If arborists and tree care workers had their own reality television show, it might be called something like “Bug Wars.” Each week the producers would highlight the banter around selecting application equipment, integrated pest management, convincing clients that regular inspections are needed and, of course, difficult-to-control pests.
Emerald ash borer (EAB) and Asian longhorned beetle (ALB) would be discussed on a recurring basis. Without a doubt, these insects are significant pests, causing millions of dollars in damage each year. Like many other invasive species, they are neither staying put nor on the decline.
Asian longhorned beetle
Native to China, Korea and Japan, ALB was first introduced to the U.S. in the mid 1990s. Most likely, the beetle was transported in wooden shipping crates from Asia. Infested areas in North America currently include Illinois, Massachusetts, Ohio, New York, New Jersey and Toronto, Canada.
Asian longhorned beetles are readily identified by their striking appearance. Most obviously, they have long (between one and two times their body length) segmented antenna marked with alternating white and black bands. The antennae of the males tend to be slightly larger than those of the females. ALB wing covers are also black and white, with the white appearing in the form of small, irregularly shaped spots. The larvae are segmented, off-white to beige in color, and can grow up to 2 inches in length.
ALB adults emerge from trees from late spring through mid-fall, with peak emergence generally occurring during midsummer. Beetles chew their way out of the trunk, leaving exit holes that range from 3/8 to 1 1/4 inches in diameter. These emergence holes are round and extend deep into the sapwood. In many cases, an accumulation of coarse sawdust and frass can be found at the base of infested trees. Tree sap may also ooze from the holes or from wounds created during egg-laying activities. Soon after emergence, females chew fingernail-sized grooves in the bark and deposit their eggs. Larvae hatch within a month, tunnel into the tree, and spend the next several weeks feeding in the phloem. Later larval stages move to the xylem. As they feed, larvae create winding galleries in both the sapwood and heartwood, resulting in significant damage to the tree’s conductive vessels and structural integrity.
Many eradication efforts have been undertaken throughout the country with varying degrees of success. In the Chicago area, ALB has been suppressed through tree removal, quarantines and insecticide treatments. Unfortunately, these efforts have often been hampered by ALBs wide host range, which includes maple, horse chestnut, elm, green ash, poplar, willow, black locust, mulberry, plum and pear.
The only assured way to eliminate ALB involves removing infested trees and destroying them through chipping or burning. In addition, adding diversity to the landscape is always a plus by creating a less-suitable environment for the beetle. Utilizing tree species that are less susceptible to ALB can be an important management strategy. Tree species such as hackberry, beech, ginkgo, honeylocust, Kentucky coffeetree, oak, bald cypress, hornbeam, filbert, serviceberry, dogwood and redbud, as well as conifers including spruce, fir and juniper, can be included in replanting programs to increase species diversity.
Emerald ash borer
Most borer species (and insect pests in general) feed preferentially on weakened specimens of a given tree species – sort of a culling of the herd or survival of the fittest. The emerald ash borer is an exception to this phenomenon. EAB infest all species of ash – blue, green, black and white – and feed on healthy as well as stressed trees. Major damage has occurred from this introduced pest in the Great Lakes states of Michigan, Illinois, Ohio and Wisconsin, with a trend for westward spread.
EAB generally has a one-year life cycle. In colder regions like northern Michigan and Canada, however, two years may be required to complete their life cycle. Adults begin emerging from infested trees in early summer, and depending on the locale, emergence continues through midsummer. Although only about .5 inch in length, adults are relatively easy to spot if found at rest with their metallic green wing covers. EABs mate during the day, especially during warm, sunny conditions. Unlike some wood-boring insects, EABs feed in both the larval and adult stages, which increases their potential to damage trees.
A few days after mating, females begin laying eggs into bark crevices on tree trunks and branches. This egg-laying activity is repeated many times for each beetle, resulting in a total of 60 to 90 eggs being deposited by each beetle. Eggs hatch in seven to 10 days, and the tiny larvae chew through the bark and move into the cambium. These larvae feed for several weeks within phloem tissues, producing a winding or S-shaped feeding pattern. As feeding continues, flattened tunnels or galleries are formed and become packed with finely powdered frass, the fecal output of the larvae.
Feeding continues until late fall, when larvae transform into a prepupal stage. Winter is spent in either the outer sapwood or bark of the tree depending on the ash species, age of the tree and bark thickness. In early spring, the prepupae pupate within pupal chambers. After about two weeks, adult EAB emerge through D-shaped 1/8-inch-diameter exit holes, and another generation begins.
Early detection is an important consideration when managing EAB infestations. Because larvae feed inside the tree, where they are hidden from view, it is often difficult to spot EAB injury. This is especially true for newly infested trees.
An early sign of EAB infestation is the presence of the characteristic D-shaped exit holes on the trunk and surfaces of the lower limbs. Also, because larval tunneling takes place beneath the bark, vertical splits are often visible directly above these galleries. Removal of this bark will reveal S-shaped, frass-filled tunnels and discolored sapwood caused by secondary infection from fungal pathogens. Later, jagged or irregular holes may appear in the bark as a result of woodpeckers seeking food.
Successful management of EAB includes the use of insecticides. Ash trees can be effectively protected from EAB using bark and canopy sprays containing bifenthrin, carbaryl, cyfluthrin, dinotefuran or permethrin; soil injections/drenches of imidacloprid; or trunk injections containing acephate, dicrotophos, emamectin benzoate or imidacloprid.
Efforts should also be undertaken to increase plant diversity in the landscape. Ideally, a diverse landscape should include many different species of both conifer and broadleaf trees with no more than 10 percent of the trees made up of the same species. Considering the aggressiveness of EAB, it is probably wise to consider alternatives to ash when establishing new plantings.