ELM LEAF BEETLE
By Dwight Scarbrough
The elm leaf beetle (ELB), Galerucella xanthomelaena (Schrank), formerly known as Pyrrhalta luteola (Müller) is an important elm-tree pest which was introduced to the eastern United States from Europe about 1834. Since then it has spread across most of North America and is commonly encountered in Minnesota. Although, elms are no longer as numerous as before the ravages of Dutch elm disease, the elm leaf beetle is still a major defoliator of elms in urban areas throughout the United States.
This beetle is often confused with other beetles of similar appearance (western corn rootworm, striped cucumber beetle), but it is distinctive in that it occurs early in the season and has three dark spots on the back of the thorax. A common nuisance produced by the elm leaf beetle is it's habit of seeking overwintering shelter in homes during the fall. Fortunately, this beetle is completely harmless. It does not reproduce indoors, bite, or feed on food or clothing as do many other species of insects found in and around human dwellings.
Hosts: The elm leaf beetle will attack all species of elm (Ulmus). In Minnesota the Siberian elm (Ulmus pumila) is the preferred host. They will also feed on American elm (Ulmus americana) and slippery elm (Ulmus rubra).
Damage to Hosts: This insect feeds on the leaves of the host tree, causing them to shrivel and turn brown when damage is severe. Under normal circumstances, beetle-feeding alone will not generally kill an elm tree. However, severe feeding will weaken a tree, making it more susceptible to attack by other insects and diseases which can result in tree demise and eventual mortality. Very young trees, recently transplanted trees, and/or trees that are under stress caused by secondary agents (i.e. insects, diseases, environmental) often die when combined with the added stress caused when defoliation occurs. Even without secondary attack by other insect and disease pests several years of repeated, heavy defoliation on mature trees can cause severe stress which may result in branch dieback and eventually tree mortality. Again, these instances are uncommon and aesthetic injury is most often the only problem.
The larva is the most destructive life stage of the elm leaf beetle, causing more foliar damage than the adult. The adults chew small, irregular holes in leaves, particularly on new growth. The larvae eat most leaf tissue leaving only veins intact (skeletonization). Most of the tree damage is done by the first generation insects. Defoliated trees may grow new leaves in the same season, but this second leaf flushing becomes subject to attack by the second generation of beetles. Fortunately, the elm leaf beetle does not transmit Dutch elm disease.
Identification: The adult beetles are 1/4" to 3/8" long and vary in color from yellow to olive-green. There is a distinct longitudinal black stripe along the outside edge of each wing cover and an elongate spot at the base of each wing cover near the center of the body. The eyes are black, and there are three black spots on the thorax just behind the head. Eggs are spindle shaped, orange-yellow in color, about 1/16" long and are laid in clusters. When full grown, the larva are about ½ " long and have a dull yellow color with two black stripes. The pupa are about 1/4" long and are a bright orange-yellow color, and have black bristles.
Life History: Elm leaf beetles overwinter as adults and are gregarious, congregating in the fall to seek a protected hibernation site. This site is often outdoors beneath piles of leaves, boards, rough bark, in cracks and crevices or some other protected location. However, beetles frequently move into homes or animal dwellings, hiding beneath floor boards, in attics, between walls or in cracks and crawl spaces. During warm winter weather, beetles may become active, making their presence noticeable to the homeowner as they crawl throughout the house. They again become active in the spring, and congregate in windows as temperatures increase.
As elm buds begin to expand in spring (early-mid May), beetles move from overwintering sites to begin feeding and egg laying on the undersides of elm leaves. These eggs hatch in about a week, and the young larvae begin feeding on the undersides of leaves for the next two to three weeks. Only the veins and the upper leaf surface are left, giving leaves a "skeletonized" appearance. Heavily infested trees take on an overall brown appearance as if scorched by fire, and often drop their leaves prematurely. In contrast, adult feeding creates only small, irregular holes through the entire leaf and is usually of minor importance compared with larval feeding. Once the larvae have completed their maturation feeding they migrate from feeding sites down the trunk to pupate in the soil at the base of the tree or in crevices in the bark.
The adults emerge in about 10 days (during July), feed again on the elm leaves, and lay eggs for a second generation. The adults from this second generation begin seeking overwintering shelter starting in late August to early September. The number of generations per year depends upon the length of the growing season which varies considerably across the range of this beetle. There are usually two generations per year in the northern parts of the range. Most of the tree damage is done by the first generation insects. Defoliated trees may grow new leaves the same season; but this second leaf flushing becomes subject to attack by the second generation beetles.
Control on Trees:. Several conventional insecticides are available for effective control of the beetle. Environmentally sound alternatives to conventional pesticides such as the bacterium, Bacillus thuringiensis var. tenebrionis (M-Trak®) are also effective. Regardless of the material used, applications should be made to the foliage as soon as eggs hatch while larvae are still small. Older larvae are more difficult to kill and produce more damage than when they are young. Thorough coverage of all foliage is essential to obtain adequate control. Therefore, it is important to use equipment that is of sufficient size and capacity to enable coverage of the tallest trees needing treatment.
Be sure to read the label on the insecticide container carefully, and use only those formulations that list the pest you want to control and the site where you wish to apply the insecticide. The following formulations of insecticides may be applied as sprays:
1. Carbaryl (Sevin) at the rate of 2 pounds 50 percent wettable powder per 100 gallons of water, or 2 tablespoons 50 percent wettable powder per gallon of water.
2. Chlorpyrifos (Dursban) at the rate of 5 tablespoons of 12.6 percent emulsifiable concentrate per gallon of water.
3. Acephate (Orthene) at the rate of 3 tablespoons of 9.4 percent emulsifiable concentrate per gallon of water.
Do not apply acephate to American elm.
4. Bifenthrin (Talstar) at a rate of 1 to 5 teaspoons of 10 percent WP per gallon of water.
5. Cyfluthrin (Tempo) at the rate of 1 to 2 ounces of 24.3 percent emulsifiable concentrate per 100 gallons of water. For use by lawn care and landscape applicators, and commercial nurserymen only.
A community-wide control program is more effective when sprays are applied to protect the trees from the first-generation larvae. When only a few trees are sprayed, later generations of beetles moving from unsprayed trees frequently will re-infest sprayed trees, thus making additional spray applications necessary.
Caution: Insecticides are poisons and should be handled with care. Read and follow all label recommendations and directions. Store pesticides safely and out of reach of small children. Carefully and properly dispose of unused portions of diluted sprays and empty containers.
safety through reduced pesticide drift and quantities. The drawback is that it won't prevent foliage damage by the first generation beetles. Because the insecticide kills larvae as they migrate after feeding, trees will still sustain feeding damage. However, second generation feeding will be reduced through fewer adults emerging from the first generation. This approach is useful if you have an isolated elm tree and are not concerned about defoliation by first generation beetles. If, however, there are many elms in the area, second generation adult beetles from other trees (unless also treated) can fly to and feed on any elm in the area.
Control in the House: Preventing invasion of elm leaf beetles into the home in the fall will forestall nuisance problems in the winter and spring. Migration from host trees to sheltered areas is gradual and may not be noticed. Make the house as tight as possible. Use caulking compound to fill in cracks around doors and windows and all other openings large enough for insects to pass through and make certain that window and vent screens are intact and properly installed.
To prevent beetles from crawling on and into buildings, spray the exterior foundation, outside walls of the house, around window wells, and around other exterior points of entry with carbaryl (Sevin) or chlorpyrifos (Dursban) using the formulations described above for tree spraying.
Beetles entering the house can also be sprayed indoors with insecticide formulations containing pyrethrins, propoxur (Baygon), diazinon or chlorpyrifos (Dursban). Repeated applications will probably be necessary. For best results, mist an entire room with the insecticide and close it off for several hours. Afterwards, dead beetles can be picked up with a vacuum cleaner. Aerosol foggers or fumigators are appropriate for attics and crawl spaces. If possible, remove accumulations of dead beetles because they may attract carpet beetles and other secondary pests. Beetles may also be captured or collected for disposal with a vacuum cleaner.
Biological Control: Several organisms attacking ELB have been studied as possible biological control agents. The most promising candidates are a parasitic fly, two species of tiny parasitic wasps, two species of earwigs and a beetle. Recently, a nematode, a small soil inhabiting roundworm, has been studied for ELB control. More time is needed for research and implementation of these different organisms before they can be of practical use to the homeowner.