Dr. Jekyll and Mr. Hyde wage war on thistles: Thistle head weevils Rhinocyllus conicus

 

Amidst defoliated leaves and deposits of beetle frass, a thistle head weevil stares back at a bug geek. Image: Paula M. Shrewsbury, PhD

 

Spiny plumeless thistles and several other thistles from abroad create major problems for agricultural crops and natural ecosystems throughout North America.

Invasive species of plants and insects levy billions of dollars of losses to crops, forests, and gardens annually in the US. Some of the most noisome plant invaders of agricultural lands are exotic thistles such as bull thistle, Scotch thistle, and musk thistle. While admiring a gorgeous pollinator meadow in Natural Bridge, Virginia, we were introduced to yet another pretty invasive thistle with the curious moniker of spiny plumeless thistle, Carduus acanthoides. These prickly rascals invaded the meadow and were crowding out flowering native plants, beauties that help support our pollinators and other beneficial insects. On closer inspection, several spiny thistles clearly were suffering from a bad case of blossom rot and defoliation. Tiny beetles behind this mayhem were weevils from afar known as thistle head weevils, Rhinocyllus conicus. How did this saga begin?

Flower heads infested with weevil larvae will fail to produce seeds.

Noxious spiny plumeless thistles are native to Europe and Asia and were first detected in the US in Camden, New Jersey, in 1878. Agricultural scientists searched the realms of several invasive thistles and discovered the thistle head weevil, which is native to parts of Europe, North Africa, and western Asia. Weevils were introduced into Canada in 1968 with subsequent introductions to California, Virginia, and Montana in 1969 for the purpose of putting a beat-down on several species of non-native thistles. Populations of thistle head weevils from Virginia were then redistributed to other states. As we wandered the meadow, every spiny thistle we encountered was under attack or bore the scars of attack by these small beetles as they munched leaves, defecated, found romance, and laid eggs on the bracts and leaves of developing flower heads. While defoliation caused by adult feeding weakens thistles, the coup de gras comes when adults deposit eggs on developing flower heads, weevil larvae subsequently hatch and then consume reproductive plant tissues before flowers can form and produce seeds. This foils reproduction in infested thistles. Scientists in Oklahoma report that thistle head weevils can reduce invasive thistles by 90% or more in 10 years or less following their introduction to an infested area. Thistle head weevils are prodigious reproducers with each female capable of laying 100 to 200 eggs during her one-year life cycle.

Spiny plumeless thistles, native to Eurasia, have invaded this pretty pollinator meadow in Virginia. But thistles are besieged by small exotic beetles called thistle head weevils, leaf munchers that deposit eggs in developing flower heads. Weevil larvae hatch from eggs and bore through the flower head, eliminating the flower’s ability to produce seeds. Over a period of several years these beetles can dramatically reduce invasive thistle populations in an area. Unfortunately, they also attack thistles native to North America, imperiling some endemic species. Watch as a mating pair is soon joined by other hungry weevils.

Thistle head weevils have a short snout. Hiding on a thistle was a similar weevil with a long snout. Could this weevil be the thistle bud weevil, Larinus planus? Like the thistle head weevil, the thistle bud weevil was imported from Europe to help control thistles in North America.

We just met thistle head weevils playing the role of Dr. Jekyll. Now let’s meet Mr. Hyde. Following the release of thistle head weevils in North America in the 1960’s, scientists discovered that the host range of the weevil included some 22 species of native North American thistles. Thistle head weevils pose a serious threat to many endemic thistles, including a California native on the federal endangered species list called the Suisun thistle. Since the time of the first introductions of thistle head weevils to the US, and with greater understanding of potential harm the weevil might cause to native thistles, in 2000 USDA cancelled all permits for interstate movement of the thistle head weevil. Nonetheless, for better or for worse, thistle head weevils are here to stay as terminators of noxious invasive thistles. The big “oops” is the thistle head weevil’s ongoing threat to our home-grown native thistles.  

Acknowledgements

Bug of the Week thanks Dr. Paula Shrewsbury for images that served as the inspiration for this episode. The following articles provided interesting facts and information for this story: “Musk thistle weevils take a stand against pasture bullies” by Lacey Newlin, “Rhinocyllus conicus (Coleoptera: Curculionidae)” by L. T. Kok”, “Invasive species: What everyone needs to know” by D. Simberloff, and “Rhinocyllus conicus” and “Larinus planus” Integrated Weed Control Program Staff, Washington State University.

Yellow snake along the trail: Spicebush swallowtail caterpillar Papilio troilus

 

Small snake resting on a stem? Nope, spicebush swallowtail caterpillar. Image credit: P. M. Shrewsbury, PhD

 

Visit Natural Bridge State Park in Virginia to experience spectacular scenery and fascinating wildlife.

A real geological treasure can be found at Natural Bridge State Park just off of Route 81 near Lexington, VA. Here Cedar Creek wends its way beneath a spectacular dolomitic arch formed when ancient subterranean streams of acidic water dissolved surrounding limestone and created an underground karst cave. Through hundreds of millennia, the cave collapsed and erosion exposed the magnificent arch. While enjoying a hike along gorgeous Cedar Creek, a fellow hiker asked if we had seen the strange yellow snake in a small shrub just a short piece down the river. Never wanting to miss a chance to see a snake, we backtracked to an impressive native herbaceous plant known as small flowered leafcup (Polymnia canadensis). There we discovered what, at first glance, appeared to be a stumpy, banana - yellow snake with large googly eyes meandering along the plant’s stem. Masquerading as a serpent, false eyespots on the body of this trickster have evolved to deceive vertebrate predators like birds. The logic behind this ruse goes something like this. What eats a caterpillar? Birds do. And what eats a bird? Snakes do. So, if you are a caterpillar, looking like a snake might just help scare away a hungry bird, right? Clearly, the “I am a snake not a caterpillar” trick worked well on a human and perhaps this works just as well on other vertebrate predators. 

Large snake-like eyespots on the body might give a hungry bird second thoughts about attacking this caterpillar.

I usually meet and enjoy spicebush swallowtail caterpillars on spicebush or sassafras trees. Several years ago, I rescued a small spicebush from a construction site and planted it in my back yard. In addition to wonderfully aromatic foliage, spicebush bears gorgeous red berries in autumn that are a favorite with birds. While attracting birds is delightful in its own right, this interesting native plant attracts the magnificent spicebush swallowtail, which uses spicebush as food for its young. After a few years of disappointment, I was rewarded with the appearance of three tiny swallowtail caterpillars hiding in neatly folded leaves. By clipping small notches in the foliage, the first instar larvae bend leaves over to create a tiny refuge. After passing through an ugly duckling phase resembling bird droppings as first instars, caterpillars molt and initiate their art of deception and visual surprises. 

With two large, glistening eyespots, at first glance a spicebush swallowtail caterpillar looks like a snake. This clever ruse may help it escape predation by birds and other predators. Young caterpillars build shelters to avoid detection by clipping and folding leaves. They too have false eyespots and their bodies resemble bird droppings. Yuck!  Older caterpillars sport eyespots on an emerald green background which may turn banana-yellow before they form a chrysalis and emerge as adults. This trickster fooled a human and is thought to fool other predators as well.  

With the body of a bird dropping and the head of a serpent, this young caterpillar presents a confusing visage to a would-be predator.

After its first molt, the spicebush caterpillar glistens and its dark brown coloration with brilliant white stripes announces that it is nothing more than a bird dropping. While a dung eating fly might find this attractive, it is unlikely that any self-respecting bird would entertain a bird dropping as a meal. The small spice bush larva has one more bit of visual trickery just in case a nosy predator takes a closer look. When viewed head-on, two beady false eyespots on the thorax and two dashed white lines along the sides create the creepy visage of a viper. As the caterpillar molts and grows, the serpent theme is repeated and enhanced with large vacuous eyespots set in a matrix of serpent green. The last stage of the caterpillar may turn yellow and move from the spicebush or sassafras to form a chrysalis on vegetation nearby before molting to the adult stage. 

The beautiful spicebush butterfly is thought to mimic the distasteful pipevine swallowtail.

If the art of deception fails to fool a predator, the spice bush caterpillar has one more trick up its sleeve. Just behind its head the caterpillar houses a specialized structure called the osmeterium. Usually, this forked orange appendage is tucked beneath the skin out of sight. When the caterpillar is threatened, it extends the osmeterium in the direction of the attacker. This glandular organ is coated with foul smelling chemicals reminiscent of rancid butter. The disturbing visual and olfactory display might be the final deterrent to hungry predators wanting to dine on this beautiful caterpillar. Deception does not end when the spice bush caterpillar transforms to the adult butterfly. Dark colored wings splashed with patches and lines of orange and white are thought to mimic the coloration of the nasty tasting pipevine swallowtail, Battus philenor. A trickster as a youth and a trickster as an adult help the spice bush swallowtail to live another day.       

 Acknowledgements

 Thomas Eisner’s delightful book “For Love of Insects” and David Wagner’s wonderful guide “Caterpillars of Eastern North America” were used as references for this episode. Several other insects including owl, promethea, and Io moths use false eyespots to confuse predators.

Vanquished vanishing columbines: Columbine sawfly, Pristiphora rufipes

 

Look out columbine! That sawfly is laying eggs in your leaves.

 

Columbines are among the most glorious perennials found in gardens and landscapes.

On a recent visit to a spectacular residential garden in Columbia, MD, I was chagrined to see beautiful clusters of columbines denuded of leaves and reduced to spindly blossom-tipped stubs. Swarming about these unfortunate beauties were what at first appeared to be tiny black flies. Upon closer inspection the presence of four wings instead of two confirmed that these were not flies, but primitive wasps known as sawflies. Flies belong to a clan of insects, Diptera, with but one pair of wings and sawflies are part of the bee and wasp clan, Hymenoptera, with two pairs of wings.

These slits in the epidermis of a leaf hold eggs deposited by the female columbine sawfly. Eggs will soon hatch and leaf-munching larvae will begin to feed.

Why are they called sawflies? Well, not because you saw them. No, these primitive wasps bear a saw-like appendage on their abdomen used to slice leaves or stems of plants to create a pocket into which eggs are inserted. This particular non-native sawfly has roots throughout much of central Europe and was first discovered in North America in Canada, the year Surfin’ U.S.A. topped the charts in 1963. By 1983 these rascals reached upstate New York and they now occur from the eastern seaboard to the Rockies. Adult sawflies feed on a variety of foods including nectar, pollen, and other insects. Ah, but larval columbine sawflies have powerful jaws to devour the leaves of columbines.

 

Newly hatched columbine sawfly larvae begin their nefarious work of defoliating leaves of columbine.

Columbines are among the most beautiful perennials to grace natural landscapes and gardens. But trouble is afoot when tiny black wasps called columbine sawflies arrive on the scene. Female wasps use a saw-like appendage to slice leaves and insert eggs into tender leaf tissue. Eggs hatch and tiny caterpillar-like larvae with black heads munch nutritious leaves. As larvae grow and develop, they remove ever larger portions of leaf tissue and may completely defoliate plants. If you discover these rascals on your columbine, pick them off, dispose of them, or feed them to the birds!

An easy way to tell many caterpillars, the larvae of moths and butterflies, from sawfly larvae is to count the number of abdominal appendages called prolegs. Caterpillars (upper image) will have five or fewer pairs of prolegs and most sawfly larvae (lower image) will have six or more pairs.

Many sawfly larvae resemble caterpillars, the larval stage of moths and butterflies. However, there is an easy way to tell most sawfly larvae from caterpillars. First, let’s have an anatomy lesson. Larval sawflies and caterpillars have a head and three thoracic segments that usually bear pairs of segmented legs. Most caterpillars and sawflies have paired leg-like appendages called prolegs on their abdominal segments. Prolegs help them grasp the plants on which they live and feed. Caterpillars with prolegs have five or fewer pairs and sawflies with prolegs have six or more pairs. By counting the pairs of prolegs, voilà, you can differentiate between caterpillars and sawfly larvae. One more tidbit, prolegs of most caterpillars bear fish-hook-like structures called crochets on their prolegs, while sawfly larvae have naked prolegs. How daring!

After defoliating columbines, sawfly larvae move to the soil and spin brown cocoons in which to pupate.

Northern states in the U.S. and Canada report a single generation of columbine sawflies annually. At the time of this posting in early June here in Maryland, a second generation of columbine sawflies is well underway. In England, three or more generations occur each year. Densities of these leaf munchers can be so great that columbines are completely stripped of their foliage. Fortunately, columbines have remarkable restorative powers and may be able to withstand severe defoliation. What can be done to mitigate damage caused by these rascals? Here in the mid-Atlantic, check your columbines within the first month that new foliage appears in the spring and then at biweekly intervals. Damage will first appear at the margins of leaves before entire lobes disappear. Larva are easily removed and crushed. For a real avian treat, collect larvae and place them on a bird feeding station where they will be gobbled up by our feathered friends. If touching insect larvae is not your thing, there are several insecticides listed by the Organic Material Review Institute (OMRI) that can bring a rapid halt to shenanigans caused by these columbine devouring critters. Oh, and when using insecticides always be sure to read and follow directions on the label. 

To see a great video of columbine sawflies and how to manage them, please click on this link: https://www.youtube.com/watch?v=HtLyWg7hV0o

Acknowledgements

Inspiration for this week’s episode comes from Ron and Leslie for allowing us to visit their garden to watch and study columbine sawflies and from Max who created the awesome video on columbine sawfly management. The enlightening articles “Common Columbine Pests: Columbine Leafminer and Columbine Sawfly” by Susan Mahr, and “Pests of Ornamental Trees, Shrubs, and Flowers” by David V. Alford, were used as references for this story.