How Milkweed Location Influences Monarch Egg-Laying and Survival
Entomology Today May 14, 2019 9 Comments
Among results of a study on monarch butterflies' (Danaus plexippus) habitat preferences for laying eggs and predation rates on their eggs, researchers at Michigan State University found that most of predation occurred in the first 24 hours after egg-laying, suggesting that weekly egg surveys are insufficient to distinguish egg-laying habitat preference from losses due to predation. (Photo credit: Steven Katovich, USDA Forest Service, Bugwood.org)
By Paige Embry
A monarch butterfly (Danaus plexippus) weighs about as much as a paper clip, but as summer ends these tiny animals begin a migration that may cover up to 3,000 miles (4,800 kilometers). The eastern population heads to Mexico and the western one splits between heading to coastal California or further south into Mexico. After huddling through the winter, the monarchs fly north, but this trip is a relay race rather than a marathon, with successive generations each taking a leg.
Paige Embry
Over the last 20 years, the eastern monarch population has declined an estimated 80 percent. One theory for the decline is the advent of herbicide-tolerant (aka "Roundup-Ready") corn and soybeans in 1996, which purged the monarch's primary host plant, common milkweed (Asclepias syriaca), from Midwestern fields. One study estimated that the Midwest lost more than 860 million milkweeds between 1999 and 2014, mostly in agricultural fields.
Surprisingly, some studies have found that monarch egg survival rates are higher on milkweed in row crops than in prairie areas. Andrew Myers, a Ph.D. candidate in the Landis Lab at Michigan State University (MSU), says it is this "intriguing mystery" that led to a study published last week in Environmental Entomology.
Myers, along with Christie Bahlai, Ph.D., of Kent State University and MSU's Douglas Landis, Ph.D., set out to learn if this habitat difference was because monarchs laid more eggs in row crops or because predation rates were lower there—or both. They had some unexpected results.
A Two-Phase Experiment
The field work took place in 2016 and 2017 at the MSU Kellogg Biological Station and was broken into two parts: habitat preference for egg-laying and predation rates in each habitat. Both phases used potted milkweeds placed in four different habitat blocks: corn, soy, bare ground, and prairie.
Andrew Myers
In 2016, monarchs laid the most eggs on milkweed in corn, followed by prairie and bare ground. In 2017, the butterflies favored prairie milkweed while the bare ground and corn habitats tied for second. Perhaps the biggest surprise was that, for both years, soybean came in last, a different result from previous studies.
For the predation study, the researchers glued eggs (with that schoolhouse favorite, Elmer's glue) onto the milkweeds. For the first 24 hours, the team checked the eggs every two to three hours, counting and identifying predators throughout the night. Myers notes, "The prairie plots were crawling with both arthropod herbivores and predators that I rarely saw during the day," and he was surprised at "how effective predator and parasitoid arthropods were at regulating herbivore populations. … [It] really increased my awareness and appreciation for the biological control services that insects provide."
Andrew Myers, a Ph.D. candidate in the Landis Lab at Michigan State University, records monarch butterfly survival data in a Michigan grassland. (Photo courtesy of Andrew Myers)
For research on monarch butterflies (Danaus plexippus), Michigan State researchers maintain a colony of the butterflies on the university campus in East Lansing, Michigan. Here, a few monarch larvae prepare to join their fellow colony mates already in their pupal stage. (Photo credit: Andrew Myers)
During a study on predation of monarch butterfly (Danaus plexippus) eggs in varying habitats, reserachers also observed a monarch larva falling victim to a spined soldier bug (Podisus maculiventris) nymph. (Photo credit: Andrew Myers)
Both predation rates and egg-laying preferences varied between habitats, but those variations weren't consistent between years or month-to-month.
Corn was the best habitat when both egg-laying preference and typical predation rates are combined, but Myers notes that corn has other risk factors, such as pesticides.
Several factors may contribute to the inconsistent habitat results, including the weather, which differed significantly between the two years.
Since most of the predation occurred in the first 24 hours, weekly egg surveys are insufficient to distinguish egg-laying habitat preference from losses due to predation.
Regarding what people should know about monarchs in general or the paper specifically, Myers says, "Monarchs are fascinating and beautiful and valuable in their own right, but I think that it's helpful to view them as a symbol for insect declines more broadly. Recent research is pointing to decreases in insect abundance as a more widespread phenomenon. This trend should really force us to question what's going wrong with the way we're approaching agriculture and our stewardship of nature more generally."
"Habitat Type Influences Danaus plexippus (Lepidoptera: Nymphalidae) Oviposition and Egg Survival on Asclepias syriaca (Gentianales: Apocynaceae)"
Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America's Endangered Pollinators and the Fight to Save Them. Email: paembry@comcast.net.
Editor's Note: This post was updated on May 15 to more accurately describe the migratory destinations of the eastern and western populations of monarch butterflies.
Andrew Myers, Danaus plexippus, environmental entomology, habitat, monarch butterfly, oviposition, Paige Embry
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Good article. I Live in Fl. We have a type of milkweed and the monarchs go crazy for it.
What are the effects of wasp realeases (feed on eggs) from agricultural areas such as Florida?
Jeffrey Cardace says:
I have now tried my first time to hatch them, i hope it works.
Hugh Dingle says:
Just a quick note to this nice article. There are NOT two NAmerican populations of monarchs. It is all one population. SOME western monarchs migrate to the California coast as we all know, BUT a significant portion also migrates to Mexico like eastern butterflies (see Biol. Journal Linnaean Society 85:491-500 (2005)). This actually explains the fact that there is no genetic distinction between eastern and western monarchs.
Elaine Toland says:
This is my 3rd year for a Monarch waystation. . My friend had an abundant amt of Monarchs feeding on a milkweed vine, so I'm adding it to my garden this year.
Sherry L Brunston says:
I live in s. Georgia, last year had over 50 monarch butterflies, hatch out. They love the milk weed I plant in flower borders. Put out more plants this year and already have caterpillars.
Lisa Draper says:
Does this research have any implications as to where to grow milkweed in our yards/gardens?
Sylvia Dennis says:
i live in Madison, Wisconsin and grow milkweed on half of my front lawn, although have migrated along western side of house to backyard & garden, I usually pull up my milkweed before winter, and wonder if it's safe to do so now?? I haven't tried raising caterpillars yet. I will have to get the book and read about it first so I know what I'm doing.
Barry Gray says:
I have put out milkweed for 4 years and the first year l raised 35 monarchs to maturity and released them. 2 years ago the number dropped to 15. Last year it dropped to 8. It is now late August and I have only 1 . What happened ? What have I doneWhat happened? What have I done wrong?
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