Does Spotted Knapweed Repel Monarch Butterflies?

| Blog, Ilse Gebhard

by Ilse Gebhard, KAWO member

This question came to my mind a number of years ago when I was raising monarchs from eggs and caterpillars found in the wild for the following two Citizen Science Projects: Monarch Larva Monitoring Project and Project Monarch Health.

Spotted Knapweed

Milkweeds are the only plants that monarch caterpillars will feed on and despite ever increasing Common Milkweed (Asclepias syriaca) in our yard, I never had enough for raising all the monarch eggs and caterpillars that I found. While milkweeds often grow along roadsides, roadsides are not dependable sources for milkweed as they are often mowed twice during the summer and some years are sprayed with herbicide. Also milkweed growing along roadsides with adjacent agricultural crops may be contaminated with drifting insecticides used on the crops and would be deadly to monarch larvae. For me a more dependable source of milkweed was a nearby fallow field where it grew among a sea of Spotted Knapweed (Centaurea maculosa), an invasive alien plant. Spotted Knapweed is said to be alleopathic, meaning that it produces its own herbicide to reduce the vegetative growth of other plants. This eventually results in a knapweed monoculture. While not yet a monoculture, the few other plant species growing in the field, besides Common Milkweed, were mostly alien as well.

Monarch egg and larval stages

Over the years I collected thousands of milkweed leaves from this field and after a while it struck me that none of these leaves ever hosted monarch eggs or caterpillars. On the other hand, in our knapweed-free yard, most milkweed plants hosted monarchs sometime during the summer.

This observation seemed interesting but before I could conclude that monarchs do not lay their eggs on milkweed surrounded by knapweed, I needed to do an unbiased study to prove it. For four years I had participated in the Monarch Larva Monitoring Project (MLMP), monitoring the use of milkweed by monarchs in our yard. Setting up the knapweed field as a second MLMP study plot might give me the answer. 

While the result was that Monarchs do lay their eggs on milkweed surrounded by knapweed, the table shows the usage was greatly reduced when compared to our yard. The difference between the two was even more dramatic if you took into account that about half of the milkweed plants monitored in our yard were Butterfly Weed (Asclepias tuberosa), which former research had shown to be significantly less used than Common Milkweed.

Date% Milkweed Usage Our Yard% Milkweed Usage Knapweed Field

Looking at the results, two questions came to mind – why had I not seen any eggs or larva on the leaves collected for food, including the year of this study, and why was the milkweed use reduced in the knapweed field versus our yard.

The first question provided an excellent example of the importance of random sampling techniques, be it in field biology or in political polls. When collecting milkweed leaves for feeding, I looked for plants with fairly large leaf size, ignoring the small plants with tender leaves as well as the big plants where the leaves were turning yellow or were all chewed up. By ignoring the small plants that monarchs often prefer for ovipositing, I missed eggs and small caterpillars, while I missed larger caterpillars by not looking at chewed up plants. 

Wild Bergamot (Monarda fistulosa); Great Spangled Fritillary (left); Aphrodite Fritillary (right); photo: Russ Schipper

The answer to the second question was less clear. Did the monarchs prefer our yard because the many native flowers provided plentiful nectar? Before eradicating knapweed from our yard, I occasionally saw butterflies land on its pink, thistle-like flowers, but I also observed butterflies exclusively nectaring on the lavender flowers of a patch of our native Wild Bergamot (Monarda fistulosa), completely ignoring a nearby knapweed patch.

The treeless knapweed field was in full sun all day while most of our milkweed got some shade, protecting it from excessive heat that can kill the eggs or caterpillars. Do Monarchs sense that when choosing where to lay their eggs? 

Were there more predators in the knapweed field than in our yard where a more diverse flora attracted birds, small mammals or frogs and toads to keep predators in check? Could one expect differences in both prey and predatory insect species between the two locations based on plant diversity?

No clear answer jumped out at me. Maybe there is something about Spotted Knapweed itself that reduces the use of nearby milkweeds by Monarchs but I propose that the more diverse eco-system of native plants in our yard was the answer for higher milkweed usage in our yard than in the knapweed field.