Round Rock Leader
Saturday, May 21, 2005
By Emsud Horozovic, City of Round Rock Forestry Manager
During the last few months we’ve had lots of rain, some hail, and falling green things from the sky. Yep, that’s right—wiggly, silky, squirmy, little wormy things that get in your hair; drop onto your salad (extra protein?), lawn chairs, cars; and get smashed under your feet. Welcome to a Texas spring!
The little green caterpillars are called “leaf rollers”. The name applies to several types of caterpillars that roll foliage around themselves as they feed, creating shelter from predators such as birds. The caterpillars spin sticky webbing as they roll new growth, attracting dust and dirt, which also helps conceal them. In our area, the caterpillar is light green, slender, and about ½-1 inch in length. They have a black spot on their heads. When disturbed, they wiggle furiously and try to escape by dropping to the ground on a thread of silk.
The species we’re addressing today are normally found on oak species, though other species may be found on pecans, hackberries, and walnut trees. The behavior and attributes are similar. The caterpillars do not necessarily appear every year, and in most cases they do not kill the trees.
Natural enemies of leaf roller caterpillars include birds, particularly mockingbirds, and parasitic wasps. If significant feeding damage is occurring, insecticidal sprays containing carbaryl (Sevin®), malathion, or the biological caterpillar sprays that contain Bacillus thuringiensis may be applied for control. The ideal time for applying an insecticide is when the insects begin their last week of voracious feeding, or around mid-April—that is right now!
When any tree is defoliated during the growing season, serious damage can occur. Green leaves manufacture sugar that is later converted into other carbohydrates that allow the tree to grow, thus keeping the plant healthy. When trees are defoliated, manufacture of sugar is essentially stopped and growth of the trees does not occur.
Trees can maintain life for short periods by using their reserve food supply but are more susceptible to disease and insect attack when weakened. Complete defoliation year after year could result in tree death, especially under drought conditions.
If the tree has received light or moderate defoliation, all that is needed is watering of the tree for several months to maintain their vigor. Fertilization is not necessary as caterpillar droppings provide enough natural fertilization.
Oak trees that receive severe leaf damage should be fertilized and watered regularly to restore vigor. Select a balanced tree fertilizer and apply at the rate of one pound per diameter inch of the trunk. Fertilizers should be applied in a circular pattern under the drip line of the tree. The drip line is the area beneath outer most limb area, away from the trunk. A second application of fertilizer should be applied six to eight weeks later for best results. The second application should be ammonium sulfate (21-0-0), applied at the same rate eg., one pound per diameter inch of trunk.
Now that you’re ready to pick up a bottle of Sevin, are you curious about how those caterpillars “just appeared” on your tree? As an adult, the oak leaf roller is a small moth that undergoes one generation each year. Eggs from this moth are laid on twig tips and buds of several types of trees during May. Eggs remain on the buds or twigs for about ten months and begin to hatch in mid-March. The young caterpillars, the topic of this article, feed on tender new leaf growth until late April. Trees heavily infested are usually defoliated by mid to late April, at which time the fully grown caterpillars form the pupa stage on twig tips, in bark crevices, or on weeds growing near infested trees. Moths begin to emerge from the pupal stage (cocoon) about the first of May and begin to lay eggs on twigs of oak, hackberry, pecan and walnut trees. These eggs will remain dormant until next March, thus completing the one year life cycle.
So, the little worms may be driving you crazy, but it is highly unlikely that your tree will die. With appropriate amounts of water and soil nutrients, your tree will probably put out a second set of leaves.
Be sure to save this article for next March.
Forestry Manager Emsud Horozovic