Space Invaders

Bugs are everywhere, right? Zillions of them crawling around in the soil, skeletonizing leaves and chewing through trees’ tasty bark.

Well, maybe not zillions. In fact, the true amount of damaging pests numbers only several hundred according to Ohio State University Commercial Horticultural Agent Joe Boggs. "Of the nearly 90,000 described species of insects in North America, only 6,700 are considered pests and, of those, only 700 species cause frequent or occasional serious damage," Boggs says. "The remaining 6,000 rarely cause significant damage. Knowing all this, dealing with pests is really a matter of perspective."

But even with perspective, Boggs notes nine specific pests, their habits and control methods that are worth contractors’ keeping their eyes on this year.

1. EMERALD ASH BORER. Found in southeastern Michigan in July 2002 by a plant pathologist, Boggs notes that the Emerald Ash Borer (EAB) is native to Mongolia, northern China, North and South Korea and Japan and was probably transported to the United States in infested wood packing material. Currently, 13 counties in southeastern Michigan are under quarantine for the pest, which is also present in northeastern Ohio. Recently, the North American infestation also was spread to Maryland and Virginia as a result of quarantine violations.

"All North American ash species are at risk from ½-inch caliper to full-size trees," Boggs says. "Though most native borers choose weakened trees, the Emerald Ash Borer is not selective." The EAB does, however, have a similar life history to its native cousins the bronze birch borer and two-lined chestnut borer. EAB larvae are creamy white to greenish-white, legless and have triangular-shaped body segments. The larvae overwinter through maturity and develop from mid to late summer. Adults emerge through distinct D-shaped holes from late May through early August. As larvae, EABs feed on the host tree’s phloem, disrupting its water transport system. A given ash tree can die three to four years after initial infestation though mortality can occur as early as the first year.

Right now, the Detroit area is the hot spot for EAB, and the infestation there will be managed with a firebreak. Specifically, more than 12 million ash trees in the Detroit area have been taken by the insect. A $43-million plan, including the removal of trees within a half-mile radius of infected species, now is underway to prevent the borer from spreading further.

Adult emerald ash borers have bright green, slender, elongated bodies measuring 7½ to 13½ millimeters in length. The females are darker than the males. The adult insect’s coloring is brassy and golden green overall with darker, metallic emerald green wing covers called elytra. The top of the pest’s abdomen is a metallic coppery red color.

Emerald ash borer larvae can reach a length of 26 to 33 millimeters. The young insects are cream-colored and dorso-ventrally flattened. Their brown heads mostly are retracted to the prothorax, and only the mouthparts can be seen from the outside.

Although research on the ash borer’s life cycle is still underway, the pest appears to have a one-year lifespan in southern Michigan but could last two years in colder regions.

Symptoms of the tree-killing pest can be tough to spy – at least until canopy dieback occurs. Such dieback appears when a tree is girdled by the serpentine tunnels excavated by emerald ash borer larvae. The result: many infected ash trees appear to lose about 30 to 50 percent of their canopy coverage in one year, and most die completely within the first three years following infestation.

Less obvious signs of emerald ash borer infestation include D-shaped exit holes on a tree’s branches and trunk. Also, callus tissue produced by the tree in response to larval feeding often results in vertical splits 5 to 10 centimeters in length on the bark above the gallery.

Emerald ash borers are ruthless when it comes to ash trees – regardless of size and vitality. The pest has killed trees of various sizes and conditions in Michigan. In their larval stage, the insects have infested and taken down trees ranging from 5 centimeters in diameter to sawtimber-sized trees measuring more than a few feet in diameter.

While treatments of imidacloprid can eliminate early instar larvae, according to Boggs, if infestations are not identified early on, many states are implementing regulations that call for the elimination of all ash trees within ¼ mile of any infested trees.

"The main confusion with that point is that contractors are wondering if they should continue preventively treating ash trees," Boggs notes. "In Ohio, and depending on regulations in other states, you can continue to treat a tree for a client but if another ash tree ¼ mile up the road is found to be infested, that customer is going to lose their tree regardless."

For contractors in this situation, Boggs recommends providing clients with a written explanation of local regulations so everyone involved is aware of efforts to head off further damage. As it stands, the EAB has claimed 6 million ash trees so far and, according to the Michigan Department of Agriculture, the pest could cause $50 to $60 billion in damage nationwide. Without strong eradication efforts, the financial impact from this pest could be devastating.

2. BAGWORM. "The bagworm can lull us into a false sense of security if we’re only monitoring evergreens," Boggs says. "Most people think of bagworm on arborvitae or junipers, but it can actually feed on 120 different plant species. Several years ago in Cincinnati we had an outbreak of bagworm on box elder and honey locust."

Bagworm, which can have two to three generations a year, are often difficult to detect early because they work pieces of the host plant into their bags of silk, Boggs says. This can camouflage the pest to plant health care technicians as the bags get larger. Still, catching bagworm early is the best control method. "If you find the bags are ¼ to ½ inch in size, these can be controlled with Bt (Bacillus thuringiensis) and this is important because Bt is a naturally occurring bacteria that has less of an impact on the parasitoids that can naturally control bagworms and other caterpillars," Boggs said, adding that some of the bagworm’s natural predators include yellow jackets, bald-faced hornets and paper wasps. "We emphasize combining the use of insecticides with natural control methods for best results."

Boggs say that timing Bt treatments for when bagworms are still small is key because larger insects are more difficult to kill with insecticides, and bagworms in particular have another trick up their sleeves. "If a bagworm is approaching the last instar stage, they’re just about ready to pupate," he says. "A caterpillar that is exposed to insecticides at this stage will stop feeding and pupate early so the technician’s solution won’t work at all. Additionally, the multiple generations of bagworm make it necessary for contractors to keep an eye on infested plants even after administering treatments."

3. EASTERN TENT CATERPILLAR. This pest makes its nest or tent in the forks of branches, but does not enclose leaves in the tent as the fall webworm does. However, Boggs says that all tent caterpillars, including eastern tent caterpillar and fall webworm, have a bevy of natural enemies.

"We’ve learned by watching eastern tent caterpillar that infestations can be worse some years than others," he notes. "Some of that can be environmental, meaning that there can be disease-causing organisms in the environment and the caterpillars die from the disease. If the disease is caused by a fungal pathogen, that may be more apparent during a wet spring than a dry spring and the size of the population will reflect that. Also, sucking pests like assassin bugs naturally feed on eastern tent caterpillar."

As with other pests mentioned here, eastern tent caterpillar must be monitored and attacked when nests are still loose. While they prefer to nest in wild cherry trees, most other ornamental fruiting trees, as well as shade trees like oaks, maples and hawthorns also are at risk. Because the pests tend to group together, one or two colonies can completely defoliate small trees, and major outbreaks in large trees will do the same. Additionally, damage can be compounded by eastern tent caterpillars’ multiple generations.

"These pests have two or three generations a year, and the female keeps laying eggs in the same areas, so the nests become very dense," Boggs says.

Shiny, gray, foam-looking egg masses about 1 inch long are wrapped around small twigs and contain as many as 150 to 300 eggs. These will hatch in early spring and the tiny black caterpillars soon move to a nearby fork in the branches. For four to six weeks as they mature, the larvae find leaves on which to feed, but return to the nest afterwards, leaving silk trails along their paths. Mature larvae usually find a new tree in which to spin cocoons. Adults, about 1 inch long and reddish-brown with two white stripes, emerge two to four weeks later and the mating and egg-laying cycle begins again. "Because the nests and egg masses are easy to spot, removing eastern tent caterpillar from trees is relatively easy," Boggs says. "You can clip the egg masses off and dispose of those when you see them in the fall. Otherwise, in the spring, you can remove tents from the trees using a broom and either crushing the caterpillars or dropping them into some warm, soapy water. Bt is also effective."

4. LARCH CASEBEARER. "This is one of the more quirky pests we’ve seen, but one that is almost always in the background," Boggs says. "It’s difficult to identify because it’s so small and they actually hide themselves within the larch needles."

The larch casebearer mines out larch needles and insert its body into the hollow needle, Boggs explains. However, the pests cannot consume whole needles early on and will instead nibble on the edges. "This causes the needle to look washed out," Boggs says. "As the needles die, you can stand back and see that the tree looks like it’s becoming a silvery color. With large populations you can see complete defoliation."

The larch casebearer does its needle mining for three to four weeks in early spring as the needles begin to grow. From there, the larvae will pupate into adult moths that are active from late May to early August. New eggs are laid on the needles, and young larvae will mine needles again from mid to late summer before overwintering. To control the pest, Boggs recommends using any insecticide labeled for larch, and that Bt also should be effective because the pest is a caterpillar.

While most reports of the larch casebearer come from Michigan and its Upper Peninsula, larch trees in Ohio and New York also are susceptible.

5. DUSKY BIRCH SAWFLY. While they look very similar to caterpillars, sawflies are different in their habits and in how they must be controlled. "Caterpillars grow up to be moths and butterflies, but sawflies grow up to be wasp-like insects," Boggs says. "One common problem that I’ve seen is that people will call me up and say they’ve been using Bt and it’s not killing these caterpillars. Of course, it won’t kill them because there’s no Bt variant that will kill sawflies – that’s why the distinction is so important. We want to advocate the proper control methods and not risk overuse of pesticides."

Distinguishing between a sawfly and a caterpillar is simply a matter of counting the number of leg pairs on the caterpillar, Boggs explains. "If you start at the caterpillar’s head, there are three pairs of legs that are separated from the rest," he says. "Those are the thoracic legs and will become the legs on the adult. The rest of the legs on the pest – the prolegs – are fleshy and will be lost on the adult. Caterpillars have five or fewer pairs of prolegs while sawflies have six or more pairs. Some sawflies can look remarkably like caterpillars, but looking for these distinguishing features is how to tell them apart." Additionally, Boggs jokes that sawflies are self-identifiers, as they roll into an "S" shape when they are disturbed.

Dusky birch sawfly has two generations per year and can start out very small to the degree that they’re hard to see, Boggs says. "What you’ll notice is that the leaves will gradually become skeletonized because early instar larvae will not eat the veins of the leaves," he notes. "They eat the edge of the leaves or eat windows in the leaves."

In addition to being difficult to see when they’re small, the dusky birch sawfly also changes color as it matures and further blends into leaves as its colors changes. Early on, the sawfly is gray-green in color and later instar larvae can turn a yellow-green. "As they mature, the dusky birch sawfly starts to consume entire leaves and they feed in groups," Boggs notes. "You’ll start seeing whole leaves disappearing and this will really become apparent when the second generation begins to feed."

The dusky birch sawfly’s primary host is gray birch, but sometimes black, red, paper and yellow birch are also attacked. A pyrithroid insecticide can control the pest or small populations can be removed by hand and destroyed. The larvae remain active to mid-Sepember.

6. WHITE PINE WEEVIL. Though found most often in the eastern United States, the white pine weevil is also found in the northern Rocky Mountains and on the West Coast where it is called the Sitka spruce weevil. Additionally, Boggs mentions that this insect is showing up in places where it had not previously been a problem. Broadly, it has made its way to northeastern and central Ohio, and Boggs notes that the white pine weevil also is becoming a factor in landscapes as opposed to only forest or Christmas trees where it is most common.

"The real challenge with white pine weevil is that it overwinters as an adult female that’s ready to lay eggs," Boggs notes. "In early April, when it warms up enough for her to start moving, the female weevil will move up the main leader of the tree or somewhere around the first whorl of branches. When she feeds, she chews distinct, round holes into the tree and then eats the phloem tissue just beneath the bark. Occasionally, she’ll turn around and lay an egg in that hole."

Though the female weevil’s damage does not do too much harm to the tree, Boggs explains that as the eggs hatch in the summer the larvae will start feeding on the tissue just beneath the bark. Leaders with expanding buds will droop and will ultimately kill the top of the tree.

"If a contractor sees the top of the tree is starting to wilt or go off-color but has perfectly healthy foliage everywhere else, he or she should check the bark at the top," Boggs suggests. "They may find grub-like larvae there and if they cut below where the larvae are found, they can eliminate the problem. If you prune the top of the tree, you can actually tie up a side lateral and train it to become a new top."

Boggs adds that the part of the tree that is removed must be completely destroyed because the larvae can continue to mature in those branches. Otherwise, the pest does respond to insecticide applications made early in the spring or imidacloprid soil drenches made the previous fall. "The soil drenches are good, but should really only be practiced by contractors who see the white pine weevil frequently," Boggs notes. "In places where the problem is rare, the excess pesticides are not really worth their time or effort."

7. BOXWOOD LEAFMINER. Leafminers get their names because the immature larvae feed and overwinter between the upper and lower surfaces of the host plants’ leaves. "You’ll start to see a bubbly appearance because the leaf is delaminating – the two leaf surfaces are separating," Boggs explains. "Because the leaves are not that big on boxwoods, the insect can be large relative to the leaf and it doesn’t take too many joining together until you have a leaf with nothing in between the upper and lower surfaces."

Infested leaves often can drop prematurely, leaving ragged-looking plants with occasional dead twigs. Boggs notes that, in the spring, boxwood leafminer infestations can often appear to be winter damage. Adults emerge in May and mate soon afterward. Females then deposit about 30 eggs in new foliage and then die. These eggs will hatch two to three weeks later and larvae continue to grow and feed inside the leaf for the remainder of the summer.

This pest can be found anywhere in the country where boxwood grows. While it also can be controlled with imidacloprid or standard insecticide applications, Boggs comments that monitoring the emergence of adults is critical for proper application. "Monitoring says that if you notice that more than half the leaves on the plant are infected, the following season you may want to consider initiating a control program." Boggs says. "This pest isn’t considered a killer because even if the population explodes, you’re not going to lose the boxwood. As long as you’re monitoring and determine when the emergence is, you can control the problem the following year." Usually, contractors can make these applications in May.

8. MIMOSA WEBWORM. Originally found in the Washington, D.C., area, mimosa webworm now attacks mimosa and honeylocust as far south as North Carolina, West to Kansas and north to Pennsylvania and New England.

"This pest can go from having one or two webs to infesting the whole tree," Boggs says. "The way it operates is that early in the season the first generation of caterpillars takes the leaflets from compound leaves in the host trees, pulls them together and starts feeding inside that webbing. As they get larger, they start pulling together entire leaves."

At these early stages, Boggs notes that Bt or other insecticide applications can be helpful, but as the pest moves into second or third generations, this becomes ineffective. "The first generation typically does not make nests so tight that you can’t spray and get insecticide to the insect," he says. "However, adult females of the first generation generally lay their eggs in the nests where they developed. Now you have a new crop of caterpillars and they keep expanding this web nest and wrapping more leaves together. Eventually, they create a nest that’s so tight that no insecticide can penetrate it."

The damage caused by mimosa webworms as they feed includes skeletonized leaves that turn brown and die. If left alone, an infestation may completely defoliate a tree by early September. Also, first-generation egg hatch usually appears in Mid-June, and mature larvae from this generation will pupate in mid-July. The second generation of adults appears from late July through mid-August and are active through September. Because of this life cycle, spraying in June when larvae are feeding is effective but, generally, prevention is key. Keeping leaf debris and webbed foliage cleared from beneath and around host trees may reduce the chances of infestation.

9. YELLOWNECKED CATERPILLAR. This pest also has more than one generation per year and is found throughout New York, the East Coast and the Midwest and Plains states.

"The yellownecked caterpillar is another pest that feeds in colonies, so you may see a full branch defoliating at a time," Boggs says. During their life cycles, this particular pest goes through multiple color phases. "It first starts as a coppery color with a tinge of yellow," Boggs adds. "As they get larger they go into the next color phase of orange-red with yellow stripes. In both of these phases, they have long, thin, white hairs. The last instar caterpillars are black with thin yellow stripes and the caterpillar’s hairs are more visible. The name comes from a bright yellow hump just behind the head."

Adult yellownecked caterpillars appear during June and July and females lay white masses of 100 or more eggs on the lower surfaces of leaves. After hatching, young larvae congregate on the foliage for feeding, and mature larvae descend to the ground and burrow 2 to 4 inches into the soil for overwintering during August and September.

While insecticides are effective when larvae are small, there are several natural enemies of the yellownecked caterpillar that can control infestations naturally. Robins and other birds feed on the larvae, and parasitic flies also may feed on the caterpillars.

The author is associate editor of Lawn & Landscape magazine and can be reached at lspiers@lawnandlandscape.com. Ali Anderson, assistant editor – Internet of Lawn & Landscape, also contributed to this story and can be reached at aanderson@lawnandlandscape.com.