The European elm bark beetle and the native bark beetle are the major elm pests, partly because they burrow under the bark as larvae and spread the Dutch elm disease.
Elm Leaf Beetle - Soon after leaves unfurl in spring, the brownish yellow, ¼-inch long beetles chew out rectangular areas in the leaves. After laying eggs on leaf undersides, their 1/2-inch long, yellow-and-black larvae skeletonize the foliage, and drop to the ground at the base of the tree to pupate. In fall, they may migrate indoors, but they normally over winter in tree bark and outbuildings.
Holes In Bark, Bud and Twig Damage
Smaller European Bark Beetle - This beetle, a principal transmitter of Dutch elm disease, frequently attacks weakened trees. Adults are reddish black, 1/10 inch long, and deposit eggs along galleries they excavate in the sapwood surface. Hatching larvae tunnel out at right angles to the main gallery. Adults emerge through the bark, leaving tiny holes, and feed on buds and bark of twigs during the summer.
Leaves Wilt, Branches Die
Dutch Elm Disease - This lethal fungus is spread from tree to tree by Elm bark beetles, as well as by natural inter-grafting of tree roots. It also enters through wounds and pruning cuts. The most obvious symptom is the wilting and yellowing of one or more branches. In cross-section, infected wood shows long brown streaks following the grain of the wood and spots or flecks near the bark. Older American Elms are susceptible, recent releases of American Elm and Lacebark Elm are highly resistant. Promptly remove diseased or dying elms before May, regardless of the cause of their problem. Isolate infected trees by trenching to sever any possible root connections with neighboring trees. (Root grafts can interconnect large elms 30 to 50 feet apart.) Trees can be injected with benomyl, a systemic fungicide, by a professional arborist. Trees with more than 5 percent of their crowns infected are probably beyond help and should be removed. Keep surviving elms in vigorous growing condition by careful feeding and watering.
Bleeding Holes In Trunks, Branches Girdled
Borers - The elm borer, dogwood twig borer, flatheaded apple tree borer and leopard moth borer all bore into weakened elm trees. Sap bleeds from borer holes, often accompanied by fine sawdust. Their feeding sometimes girdles branches or even main trunks. Borers are white, pinkish or yellow grubs, 3/4 to 1 1/4 inch long. Keep trees well watered, watch for signs of injury and dig borers out with a wire or inject holes with a BT (Bacillus thuringiensis) solution. Prune off and destroy infested branches.
Branches Encrusted With Small Bumps
Scale Insects - At least a dozen scale species attack elms. They all form groups of small bumps or blister-like outgrowths on tree stems and leaves, from 1/10 to 2/5 of an inch across. These white, yellow, brown or black bumps are waxy shells that protect the insect beneath. When the scale infestation is heavy, many branches may turn yellowish brown, foliage turns gray green and wilts.
Swollen Lesions on Stems and Trunk
Cankers - At least eight species of fungi cause cankers and dieback of twigs and branches of Elms. Many cankers can be removed by pruning well beyond the affected area to ensure complete removal of infected tissue. Be sure to burn the pruned limbs. Mildly infected trees have been known to recover without any special treatments, but once a tree is heavily infected, it should be removed.
Leaves Curl and Wither
Elm Phloem Necrosis - This disease is thought to be caused by a mycoplasma-like organism (MLO), similar to a virus. Affected trees show drooping and curling leaves, which become yellow, then brown, then fall off. Usually all the branches on the tree show symptoms at once, but sometimes the symptoms develop branch by branch. The phloem tissue of the inner bark develops a butterscotch color and a distinctive wintergreen scent. Highly susceptible trees are killed, while survivors become stunted and sometimes develop witches brooms. The disease is spread from tree to tree by natural root grafts and by leafhoppers. The organism also over winters in infected tree roots. Spray with rotenone or pyrethrum to control the insects. Injections of tetracycline into trunks may stop the disease, but this method is too expensive except where valuable specimens are concerned.
Leaves Spotted, Turn Brown, Drop Prematurely
Leaf Spot - A number of leaf spot fungi attack elms. Often, small white or yellow flecks appear on upper leaf surfaces. These enlarge and turn black in the center. In heavy infections, leaves may drop prematurely. Usually the disease predominates in late fall about the time the leaves drop normally, so little damage to the tree occurs. Gather and destroy the fallen leaves in the fall. A copper fungicide applied 3 times at 10 to 12 day intervals, starting when the leaves are half grown, gives control.