Webworms in General and on Pecans in Particular
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Q. Emily in Edmond, Oklahoma writes: "I have a beautiful large pecan tree that came with our new home. The first couple of years we lived here, it didn't really do anything. The third season, the tree was so full of nuts the branches were dragging on the ground! I would stare at it and dream of pecan pie, but before they started to drop, the tree was picked clean! Not sure if it was the squirrels or crows, but I did not get one single pecan!!! Now I'm afraid I have bigger problems than creatures stealing my nuts. At the end of the summer last year, I noticed two small areas of fall web worms. Didn't think much about it until we returned from a long vacation this summer to find webs all over the tree. So I have three big questions.
#1: "Can I just cut out the infected branches and throw them away? Some are pretty high up and I'll have to hire someone to do it. I really want to know what you think before I get someone out here trying to convince me to spray the tree with Sevin."
A. These pests are very similar to the tent caterpillars that build big nasty-looking nests in trees in the Spring. Despite their common name, fall webworms are caterpillars—any horticultural pest with the word 'worm' in its name is a caterpillar…And they give real worms a bad name!
The 'fall' part is also misleading. As Emily has learned, 'mid-to-late summer' is more like it. Now 'the book' does say to prune out any infested branches you can reach, but when they appeared in my crabapple tree a few years back, I just ripped the nest apart with a long pole pruner and birds made short work of the exposed caterpillars.
And, of course, she can spray the tree (or have it sprayed) with the original strain of Bt—BTK—it only harms caterpillars that chew on the sprayed leaves. (Which means that you have to spray it while the caterpillars are actively eating. And it would be nice if, again, you could bust up at least some of their tent cities a bit first.)
But you have to realize that whether you use a bird, bee and people-safe pesticide like Bt or a nasty chemical one, the ugly webs are still going to be there. And in most cases with homeowners, it's the aesthetic—or lack of it; you know, the nasty messy look of the things—that's the biggest issue.
But don't the caterpillars harm the tree? Not generally. Trees typically grow new leaves after tent caterpillars stop feeding in the Spring, and fall webworms attack after a lot of photosynthesis has already occurred. But this tree does sound like it might have an unusually high number of webs….
Question #2: "Can I collect and compost the leaves from the tree or will they be infected with worms? I would hate to spread them any further."
A. A little bit of a trick answer here. The eggs of the next generation are either placed on or just under the bark, in the soil itself or in leaf litter under the tree. So it's a GOOD idea to rake up and compost the leaves—to get rid of one 'hiding place'. But just to be safe, you want to make sure you shred those leaves really well. And then compost them in a hot pile—just shredded leaves mixed with coffee grounds or horse manure for Nitrogen; no kitchen waste to cool things down. (Put that stuff in a worm bin where it belongs!)
Question #3: "This is the first year the nuts are falling off early—in a green underdeveloped state. This has been a pretty mild summer with some good rain early, so I would have thought that the tree would be doing awesome. Do you think the worms are the reason?"
Yes and no. An excellent Texas A & M Extension Bulletin on pecan growing notes that "any stress" can cause developing nuts to drop. And an unusually large number of hungry, hungry caterpillars can be very stressful—especially if the tree is also dehydrated. (Pecans are thirsty trees that are going to do best with some irrigation during long dry spells in summer.)
But nut drop can also be caused by a hard freeze the previous winter (pecans are warm weather trees whose very survival is dicey even in the normally-warmest climes when cold weather hits)—or attack by weevils and other pests; she should inspect the dropped nuts for distinctive little holes (this could be a multiple issue tree).
Taking all of the above (and more we don't have room for) into consideration, I suggest that she:
- Hack away at the nests she has;
- clean up her leaf litter promptly;
- and hang suet feeders in the tree but let them go empty for a week or two at a time so the meat-eating birds attracted by the suet look for eggs in the tree bark in between the human-provided meals.
- Think about watering deeply once a week in the summer if rain is scarce.
- Mulch with two inches of compost beginning a foot from the trunk and going out as far as the furthest branch to provide a gentle feeding and reduce competition from grass and/or weeds.
- And begin spraying the tree with Bt next summer as soon as you see the first sign of 'worms' If she handles these pests, she might need to research some squirrel and bird deterrents next fall. (Hint—she'll find good ideas in the Texas A & M Bulletin, as well as our previous Questions of the Week about birds and Evil Squirrels).