Invader, Meet Invader – Garlic Mustard by Julie Craves
The non-native, invasive plant Garlic Mustard (Alliaria petiolata), is such an abundant part of my urban landscape that I often don’t even notice it. So I surprised myself one day walking through my field site because, although my thoughts were elsewhere, I stopped in my tracks when I saw a Garlic Mustard plant with chewed leaves. Part of the success of many invasive, non-native plants is that they flourish in regions where they have no natural enemies. Not much eats Garlic Mustard in North America. Apparently I was so used to seeing intact leaves that these obviously browsed ones triggered a response in my subconscious.
When I stopped to take a look, I saw two small Cabbage White (Pieris rapae) butterfly larvae feeding on the leaves. That’s another non-native, widespread species that’s so ubiquitous I hardly register their ever-presence. I found two more Garlic Mustard plants nearby that each had a Cabbage White caterpillar feeding on it.
Here, the plot thickens.
Cabbage White larvae feed on many species of plants in the mustard family (Brassicaceae). This includes crop species introduced into North America, including Brassica oleracea (from which cabbage, broccoli, kale, and other vegetables are derived), mustard greens, and turnips. In Europe, Cabbage Whites also use Garlic Mustard as a host plant.
Both Cabbage Whites and Garlic Mustard were first recorded in North America in the mid-1800s, in Quebec and New York, respectively. Despite long association in both their native and introduced ranges, there are relatively few reports of Cabbage Whites using Garlic Mustard as a host plant in North America.
Studies have suggested this may be due to the fact that the butterflies have so many delicious choices of host plants here that they can choose those that they prefer (mustard greens are a favorite), and don’t need to resort to Garlic Mustard. There is an organic garden and lots of “preferred” choices within 100 yards of where I found these caterpillars. Why Garlic Mustard was chosen instead in this case is a mystery.
I brought the four Cabbage White caterpillars indoors and successfully raised them to adulthood on wild Garlic Mustard. I released the adults, hoping that perhaps they might go on to begin a population with a taste for a host from the homeland that wasn’t also destined for own our dinner plates.