One of many invasive plants in Pennsylvania, garlic mustard (Alliaria petiolata) was introduced on the east coast in the 1860s as a valuable food source and its proclaimed medicinal properties. It has since spread throughout the Northeast and Midwest. This shade-tolerant invasive plant out-competes against native vegetation. It can be especially detrimental at Howard Falls, as it also acts as a toxic decoy to the West Virginia White Butterfly (Pieris virginiensis), a delicate and rare spring-flying butterfly native to Pennsylvania and nearby regions.
In recent years, a thriving colony of West Virginia white butterfly was identified by Dr. James Bissell of the Cleveland Museum of Natural History, during one of his many site visits to the Howard Falls Gorge. The colony he identified lives in one of the most remote areas of the Gorge and likely has thrived there for ages. Luckily, garlic mustard has not invaded that isolated habitat.
The West Virginia white butterfly was first described by Henry Edwards in 1870 as a subspecies of the holarctic mustard white butterfly. Historically, the butterfly was known to occur from Vermont west to Wisconsin and southeast (primarily in the Appalachian Mountains) to extreme northeast Alabama. The habitats for this butterfly are mature, moderately moist, hardwood forests, typically dominated by basswood, beech, birch and/or maple, and the butterflies rarely venture outside the confines of a closed canopy forest.
The most serious of several threats that garlic mustard poses to these butterflies is when it acts as a toxic decoy. The butterfly’s preferred plant, toothworts, is in the mustard family, the same family of plants that contain garlic mustard. In the Howard Falls region, they mostly rely on the Large Toothwort (Cardamine maxima), which can be found along the slopes of the gorge. Garlic mustard and toothworts are similar enough in chemistry that butterflies become confused and lay their eggs on garlic mustard. However, the chemistry of the plants is different enough that the hatched caterpillars invariably die when on garlic mustard.
Like all other Lepidoptera, the West Virginia White is a holometabolus insect, meaning it passes through four life stages: egg, larva, pupa and adult. The species is single brooded, with the adults appearing from mid-April through early June. The life of the West Virginia White is fairly short with adults typically surviving 5-10 days. They mate quickly upon emergence from the pupa and females lay eggs on the leaves of toothworts. The eggs are small (~0.003 inches in diameter) white and spherically conical in shape. The brief, 5-10 days, egg stage is followed by a relatively short larval stage at around 10-20 days.
The young caterpillars are yellowish green and become a dull grass green with maturity. The skin surface is covered with a short, dense layer of hairs, making the caterpillar feel fuzzy to the touch. There are also scattered, somewhat longer hairs, translucent or whitish in color, all over the body and giving it a silvered appearance in the light. The caterpillars blend in very well with the leaves on which they feed and they thrive on several species of spring ephemeral plants in the mustard family.
Pupation occurs on either the food plant or adjacent vegetation, typically within one foot of the ground surface and begins the extended pupa stage. After 10-11 months, the relatively small, measuring approximately 1.5 inches in wingspan, dusky white, somewhat translucent, winged adult emerges and the cycle continues. The white pigment in their wings advertises that these butterflies are distasteful because of the mustard oils they retain from their food plants. Garlic mustard also affects the West Virginia White by displacing its host plants and its nectar sources. Even though garlic mustard itself is a good source of nectar, it does not bloom when the butterflies first emerge. The butterflies require a diverse and abundant range of wildflowers with staggered blooming times to provide nectar throughout their short-lived life while they fly. Unfortunately, West Virginia White populations have decreased, mostly due to the spread of garlic mustard; however, they also face threats around loss of habitat, primarily through logging and clearing for agriculture or residential development.
May is one of the best times to spot Garlic Mustard and help control its spread. This year, most of the plants have already begun to flower and set seed and should be easily identified. Garlic mustard leaves are quite distinct and vary from rounded, to kidney-shaped (reniform), to triangular, with a slightly rounded or serrated edge. The foliage is often a deep and distinct shade of green with a very pronounced leaf venation. Early in the season Garlic Mustard grows as a small, squat herb on the forest floor. In April or early May, it quickly sends up a flower stem that can reach as tall as three feet. This flower stem terminates in a small cluster of tiny white flowers with four petals. The stem eventually elongates into a seedpod, featuring many stringy pods reaching up to the sky. The plants are relatively easy to pull, but they are brittle, so be sure you are lifting the entire plant out of the ground, not just breaking off the top, and and not dropping any seed capsules as you remove them. Plants should be bagged or burned since research has shown that composting is not a consistently good option.
Howard Falls is committed to combating the spread of garlic mustard within its forest with the goal of protecting this important butterfly.