, in September, making the trek to Mexico for the winter, and then flying north in the spring to start the cycle over again. My question is about the timing of the mowing of the fields. If it is mown at any time during the breeding season (May – September?), isn’t the current generation almost or completely wiped out? Do you  know when the best time is to mow for the butterflies? I assume it is late fall, but since the mowing of meadows has been happening for centuries, is it perhaps ok to mow during the season? (or perhaps this is one contributing reason for the demise of these lovely creatures….) Thank you in advance for any info!

1 thought on “When to Mow Fields for Butterflies and Beneficial Insects

  1. kimsmithdesigns

    From Eric:

    Hi Kim,

    The flowers, grasslands and milkweed are proliferating at the Loop Pond site. For the past couple of years…and likely into the future I do all the cutting no earlier than October. A few reasons for this approach…to better serve the insects, cooler weather to do the back breaking work, and the fact that any nests of stinging bees are reduced! My target has actually been not to mow until after the first frost. Why destroy the fall flowers?

    Come take a look sometime. Flowers all growing season long.


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