If you grow Milkweeds to attract Monarch Butterflies, do you ever wonder why some plants get lots of Monarch eggs and caterpillars while others are ignored? This is the question, more or less, that some scientists tried to address with research published in the journal Frontiers in Ecology and Evolution.
Entomologists Adam Baker and Daniel Potter at the University of Kentucky measured the number of Monarch eggs found on Swamp Milkweed (Asclepias incarnata) scattered among grasses and nectar plants versus those planted around the perimeter. They also looked at Swamp Milkweed planted in a mass surrounded by grasses and nectar plants.
The results were pretty clear:
… monarch eggs and larvae were 2.5–4 times more abundant in gardens in which the milkweeds were planted around the perimeter, surrounding the nectar plants and grasses, than when the layout was reversed, with milkweeds in the garden interior, or when the milkweeds were randomly intermixed with the other plants.
Female monarchs foraging in an open-field setting laid significantly more eggs on single milkweed plants that were accessible from top to bottom, without visual obstruction, compared to single plants surrounded by, but not touching, ornamental grasses of equal height.
Another thing the researchers found: Monarchs are less likely to use Milkweeds located next to buildings that block north or south access. Turns out courtyards are an especially bad place to put your Monarch Waystation.
These results kind of confirm my own experience in the garden. I’ve been frustrated by the scarcity or Monarch eggs and caterpillars on my Milkweed plants. When I do see caterpillars they are on Butterflyweed plants (A. tuberosa) that have been planted on the perimeter of a bed or border.
I have a whole bunch of Swamp Milkweed and also some Prairie Milkweed (A. sullivantii) that’s scattered among other tall plants. These get no Monarch eggs at all, though they are used for nectar.
After reading this article, I will try to encourage Milkweeds to take over space along the edge of some of the borders. This is feasible mainly along the back of the Sidewalk Border and along the west edge of the Front Island Bed.
Anyhow, read the article yourself. The writing is a bit dense, but not impenetrable. Does this research fit with your own experience with Monarch eggs and caterpillars?