Most gardeners are familiar with spider webs, which are ornately designed and sticky enough to capture most unsuspecting insects. However, one group of webs commonly seen this time of year is nowhere near as sightly and hardly functional when it comes to catching a six-legged meal. The protective webbing of fall webworms can be found in many trees lining city streets and country roads. The recent and sudden appearance of these webs gives the impression that an overnight infestation has occurred. In fact, worms have been actively feeding and spinning for the past four to five weeks and are just now starting to mature. Here is what you need to know about fall web worms and what can be done to untangle their massive webs of defoliation.
Kansas has two races of web worms. For the blackheaded race, moths emerged from overwintered pupae back in mid-May. A few weeks later, in mid-June, moths emerged for the redheaded race. Both sets of moths laid egg masses on the underside of leaves and protected them with white hair from their bodies. Young larvae constructed web masses on the ends of branches and feed on leaves enclosed within the web mass. Initial web masses went unnoticed because of the small size of the larvae and accompanying web. They basically got lost among the lush spring foliage. By the beginning of July, larvae started to mature and pupate into adult moths to start the cycle over and create a second generation. This second generation is larger and more hungry than the first. And now, as they approach the completion of their feeding cycle, they require huge amounts of foliage to satisfy their ravenous appetite. As leaves are consumed, larvae continue to extend the web mass to enclose additional foliage. This accounts for the "sudden appearance" of large web masses.
Tree leaf defoliation by fall webworms can startle most people. And many may think that trees are in danger of dying. However, fall webworm feeding poses no threat to well-established trees and shrubs. In fact, there is no evidence to prove that long-term health of a tree or shrub is jeopardized. Leaf buds for next year are not harmed, so trees will leaf out as usual come spring.
If homeowners feel they need to take action against fall webworms, physically remove the web mass. If within reach, merely gather in the webbing. If the thought of touching the webbing repulses you, wear gardening gloves, or use a stick or broom handle to break up and remove the webbing. Once collected, dispose of the webbing and larvae. Be cautious not to remove large web masses by pruning out the branch or branches as "holes" may be created in the tree canopy next spring. In large trees where web masses are beyond reach, physical removal may not be possible.
If gardeners are determined to utilize insecticides, there are many registered for use against fall webworms and/or defoliating caterpillars. Keep in mind that fall webworms remain in their web mass as they feed, so insecticides merely sprayed on the outside of the mass will do little to kill them. To ensure control, thrust the spray wand through the outer web and directly spray the caterpillars. As with physical removal, large trees may not lend themselves to spray treatments as the spray may not penetrate the web mass.