Twig Borers and Girdlers on Oaks

By Susan Clark, Board Member, Massachusetts Chapter of the American Rhododendron Society

Photo by bert Cregg, MSU

In mid to late summer the ground under our oaks is usually littered with terminal leaf clusters that seem to have been snipped off.  I always wondered what caused this loss of healthy twigs as it didn’t seem to be the wind twisting them off; the stems seemed smoothly cut, as though by a mysterious vandal high in the tree tops. I resolved to figure this mystery out and, once again, it’s amazing what a clumsy Google search can turn up. ‘Oak twig loss’ eventually explained the mystery and, no surprise, it turns out to be insect damage.

Two different beetles cause this twig loss in the Northeast. Both are members of the huge Longhorn Beetle family, Cerambycidae, with more than 35,000 species found world-wide. Most species, including the two in the Northeast, have distinctly long antennae (horns?) and eat plant tissue like twigs, stems, and roots. The taxonomy of these beetles is predictably complex and disputed. Curiously, the family name comes from Greek mythology, a story about an unfortunate shepherd, Cerambus, who was a master musician and probably invented the pan-pipes. He became dangerously conceited and eventually lost an argument with some nymphs he had offended with his song lyrics. He was transformed into a large wood-eating beetle with horns.

Photo credit: Tim R. Moyer, bugguide.net

Twig Pruner Beetles (genus and species names of this beetle are uncertain and different sites give very different names), are native long-horned beetles, ¾” long, brownish, slender and elongate. They have two posterior spines on each wing cover and their antennae are longer than their bodies. The Twig Pruner prefers oaks but also attacks other hardwoods, like dogwood, elm, honey locust, hickory, but around here it really favors our Red Oaks. When the oaks begin leafing out, these beetles lay their eggs at the tips of the host tree branches up to 1 inch in diameter, one egg per tip.  When the eggs hatch, the growing larvae, called roundheaded borers, which are the usual legless white with black heads, bore into the stems and tunnel down them. They winter over inside the twigs, continuing to feed again the next spring. In midsummer they cut around the inside of each branch, but leave the thin cambium layer and bark intact. Eventually the twigs’ weight or the wind snaps them off. Look at the end of a leaf cluster with a Twig Borer and you should see a hole in the inner pith plugged with frass (the beetle poop) to protect the overwintering larvae from predators. The larvae remain inside their now fallen stems, eventually pupating in the twig over the winter, to emerge as adult beetles the following spring when they will fly up into the host tree to lay the next generation of eggs.

Twig Girdler, Photo from Mississippi state university

A different long-horned beetle, Oncideres cingulata, girdles rather than cuts off the leafy twigs and it has a similar life cycle as the Borer. The Twig Girdler is about ¾” long but it is grey-brown with a lighter stripe across its wing covers (its elytra) and its antennae are about the same length as its body, not longer. Adult beetles emerge in late summer when the females girdle a twig and lay a single egg in the cut portion so their eggs can feed on the dying wood; these larvae don’t eat live wood, unlike the Twig Borer. The girdled terminal quickly falls to the ground that first summer. The Girdler Beetle can be identified by looking at the cut end with its smooth V-shaped outer cut and ragged torn inner wood, quite distinct from the Twig Borer’s cut appearance with the visible tunnel inside. The Twig Girdler larvae bore further into the fallen twig, spending the winter and the next spring inside until they pupate and emerge as adults to start the cycle again.

During late summer, the cut-off twigs, with their green leaves attached, litter the ground under infested trees or worse, they stay snared in unreachable branches, very noticeable and often long-lasting, and to the tidy landscaper, frustrating. Their remarkably persistent green leaves eventually brown but don’t drop off since no abscission cells form as they normally would each autumn for leaf drop. You can diagnose the source of these branches by their ends, a rather smoothly cut surface, very unlike any branches ripped off by the wind.

I have to confess I have never noticed nor identified either beetle; I only saw their littered handiwork. And in a cursory examination of the fallen leafy terminals in my yard, they all seem to be from the Twig Girdler, not the Twig Borer, so each clump of leaves lacks a central eaten-out tube and a frass plug. But I will look more carefully in the future. These beetles do not do much damage to healthy trees, only to the gardener’s peace of mind. The natural world continues to amaze and mystify, but at least I have finally solved another minor mystery.  

In Defense of Weeds

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By Joan Butler

One summer day a few years ago, I noticed a very large spotted white moth resting on a leaf in my crabapple tree. I was amazed by its size and distinct markings, unlike anything I had ever seen before. Eventually, I tracked down its name – it was a Giant Leopard Moth. And I learned that in its caterpillar stage, it dines on broad leaf plants such as plantains, dandelions and violets. These are usually considered weedy plants, and are aggressively eradicated from lawns and gardens. But here was a creature that relied on them as a food source!

Giant Leopard Moth

Giant Leopard Moth

This past summer, I had two new types of butterfly visit my garden. Once again, I had to do some research to figure out what they were, and what their larvae fed upon.

Red Admiral

Red Admiral

The first were Red Admirals. They busily worked on the pink coneflowers in my backyard – what a joy! When I read what their caterpillars ate, though, I was puzzled: nettles and hops, which were not plants I had growing in my gardens, nor did my nearby neighbors.

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clearweed

I did, however, have a weed that had gone crazy in one corner of the yard. I don’t use herbicides, and I was too busy to contend with it manually. I finally identified it as Clearweed, and learned that this eastern US native is a non-stinging member of the nettle family! The hand of Mother Nature seemed clear in the relationship between the Clearweed growing out of control and the appearance of Red Admirals in my gardens.

Common Buckeye

Common Buckeye

The second new visitor arrived just as my Sedum ‘Matrona’ was entering its prime, and it busily worked the flowers alongside scores of honeybees and native bees. I came to learn that this newcomer was a Common Buckeye, but there was nothing common about it as far as I was concerned - I was thrilled. I’m sure you have already guessed that, once again, one of the preferred foods of the larval stage is a weed: plantain.

plantain

plantain

Every summer, I await the reappearance of Fritillary butterflies. They seem especially fond of the nectar of Verbena bonariensis and coneflower, but they also require the presence of violets. The female lays eggsin late summer on or near violets. The eggs hatch in fall. The young caterpillars overwinter until spring, when they begin dining on violet leaves. Without violets, which are the only plant the caterpillars consume, there would be no Fritillary butterflies.

Fritillary butterfly

Fritillary butterfly

common violet

common violet

Over the last decade, there has been a surging interest in including native plants in our gardens, as we have come to recognize their vital role in our ecosystem. A functioning food web is created by using plants that evolved with our native animals - and insects. And we probably should allow some weeds in the mix.

Planting Natives for Wildlife

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Guest Post by Trista Ashok

For the next two years, garden clubs across Massachusetts are joining an initiative to encourage the planting of native plants. There are some great reasons to choose native plants for our gardens.  Native plants are adapted to our local climate and soil conditions and thus can be easier to grow.  They are also a very important part of the local food web, providing seeds, pollen, nectar, and forage for our wildlife.  The leaves of native trees and plants play host to many butterflies and moths, and many birds rely on those caterpillars for food, especially for their babies.

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While most might think that a garden full of plants with pristine, uneaten leaves is prettier and more desirable, this is a detrimental environmental landscape. We actually need leaf-munching caterpillars and other insects in our garden. Dr. Doug Tallamy, the author of Bringing Nature Home, found that a pair of Carolina Chickadees feed their babies up to 9,000 caterpillars before they fledge. Most songbirds do not reproduce on a diet of berries and seeds, but instead need a protein-rich diet of caterpillars and insects during the breeding and nesting season. And the vast majority of our native, plant-eating insects only feed on particular plant species that they have adapted to over time – which means they need native plants. Our native oak trees are hosts to over 550 different species of caterpillars, for example, while the non-native ginkgo only hosts 5. Our native trees and shrubs act as giant birdfeeders in addition to being hosts for our butterflies and moths. Unfortunately, it is estimated that the average urban/suburban American garden is made up of only about 20% native plants – far less than the 70% that birds such as the Carolina Chickadee need to provide enough insects to sustain their population.

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So what are the best plants for wildlife to put in your garden? The National Wildlife Federation has built a database of plants and the number of butterfly and moth species that use them as host plants.  You can go to their website at www.nwf.org/NativePlantFinder/Plants to see which plants support the highest number in your zip code.  For example, here are the top five plants listed in each category for Holliston, Massachusetts, along with the number of butterfly and moth species that they host:

Flowers and Grasses:

1.  Goldenrod (Solidago) – 125

2.  Strawberry (Fragaria) – 81

3.  Sunflower (Helianthus) – 58

4.  American Trefoil (Lotus) – 32

5.  Joe-pye Weed (Eupatorium) – 31

Trees and Shrubs:

1.  Oak (Quercus) – 473

2.  Beach Plum, Cherry, Chokecherry (Prunus) – 411

3.  Willow (Salix) – 399

4.  Aspen, Poplar (Populus) – 335

5. Crabapple, Apple (Malus) – 291

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What about cultivars?  Many of the native plants we find at nurseries are not straight species, but are instead cultivars, often called ‘nativars’.  These nativars can be a particular variety found in the wild, or they can be bred for a particular feature. But are they as attractive to wildlife as the straight species?  Research is ongoing, but they have found that it really depends.  Plants that were bred to have different leaf colors, such as leaves altered from green to red, were found to deter insect feeding, as different colored leaves have different compounds in them. Also plants that were chosen for showier flowers or doubled flowers with more petals were often found to be less attractive to visiting pollinators. Take for example the ‘Annabelle’ Hydrangea.  ‘Annabelle’ is a naturally occurring cultivar of our native Hydrangea arborescens that was discovered in the wild near Anna, Illinois. It was chosen for its huge flowers, but those ‘flowers’ are actually full of sterile bracts and even the few fertile flowers they have are very poor in nectar.  It is found that pollinators visit ‘Annabelle’ and other mophead varieties much less than they do the lacecap varieties that more resemble the straight species and are full of tiny, fertile flowers in the middle of the showy, non-fertile bracts.

 Another issue is that some of these nativars are developed by European breeders, and resistance to North American plant diseases is not considered. This is how we end up with Bee Balm (Monarda) varieties that are so prone to powdery mildew. Researchers at several botanical gardens are now starting to investigate which varieties of nativars are best for our garden and for wildlife. Many of the cultivars that are chosen for size or compactness are found to be just as beneficial, but much more information is needed about plants bred for different flower sizes or colors.  Many nativars still support more local wildlife than a lot of non-native plants, however.

So should you tear all of the non-natives out of your garden and plant only straight native species? While some may choose that route, I am personally going a more balanced approach, as it was the non-native Clematis ‘Rooguchi’ that sparked my gardening passion in the first place.  When choosing new plants for our garden, however, we should consider native varieties that will benefit our local wildlife and ecosystem.

Trista Ashok is past president of the Holliston Garden Club.

A Walk on the Wild Side

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By Joan Butler

My husband and I are avid hikers and campers. We love hiking on trails and woodland paths, and along streams and lakes. Another love is gardening. But gardening is a bit of a curated affair, whereas hiking offers us the chance to connect to the wild world, and take in the beauty and complexity of life around us. 

The New England landscape is uniquely beautiful in every season:  from the frosted silences of winter, to the exuberance of spring, to the lushness of summer. Autumn and late summer have their own special charm, with a gentler, but no less beautiful, aspect. 

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Meadows and fields of goldenrod and other wildflowers staged beneath a clear blue sky, with the angle of the sun low, are breathtaking. Goldenrod (Solidago sp.) is stunning, and is a workhorse throughout the growing season, typical of many of our native plants. Its foliage feeds a variety of butterfly and moth larvae and its bright flowers (a textural delight!) are an important food source for migrating Monarchs and dozens of pollinators.

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Late summer is the season for the beautiful blooms of asters. Most asters are sun-lovers, but the white woods aster (Eurybia divaricata) is unusual, growing in shade in dry open woods. It is very tough and very showy, with dark stems and white flowers. It readily reseeds, creating stands of misty white in the woods.

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Indian Pipe (Monotropa uniflora) may look like a fungus, but it’s not. It is a small white herbaceous perennial with an interesting survival strategy. Lacking green pigment, it is unable to photosynthesize (produce food). It survives by sapping nutrients and carbohydrates from tree roots using an intermediary: myccorhizal fungi. Also called Ghost Pipe, it can be found rising through leaf litter on the forest floor.

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The bright flowers of Steeplebush (Spiraea tomentosa) are noticeable from a distance, and their form certainly references their common name. Its pink spires are a late-season source of food for butterflies and other insects. This suckering native shrub can be found basking in the sun in moist soil near lakes and streams.

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Nearby, you may also find the unusual flowers of white turtlehead (Chelone glabra). Easy to see how it got its common name! Its primary pollinators are bumblebees strong enough to pry open the two-lipped corolla in order to reach the pollen and nectar inside. And they really have to work at it: a bumblebee I recently watched made numerous attempts and false starts before it got the hang of it. And I have to mention that exiting seemed a challenge, too! Turtlehead has a particularly important eco-function as a primary larval host plant of the Baltimore Checkerspot Butterfly.

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Another lover of moist conditions is cardinal flower (Lobelia cardinalis). Ahh…that brilliant red! It draws us in like a magnet. The long tubular flower form is difficult for most insects to navigate, but the flowers are very attractive to hummingbirds, hummingbird moths and butterflies. 

These are just a tiny sampling of the fascinating, beautiful plants to be found in our woodlands, meadows and lowlands. Initially attracted by their beauty or color or location, I eventually come to marvel at the overall picture: the inter-relatedness of life around us. Just by taking a walk on the wild side.