ANSWERS TO GO with Susan Smith
Q. What is up with the tiny caterpillars hanging by a thread from my oak trees?
A. I found a relevant article, “Oak Leaf Roller and Springtime Defoliation of Live Oak Trees,” by Bart M. Drees. Drees is a professor at Texas A&M and an Extension Service entomologist.
Drees writes: “In early spring, many species of caterpillars can defoliate oaks and other trees. The caterpillars feed on the early spring growth and occasionally will eat all of the newly emerged leaves.
“Two such caterpillars are the oak leaf roller, Archips semiferana, and an associated species, Sparganothis pettitana. These insects occur throughout Texas but are most destructive in the Hill Country and South Texas. Damaging populations have been reported in Fredericksburg, Kerrville, Boerne, New Braunfels, San Antonio, Floresville and Goliad.
“These species spin silken threads from which they hang when dislodged from leaves and branches. Other caterpillars that dangle from silk threads include the spring and fall cankerworms (often called ‘inchworms).
“Trees that are heavily infested will usually be defoliated by mid-to late April, when the fully-grown caterpillars form the pupa (cocoon) stage. At about the first of May, moths begin to emerge from the pupal state and begin laying eggs on the twigs of oak, hackberry, pecan and walnut trees. These eggs will remain dormant until the following March, thus completing the 1-year life cycle.
“When a tree is defoliated during the growing season, it becomes stressed and occasionally serious damage can occur. Green leaves manufacture energy (sugar that is later converted into other carbohydrates) that allows the tree to grow and maintain its natural vigor. Once a tree is defoliated, it essentially stops manufacturing sugar, which slows tree growth.
“More importantly, these stressed trees are more susceptible to attack by various diseases and insects. Weakened trees are extremely susceptible to wood-boring insects such as longhorn beetles, flat-headed tree borers, shot-hole borers and many clear-winged tree borers.
“If the trees are completely defoliated year after year, they will die.
“For most oak trees throughout the Hill Country, it is not feasible to use insecticides to control the oak leaf roller and related species. Often the best course of action is to simply do nothing. Oak trees have survived in South and Central Texas without human intervention through time, despite periodic pest outbreaks and resulting defoliation. Caterpillar pest populations are generally cyclic, and they rarely occur year after year.
“In residential situations, however, valuable landscape or orchard trees may need to be protected from caterpillar outbreaks. Valuable landscape trees with a history of leaf roller infestation and defoliation should be watched closely from mid- to late March.
“If heavy leaf-feeding damage is observed, the tree leaves can be sprayed with a registered insecticide containing either carbaryl (such as Sevin®), the insecticide that contains Bacillus thuringiensis, or any of several other products labeled for control of ‘caterpillars’ or ‘oakworms’ on ornamental trees and shrubs.
“To minimize stress and to restore plant vigor, oaks or other trees that have been severely defoliated in the past should be fertilized and watered regular. Choose a balanced fertilizer (one that has equal amounts of the three major elements — nitrogen, phosphorus and potassium) and apply it at the rate of 1 pound of nitrogen per diameter inch of the trunk at waist height.
“Apply the fertilizer in a circular pattern within the tree’s drip line, which is the area under a tree from the trunk to the outermost limb. For best recovery of the trees, apply ammonium sulfate (21-0-0) at the same rate 6 to 8 weeks later.”
