Most Unwanted Pest #5: Elm Bark Beetles

Our reverse countdown of the 10 Most Unwanted Pests continues with #5: the Elm Bark Beetle.

Elm Bark Beetles are mass murderers, responsible for the devastation of the American Elm population – up to 99% of elms have succumbed in some areas. Once the go-to urban tree, the rows of stately elms that lined our streets and shaded our parks have disappeared due to the Dutch Elm Disease transmitted by these beetles.

European Elm Bark Beetles (Scolytus multistriatus) and the Native Elm Bark Beetle (Hylurgopinus  rufipes) make their home – and their meals – beneath the bark of trees. The beetles penetrate the bark and bore through to the sapwood. Once inside, they gouge out galleries for their eggs. The larvae develop beneath the bark of the tree and expand the galleries to eat the underside of the bark.

The hungry Elm Bark Beetle larvae gouge out
these galleries as they feed.
Image: Joseph O'Brien, USDA Forest Service, Bugwood.org

 

Unlike most bark beetles that target soft-wood trees, the Elm Bark Beetle targets the hard-wood American Elm. The beetles typically lay their eggs in elm trees that are already weakened or beginning to decay. This means that Elm Bark Beetles would never have made our Unwanted list on their own: their larvae rarely do substantial damage to a healthy tree, and controls are available to fight these infestations. It is the adult Elm Bark Beetle that is causing so much such fatal damage. Although most of their lives are spent in weak trees, they fly to feed on healthy trees as part of their life cycle. They carry the fungus that transmits Dutch Elm Disease from the failing tree to its healthy neighbor.

Once the fungus enters the elm tree it starts to clog up the tree’s xylem – the aquaduct system inside the tree. The tree responds by trying to seal off the infected xylem tubes with its own clogging mechanism: this is why one of the first signs of Dutch Elm Disease is a yellowing patch of canopy as a branch stops providing water to it.

Elm tree branches affected by Dutch Elm Disease.
Source:  USDA Forest Service - Northeastern Area Archive,
USDA Forest Service, Bugwood.org

Can Dutch Elm Disease be stopped? Not currently. But individual trees can be maintained in good health for a long time – and a fortunate few never succumb. The first step in prevention is keeping elm trees healthy. Beetles are most attracted to trees that are already stressed, so a regular program of watering and fertilization is important.  Dutch Elm Disease also spreads less quickly in a well-watered tree – greater hydrostatic pressure can help slow it down.

There are also inoculations that can slow the progress of Dutch Elm Disease. The earlier the better.  So it’s extremely important to monitor elm trees for any signs of infestation or canopy yellowing. In the early stages, pruning can remove infected branches before the disease spreads to the rest of the tree.

Finally, if you have an elm tree that is beyond saving, it should be removed immediately. The wood must be disposed of appropriately and the stump removed. The roots should also be severed if other elms are near, to prevent the disease spreading through root connections. 

Unwanted Pest #6: The Evil Weevil

Number 6 in our countdown of America’s Most Unwanted garden pests is the Black Vine Weevil.

The Black Vine Weevil (Otiorhynchus sulcatus ) is a root feeder. Although adults will damage foliage, the most serious damage is done by larvae feeding on roots. The insects’ diet includes hundreds of shrubs and ornamentals, including yew, hemlock, some rhododendrons and other broad-leaf evergreens like azalea, mountain laurel and euonymus. Adults can also feed on deciduous and herbaceous plants.

When shrubs fail to put out new foliage in the spring or new foliage quickly yellows or dies, one of the first suspects is this evil weevil. Another indication is the characteristic crescent notches on the leaf margins made by adults feeding. The adults are active at night throughout the summer. The non-flying insects are about  ½” long, slate grey to blackish-brown with pitted wings and a short snout.

Prevention and Control

One important step to prevention is carefully examining new plants before bringing them home. Since adult insects are unlikely to be seen, the leaves should be checked for signs of damage.

If you begin to see leaf damage on your shrubs – or stunted growth – call your Almstead arborist for an inspection. Often, by the time the damage is noticed, the plant’s health has been seriously compromised.

Biological controls for Black Vine Weevil are beginning to be available. Nematodes (little organisms that make weevils look big) are ingested by the weevils; they then execute an Alien-worthy switch and kill their hosts. Although this biological control is promising, chemical controls are presently the usual choice for dealing with these pervasive pests. Treatments are most effective on adult populations before they begin to lay eggs. That puts the timing of treatments around late May, with repeat applications through the summer if necessary. (These weevils can have several generations per summer.)

If you didn't treat for Black Vine Weevil this year and are wishing that you had, consider a fall treatment to reach overwintering larvae, which are found in the soil.

Unwanted Pest #7: Aphids

#7 in our series on America’s Most Unwanted garden pests is the all-too-common aphid. Aphids otherwise known as plant lice, are tiny soft-bodied, sucking insects. With over 4,400 types of aphid worldwide, they are the most common garden pest. Most aphids only target one kind of plant, while others have a much broader diet.

These tiny (1/32”) pear-shaped insects can be winged or wingless and almost any color. A colony of aphids can cause substantial damage to plants, trees and shrubs including yellowing or curling leaves and stunted growth. In addition to sucking the juices from plants, their saliva is actually toxic to many plants. Aphids often transmit fungi and disease to the plants they infest. 

Aphids with their ant bodyguards
Source: E. Bradford Walker, Vermont Dept. of Forests,
Parks and Recreation; Bugwood.org

In the process of feeding, aphids secrete a sticky, sugary liquid called honeydew -- which can rain down from the trees on anything below it, like your car or lawn furniture. Given the size of an aphid, when there’s enough honeydew to drip from a tree, you know that there is a huge population feeding above.  

The sweet honeydew is attractive to ants. Many sugar-eating ants actually act as body guards to aphids: they protect them in order to milk the aphids for honeydew. In addition to the damage aphids cause directly, the honeydew can make plants more susceptible to fungi, like sooty mold.

Potato Aphids
Source: David Cappaert, Michigan
State University:Bugwood.org

Inspecting plants for aphids is the first step towards controlling this pest. Many aphids are well camouflaged to blend with the plants they target. Their coloring – along with their tiny size – means aphids are often overlooked until they are so numerous that they coat the plant. Aphids are a preferred food of beneficial insects like ladybugs, predatory wasps and lacewings; a sufficient population of these species can keep aphids under control.

If aphids have populated a plant or shrub, a stream of water from a garden hose can sometimes dislodge them; although flying aphids are likely to return, crawling ones often won’t make it back to the plant.

Because of the potential damage from aphids, as well as the diseases they transmit, it is wise to begin controlling aphids at the first sign of infestation. Almstead arborists offer both organic and traditional methods for control. Often a single application in spring is sufficient to keep aphids in check.

Most Unwanted Pest #8: Tent Caterpillars

Our countdown of the Most Unwanted arbor pests continues with #8: the Eastern Tent Caterpillar (and a similarly domiciled pest, the Fall Webworm).

Although many insects can be difficult to identify – or even detect – the tent caterpillar is not. One day you look at your tree and notice that something has actually pitched a tent there. The tent looks like an industrial version of a spider web: thick, white and slightly opaque. If you look carefully, you may see the newly emerged caterpillars celebrating their good fortune.

A single tent is not an infestation – yet. But often, if you continue your examination of the tree, you’ll find that it’s turned into a caterpillar campground. An infestation of tent caterpillars can defoliate – or eventually kill  –  a tree.

These insects are native to our (NY/NJ/CT) region. The larvae emerge in spring and construct a tent for their dormitory. They emerge daily to feed on the leaves of their host tree. Their favorites are black cherry, choke cherry, scrub apple and many species of ornamentals in the family Rosaceae (including serviceberry, hawthorn and many more).

A tree with several colonies of eastern tent caterpillars can be completely defoliated by the hungry grazers. Most trees can withstand this for a single season, but a repeated assault can leave the tree without the resources to survive. 

For a residential tree, the unattractive appearance of caterpillar tents and the resulting defoliation are reason enough to try to remove the pests. For a smaller tree, Almstead arborists can selectively clip off the egg masses in the fall, or remove the caterpillar tents in spring. For a large tree, it is more practical to apply a control to prevent the larvae from emerging.

Just as tents in your tree in spring usually indicate tent caterpillars, similar tents in the fall are usually a sign of Fall Webworms.  Black cherry is their preferred tree but they are also common on alder, apple, beech, birch and oak.

Fall Webworm Damage
Courtesy of  Ronald F. Billings, Texas Forest Service, Bugwood.org

In June and July, moths lay their larvae on the underside of leaves and cover them with scales. As the larvae grow, they eat the leaf tops; the tents are expanded as they move to new branches. It’s possible for the tent to envelop an entire tree and for the webworms to defoliate it completely.

The webworms don’t typically kill trees, since trees have already stored up most of their nutrients by the time the defoliation occurs. However, the effect can be grotesque. Your Almstead arborist can discuss alternatives for shutting down the tent city in your tree. 

Unwanted Pest #9: Japanese Beetle

This is the second villain in our countdown of the Top 10 landscape pests in our area:

Beautifully glowing with green and blue iridescence and coppery wings, the Japanese Beetle doesn’t look like it should be despised.  But Japanese Beetles have earned their unsavory reputations by having populations that quickly grow out of control and damage almost every part of our landscape.

And summer is Japanese Beetle time: this is when the adults emerge from the ground and start dining on our plantings. With a diet that includes over 300 different plants, our yards are their buffet. They devour the soft leaf tissue of our plantings, often leaving only the leaf ribs behind. Since July is also their time to mate, a few Japanese Beetles can turn into a mob when they release pheromones that attract other beetles from a mile away or more.

In late July through August, the females lay their eggs in our lawns. It takes less than two weeks for the larvae to develop into grubs and start gnawing their way through the grass roots. These grubs are the single most damaging lawn pest we have.

There are a few different ways to control Japanese Beetles. Certainly the simplest is to remove them by hand -- and if the beetles have confined themselves to a single rosebush, it’s possible to curtail their population this way. However, if they’re munching through your Birch tree – or you have no interest in hand-picking beetles – you probably should consider a more sophisticated solution. We usually recommend applying a low-toxicity spray to the most susceptible plantings, repeated 2 or 3 times over the course of the summer.

The grubs represent a different challenge. Because Japanese Beetles are very mobile, it’s possible to have lots of grubs beneath your lawn, eating the grass roots and creating dead brown patches, without seeing many adults. We have several options for treating grubs. When we confirm grub activity (we do this by lifting the soil and looking), we can apply a grub treatment. One annual application usually controls all kinds of grubs, including Japanese Beetles.

There is also a purely organic approach that targets only Japanese Beetles.  Milky disease is caused by a bacterium that infects Japanese Beetle grubs. We can apply Milky Spore powder to a lawn to control the Japanese Beetle grub population.  The Milky Spore continues to infect the beetles for several years.

One warning: many sources sell pheromone traps for Japanese Beetles. These traps work well – at attracting Japanese Beetles to your yard from all around the neighborhood. So don’t buy these traps – unless they’re a gift for your neighbor!