Managing Milkweeds for Monarchs

September 29, 2013

Nature Notes by Bill Dunson


In early fall the naturalist’s thoughts turn mainly to the southerly migration of animals.  In addition to birds, millions of insects including butterflies and dragonflies fly south.  The most famous of these is the monarch butterfly whose populations in the eastern US seem generally to be at a low ebb.  However due to the abnormal amount of rain received at our VA farm this year, I have hopes that more than the normal number of monarchs will be heading south from our locality.  One of the major contributors to this success seems to be a change in the cutting cycle of our grasslands caused by heavy rainfall.

Monarchs in the east are heavily dependent on a single species as a larval food plant, the common milkweed, Asclepias syriaca.  Most of these plants grow in pastures or hayfields which are grazed or cut several times a year.  The timing and frequency of cutting will determine whether these plants will feed monarchs and a wide variety of other insects which utilize the flowers and seeds.  Farmers generally will want to cut their fields when the grass is ripe and as many times as possible.  This can result in a sterile environment for wildlife.  A common recommendation for protecting breeding grassland birds such as meadowlarks is to cut only once at the end of August.  This has the problem that milkweeds are removed and there is no cover or food left for wildlife during the fall and winter.   I prefer to cut only once at the end of June, sacrificing a few nests, but allowing for birds to re-nest.  Then a re-growth of summer grasses and other plants will provide food and cover.  But this year my best laid plans were derailed by abnormally heavy and long-lasting rains which prevented mowing on my usual schedule.  Thus our last field was cut on August 12 and I was afraid this would not be good for monarchs.  But to my surprise we have more monarch caterpillars in that field than ever before.  So what has happened?

The fields that were cut Aug. 12 included thousands of mature common milkweeds.  At this stage they had seed pods and the leaves were hard and not very palatable to monarch caterpillars.  Indeed such mature plants are attractive primarily to milkweed tussock moth caterpillars.  But due to the unusually wet conditions, by Sept. 27 there were thousands of small tender milkweed plants re-growing and a considerable number of late instar monarch caterpillars were feeding on these small plants.  I have not previously observed such a large number of monarch caterpillars, presumably since the plants in a more normal year would have been much older and less attractive to monarchs.

In managing land for wildlife one quickly learns that few decisions are easy and that every year teaches new lessons about how to better improve the habitat.  It is wise to be flexible and to learn from the effects of variable natural cycles of weather that ebb and flow across the remarkably complex habitats that make up even small parcels of land.  So this year taught me once again to reject the “one size fits all” recipe for land management and to experiment constantly to see what works best under different conditions.

Bill Dunson, Englewood, FL, and Galax, VA

Bill Dunson

About Bill Dunson

Bill Dunson, born in rural Georgia, skipped 12th grade and went directly to Yale. Bill subsequent-ly received a PhD in Zoology from the University of Michigan, studying softshell turtles. Bill is Professor Emeritus of Pennsylvania State University thanks to a career spent entirely at that institution, teaching and doing research on the physio-logical ecology and ecotoxiciology of reptiles, amphibians and fish. Always curious about nature, Bill has dedicated his life to learning and sharing his knowledge with others. He has served on many advisory boards here in Southwest Florida to preserve the water that gives life to our region.

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