During our guided hike to Glendon campus with Dawn, we discovered some non-indigenous plant species like Canada thistle (not a Canadian species, despite its name) and, as I have a keen interest in butterflies, the case of the swallow-wort (or dog-strangling vine) drew my attention; these plant species, the black swallow-wort (Cynanchum louiseae) and the pale swallow-wort (Cynanchum rossicum) are native to Europe (introduced in North America in the 1800s) and belong to the Asclepiadacea family (milkweed), the same family of host plants of Monarch catterpillars. They can easily be mistaken for Cynanchum laeve, which however IS a suitable host plant for caterpillars, and is a native species. They spread by seeds (each pod contains hundreds of seeds that are dispersed by animals and the wind) and by rhizomes, and are found in open areas (old fields and pastures) and even some forested areas. It is today present in some parts of Ontario, southern Quebec and several American states.
Swallow-wort, aka dog-strangling vine
Monarch butterflies (Danaus plexippus) are lepidoptera insect belonging to the Nymphalidae family and native to North America. They are famous for the noteworthy migratory phenomenon between southern Canada and Mexico forests where they hibernate (a journey of up to 5,000 kilometers!), that they accomplish between August and October, before heading back north after the winter. But recently, the number of butterflies present in November is at its lowest in 20 years, as stated in the WWF’s 2013-14 report from Mexico. This reduction is due to illegal logging, climate change and lack of milkweed plants. I will treat the latter in this blog post.
The problem is the following: if the female Monarch lays her eggs on a swallow-wort plant, when they hatch the caterpillars will not feed on this plant and will die. Usually female butterflies locate the appropriate host plants with chemicals signals and oddly enough, they will preferentially lay their eggs on swallow-wort plants even if appropriate milkweeds are present in the same area 20% of the time. Furthermore, swallow-wort species are invasive besides being non-native: indeed if present in the same area, Cynanchum sp will outcompete the native milkweed, reducing the number of food resources for caterpillars. Swallow-wort have no ennemies because produce glycosides, toxic for grazing mammals (so even the voracious deers can not eat it) and are allelopathic: they can release chemicals that hinder the growth of surrounding plant species by altering the soil and helping cultivate soil fungi that benefit swallow-worts but deter other plants, including milkweed species appropriate for Monarch caterpillars to feed on.
Several actions have been considered to eradicate, or at least reduce, these invasive species:
- mechanical control, by pulling or digging out the plant, but it is not the best solution because sometimes seeds are released in the process;
- chemical control by using herbicides (but nothing is said about the possible collateral damages on untargeted plant species with the use of such products);
- in Canada, biological control have been tested by using a leaf-eating moth species, Hypena opulenta, feeding exclusively on swallow-wort.
We can see that the introduction of invasive plant species can impact native plant species but also animal species as well, and lead to the reduction of population size of one if the most inconic butterfly of North America.
For more information and sources: