EAST LANSING, Mich. — Throughout southern Michigan common milkweed is starting to emerge and adult monarch butterflies have arrived. At the start of this new monarch season it’s helpful to review what we do and don’t know about monarch biology and conservation. Michigan is an important state for the Eastern migratory monarch, and supporting the summer breeding population is key to monarch conservation.
Recent research in our lab has focused on understanding where monarchs prefer to lay eggs and the fate of those eggs and young larvae (caterpillars). In our studies, monarchs have laid most of their eggs on common milkweed, although butterfly milkweed, swamp milkweed, and whorled milkweed are also utilized, especially later in this summer. When given a choice of laying eggs on common milkweed in different types of habitats, monarchs showed varying preferences. In one year, monarchs laid more eggs on milkweed growing in corn fields while in another they laid more eggs on milkweed growing in grassland habitats. One of our key findings is that monarch eggs can experience very high rates of predation. In one study 90% of the eggs were consumed by predators before they hatched.
We’ve also been very interested in learning who eats monarch eggs and young larvae and what time of day these predation events occur. We collected a total of 75 insect and spider species from common milkweed and gave them access to eggs and young larvae in the lab. Of these, nearly half readily consumed one or more immature monarchs. We followed up the study by monitoring eggs in the field using video surveillance cameras, confirming that a wide diversity of arthropods consumed monarchs. Most surprisingly, was that 74% of the predation events occurred at night! Common predators include: spiders, earwigs, tree crickets, and even the primarily plant feeding small milkweed bug.
Finally, we’ve been interested in what we can do to enhance monarch egg laying and survival. Longtime observers of monarchs have noted that when given the choice females greatly prefer to lay their eggs on very young plants. Our studies have shown that by late June common milkweed around East Lansing is becoming relatively unattractive for monarch egg laying. In addition, once it starts flowering, common milkweed attracts large numbers of ants and other predators. These observations led us to consider a new approach to managing milkweed for monarchs.
As every gardener knows, cutting off milkweed stem above ground simply causes it to regenerate from the roots. We have found that by mowing approximately one-third of the common milkweed stems in a patch in mid-June and a second third in mid-July, the regenerating stems are more attractive, increasing monarch egg laying up to 10-fold and simultaneously reducing predator numbers by half or more. Our initial studies showed the monarch survival stays the same or increases, but we are repeating the study to gain more information.
To be clear, we are not suggesting indiscriminate mowing, but if your milkweed stand is starting to flower and the leaves are looking a bit tough, consider trimming back a few stems to near ground level and seeing if the new shoots that come up have more monarch eggs and caterpillars on them. If you’d like to share your observations send a report to email@example.com.
— Douglas Landis, Nathan Haan and Andrew Myers, Michigan State University Department of Entomology
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