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Schmidt: Fall webworm starting to appear

By Staff | Aug 5, 2016

Fall webworms have begun to emerge. Although the silken nests that are appearing in the trees of yards and public grounds this time of year can be very unsightly, fall webworms cause more of a nuisance than a threat to tree health.

Fall webworm (Hyphantria cunea) will eat the foliage of many trees and shrubs. There are a number of trees that are likely hosts for fall webworm including: alder, aspen, birch, chokecherry, cottonwood, elm, fruit trees, maple, pin cherry, poplar, willow, and other hardwood trees, but chokecherry, cherry, birch, and elm leaves seem to be preferred.

There is no need to worry about the defoliation caused by the feeding of fall webworms. The leaves have pretty much done their job for the year and you can let the webworms eat. When they get full, they will leave the nest and find a sheltered place to form a cocoon and spend the winter. The trees will be fine. Often times the defoliation is more distressing to the homeowner than it is to the tree.

Since the caterpillars do their feeding within the web, control with insecticidal sprays is difficult as the pesticide must contact the webworm larvae. This can be achieved by opening the nest- a tedious task- before applying the insecticide. Insecticides containing the active ingredients: carbaryl (Sevin), malathion, bifenthrin, and permethrin can kill fall webworms. Safer insecticides like Bacillus thuringiensis var. kurstaki, horticultural oils, and insecticidal soaps can also be used, but are most effective before the worms become enclosed inside their webs.

It is justifiable to spray young trees to protect them from webworms since these trees don’t have a lot of leaves and they need them to overcome transplanting shock and to prepare for winter. It is hard to justify killing webworms in tall trees such as elms and birches. These mature trees are under minimal stress due to the feeding of webworms and you are more likely to get the toxic insecticide on you when spraying.

Some gardeners choose to prune out nests as soon as they appear, but it is not totally necessary and can cause more harm to the tree in terms of the open pruning wound being an entry point for other infections. The use of propane blow torches to “fry” the worms is not recommended for obvious fire risk potential and can also cause more damage to the tree than the webworms themselves.

Fire Blight Symptoms Also Occurring Now

Now is also the time to be on the lookout for fire blight, a serious disease of apples, crabapples and pears. Blossoms, blossom spurs, and branch tips are infected, turn brown and die rapidly. The tips of branches often curl to form what resembles a shepherd’s crook, or the curved end of a walking cane. The dead leaves remain brown to almost black and may remain on the branch all winter. The disease is caused by a bacterium that lives in the conducting or vascular tissues of the plant. The bacterium progresses down the diseased shoots and spurs, and sometimes reaches a larger branch where a dark or discolored area is formed. This discolored area is called a canker. The bacterium lives over the winter in young cankers with smooth, indefinite margins. In the spring, bacteria ooze from the canker, forming a sticky, thick liquid that insects feed on. These insects transmit the bacteria to blossoms, where new infections start. Later, the disease may spread from infected blossoms and old cankers to the shoots. Rain, wind, and hail help spread the disease.

Successful fire blight control requires a combination of actions. First, overwintering cankers should be pruned away by cutting at least eight to twelve inches below the edge of the canker. Smaller diseased branches should be pruned about twelve inches below the edge of the diseased area. Pruning is best done in late fall after leaf drop, when the diseased shoots can be readily seen since they retain their leaves. Or, pruning can be done in late February to early March. Dormant-season pruning is the best option for treating fire blight. Pruning tools should be sterilized between each cut, Lysol diluted 3/4 cup per gallon of water, or household bleach diluted 1 cup in each gallon of water. Household bleach works very well, but corrodes tools, which must be carefully washed and oiled after use to prevent rusting.

Second, apple or pear trees can be sprayed with streptomycin at blossom time. Use streptomycin at 50-100 parts per million every 3-5 days during blossoming. It can also be used every 14 days after blossoming, but streptomycin is not as effective for control of shoot blight as it is for control of blossom blight. After a hail storm, streptomycin can be sprayed immediately to reduce infection in the hail-induced wounds. Uptake of streptomycin is improved if applied during the evening. Streptomycin should not be applied within 50 days of harvest for apples or within 30 days of harvest for pears. It is not registered for use on crabapple, cotoneaster, or mountain ash.

Fire blight is most severe on succulent shoots. Avoid over fertilizing trees, and do not fertilize in late spring or early summer. Dormant-season pruning is the best option for treating fire blight.

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