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Is Black Swallow-wort* a trap plant for monarchs?
Black swallow-wort, Vincetoxicum nigrum, is an introduced species in the milkweed family, Asclepiadacae (now Apocynaceae). This is a twining species introduced from Europe late in the last century presently ranging from Kansas and Nebraska east to Maine where it appears to be replacing native vegetation. This species has become well established in the northeast and is a noteworthy pest. If you search for black swallow-wort on the Internet, you will find several web sites dedicated to telling you how to rid your property of this plant. Black swallow-wort is also of interest because it may be a trap plant for monarchs, that is, one on which the females will lay eggs due to the chemical similarity of the plant to other milkweeds but on which, for various unknown reasons, the monarch larvae cannot mature. Placing eggs in the wrong place could reduce the reproductive success of the population. In the spring of 2000 monarch enthusiast Gary Stell observed about thirty monarch eggs on V. nigrum in upstate New York but was unable to establish whether the monarch larvae fed on this plant. The obvious question is - "Is black swallowwort a suitable host for monarch butterflies?" Sounds like an easy question to answer and it may be, but we need to be cautious.
Monarch larvae will sometimes feed on cut foliage of plants that they are unable to use when intact. Such seems to be the case with Asclepias sullivantii. Monarchs will oviposit on the blossoms of this relatively common tall grass prairie milkweed but the larvae disappear from the plants in the third instar. Damage to the foliage is virtually absent and the only way to rear monarchs on this species is to remove the leaves from the plants. A. sullivantii produces profuse quantities of latex, which may be sufficient to deter the larvae from feeding on the leaves. In effect, A. sullivantii has the attributes of a "trap plant" and it is a native species, although one with a relatively limited range.
Knowing this, how should we go about testing to see if monarchs can be reared on Black Swallow-wort? Clearly, we have to rear the monarchs on intact plants and we need controls. One way to do this is to place larvae or eggs in sleeves that cover the plants. The sleeves should eliminate complications caused by predators and parasites. Simple sleeves can be made from 5-gallon paint strainers that can be purchased from many paint stores.
The larger question is why do female butterflies make mistakes in their choice of host plants? In other words, why hasn't selection eliminated this type of behavior? Could it be that the chemoselective means by which females select plants is specific to the milkweed family but not specific enough to exclude species which are unfit for larvae?
* Dog strangle vine or pale swallow-wort, Vincetoxicum rossium is another invading species with similar properties and potential as a "trap plant" for monarchs.
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