'Unusually high' population of fall webworms attacking Long Island trees

DEAR READERS: Over the past few weeks, I've received dozens of emails, letters, social media messages and phone calls from residents concerned about large, mysterious webs in their trees — and in trees around their neighborhoods. Many fear an epidemic and worry for the health of their trees, and some are confused because the eastern  tent caterpillar, known for producing similar webs, is active during spring, not late summer or autumn.

What they have been reporting are webs left behind by fall webworms, whose population is, in fact, "unusually high this year," according to Cornell Cooperative Extension of Suffolk County entomologist Dan Gilrein. 

Whereas "eastern tent caterpillars form their webbing 'tents'  in spring in branch crotches on cherry, apple and related host plants, fall webworms form webs in summer farther out on branches," Gilrein said, adding that the pest has a "very wide host range of over 400 kinds of plants."

Gilrein suggests four options if catepillars and webs are still present: Ignore them, remove them by hand, prune away affected branches or apply an insecticide. If you determine pesticides are necessary, be aware that the caterpillars are only susceptible to such measures when they are in their feeding stage. So if you see caterpillars on the webs, watch to see if they're feeding on leaves. If they are, treat the tree with a high-pressure spray of DiPel   or Thuricide,  both brand names of a strain of the bacteria Bt,  a natural remedy. You might need to hire an arborist or other landscape professional for the job.

Although the caterpillars may be munching on foliage, damage to plant health should be minimal because by this time of year, leaves have already served their purpose by photosynthesizing, or producing food, according to Gilrein. He added that "trees with a history of good care probably can tolerate much more late-season defoliation without a problem." Trees that were recently planted, already under stress or in poor condition may be less tolerant to the stress of attack, he said. 

Proper care goes a long way toward helping plants withstand such stressors as pests, adverse weather conditions and diseases. Best practices for keeping trees strong include watering deeply, especially during drought, and mulching properly (no more than 3 inches deep and starting three to four inches from trunks) to retain soil moisture, minimize competition from weeds and keep soil temperature even. In addition, it's best to avoid overhead watering, as it wets foliage, encouraging mold and fungi growth; opt instead for drip irrigation, which directs water at the roots. In addition, prune diseased or broken branches, and dispose of plant material properly, cleaning up fallen leaves and debris from the soil surface. Before planting, ensure the area offers the sunlight exposure, soil drainage and pH requirements of the specific plants. Then space plants properly, accounting for their mature size, to ensure sunlight and air circulation can reach all plant parts.

DEAR JESSICA: My Snow Fountain weeping cherry tree died last year, and this spring, I replaced it with a new one. After three months, the tree is doing poorly. The leaves are curled, many are turning brown, and one or two branches have all-dead leaves. I sprayed with garden oil after the first month when I saw little holes in the leaves and suspected insects. I also sprayed with neem oil at least twice in the past two or three months. I took some leaves to the garden center, and they told me to water more and spray for insects again. It doesn’t seem to be working, and I don’t want to lose another beautiful cherry tree. Can you help? — Mary Kriner, Wantagh

DEAR MARY: It looks like your tree has shot hole disease, which is usually caused by a fungus, but sometimes a bacteria. It’s important to clean up any fallen plant parts (leaves, twigs, buds, etc.) from the soil because the disease can survive a long time in plant material and reinfect your tree next year. It’s possible debris from your first tree is what infected the new tree if it was planted in the same spot.

There’s nothing you can do now except rake up and discard (in a sealed bag in the trash) plant material that falls onto the soil, and keep the area clean. In late fall, spray the tree with Mancozeb, a fungicide and bactericide, and repeat that treatment next spring, just as leaf buds are opening, and once more when leaves reach full size. If you cannot find that product, treat the tree with a copper fungicide or one containing chlorothalonil according to the same schedule.

If there are any galls, dark and swollen growths, on branches, prune those branches about 6 to 8 inches above the gall, disinfecting pruners with a 1-part-bleach to 9-parts-water  solution in between cuts to prevent transporting the infection from one part of the tree to another. Be sure to properly dispose of those branches properly.

As your tree is obviously stressed, be sure to water regularly, providing an inch of water per week (accounting for rainfall). And because a stressed tree is more susceptible to insect attack, keep an eye out for pests (especially under leaves and on branches), and apply a product containing the insecticide acetamiprid according to package directions, if necessary.