Webworm Mounts Attack On Area Trees

Homeowners urged to attack webs harboring caterpillars on alders, sycamores, walnuts and birches before infestation spreads


Fall webworm, a native insect, is again active in the Payson area principally on Arizona walnut trees. Most of the webs are still small and few in numbers. And now is the best time to treat the small caterpillars located within the webs before they increase in size and numbers.

The webbing is loosely arranged in layers within the foliage, and the caterpillars enclosed within the webs. The insect is most visible in the Tonto Creek area north of Kohl’s Ranch.

Although Arizona walnut is the most common host, other shade, fruit and ornamental trees are also attractive to the caterpillars. The insect has been identified on apple, ash, cherry, cottonwood, mulberry and poplar trees. Purple-leaf plums, sweetgums and willows are also hosts. In 2009 they were also identified feeding on Arizona alders and sycamores, chokecherry, English walnuts and ornamental birch.

On small trees, webs should be properly pruned out and disposed of in plastic bags. Begin pruning now and utilize a pole pruner on larger trees. Caution: Caterpillar hairs can cause irritation if accidentally rubbed into the eyes. Check trees in late August and September if a second generation occurs and prune out webs properly.

If chemical control is needed, a biological insecticide, Bacillus thuringiensis (B.t.) is available (as a foliage spray) under several trade names. It kills only caterpillars feeding on the foliage and is most effective on the younger caterpillars. Spot treatments of the webs and surrounding foliage with B.t. is recommended. Caterpillars usually stop feeding two to three days after treatment.


Specific chemical recommendations are given with caution. Whenever using chemical pesticides, always check the pesticide label to be certain it’s registered for use on the site to be treated. Chemical registrations and chemical use regulations frequently change. Contact manufacturers for the most current product information, including supplemental labeling and special local needs regulations.

It is a violation of federal law to use any pesticide in a manner inconsistent with its labeling. Check with the Arizona Department of Agriculture, County Extension Agents, and state or federal forest health management specialists for current information on available insecticides.

For further information about this insect, go to www.ento.okstate.edu/pddl/2009/PA8-17.pdf. And for additional information about this insect or other forest health concerns in the Payson area, contact Bob Celaya, Forest Health Specialist, Office of the Payson Field Office at (928) 474-2689.


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