Invasive thistles are a perennial problem for gardeners where they pop-up along the edges and sometimes in the middle of cultivated beds, especially this time of year when they grow like, well, weeds. Invasive thistles compete with garden plants for nutrients, sunlight, and water and are downright nasty when you kneel on their thorny leaves and branches or try to remove them with an ungloved hand. While many species of native thistles are important food sources of our pollinators, some species like Canadian thistle, a member of the aster family native to Europe, have invaded other realms, including North America, and become major agricultural pests. To help control pestiferous thistles, plant-eating insects called biological control agents have been imported from their native lands and released in the invaded realm to put a beat-down on invasive thistles of agricultural importance. While visiting a community garden we encountered two interesting biological control agents putting a serious dent in the happy existence of thistles.
Our first encounter was with the thistle tortoise beetle, so named for the tortoise like plates that cover the upper surface of its body and head. This native of Europe was accidently introduced to Canada, first discovered in the New World in 1901, and had spread to Virginia by 1973. Like its namesake, this beetle often moves rather slowly as it grazes the surface of a thistle leaf, however, it can skedaddle with hare-like speed across a leaf or quickly drop out of sight when harassed by a predator or bug geek. Both adult beetles and their offspring chew the leaves of thistle, removing precious photosynthetic tissue and reducing plant vigor and its competitive ability. Tortoise beetle larvae have a really neat, although somewhat nasty strategy for holding predators at bay. At their rear end, tortoise beetle larvae have a fork-like appendage upon which they pile their feces to form a mobile “fecal shield” capable of being shoved in the face of an attacker. The fecal shield is most effective against predators with chewing jaws like lady beetles but not so much against predators with piercing mouth parts like spined-soldier bugs. Who wouldn’t have second thoughts about eating a beetle where the appetizer it serves is a pile of feces. Yuck!
Adult thistle tortoise beetles blend in nicely with the background provided by their prickly host plant. Larvae of the tortoise beetle carry a nasty shield composed of feces, potent defense against a rear attack from an enemy like a lady beetle.
The second biological control agent we witnessed was the Canada thistle bud weevil. This second thistle eater is another native of the Old World, first identified in the New World right here in Maugansville, Maryland in 1971, apparently having established in our region sometime in the 1960’s. Adult weevils have tiny chewing mouthparts at the tip of their elongated schnozzle. After emerging in spring they feed on the developing bud, wreaking havoc with the developing plant. The more serious setback for thistle comes when the female drills tiny holes in a flower bud and deposits an egg within. The egg hatches into a white, legless, C-shaped larva that devours the inside of the flower, destroying almost all of the developing seeds. Without setting seeds, the Canadian thistle population declines, much to the benefit of the farmer. Ah, but these benefits do not come without peril. It turns out that the thistle bud weevil is somewhat adventurous in its choice of food and has incorporated some native species of thistles into its diet. Conservationists are rightly concerned that introductions, be they accidental or deliberate, imperil our native species of thistles, which are valuable sources of nectar and pollen for many of our indigenous species of insects.
“Classical Biological Control of Nodding and Plumeless Thistles” by L. T. Kok, and “Effectiveness of Tortoise Beetle Larval Shields Against Different Predator Species” by Karen L. Olmstead and Robert F. Denno, and “Unexpected Ecological Effects of Distributing the Exotic Weevil, Larinus planus(F.), for the Biological Control of Canada Thistle” by Svata M. Louda and Charles W. O'Brien, were consulted in preparing this episode. We thank our friends at the Howard Conservancy for providing the opportunity to observe thistle tortoise beetles and Canada thistle bud weevils at work and Dr. Shrewsbury for the delightful images of these interesting herbivores.