Issue: June 8, 1996

Horn-tail tree borers

Question: I saw the strangest thing. I found a large wasp stuck to a tree by its stinger. What was that and why did it sting the tree?

Answer: You probably saw what is called a horn-tail, or horn-tail wasp, as it was laying its eggs under the bark of the tree. The "stinger" is its ovipositor, or egg laying organ, with which it can inject the eggs like it was a hypodermic needle. The larvae of this wasp hatch under the bark, feed on the cambium layer under the bark, bore into the trunk and pupate until they emerge from the tree as mature wasps. Yes, this is a wasp which damages trees. You can treat with any of several products labeled for borer control at the time the horn-tails are laying eggs or when the adults are emerging from their pupae.

According to Dr. Charles Ward, NMSU Cooperative Extension Service Entomologist, appropriate products include those which contain the insecticides lindane or chlorpyrifos. As with most borers, the tree was probably under some environmental or cultural stress which made it more susceptible to attack by the horn-tails and other borers. Stressed trees produce chemical signals, known as kairomones, which alert the insects to the fact that the tree is in trouble. Such insects are natural agents with the responsibility for removing weak or declining trees from the environment so that younger, more vigorous trees may occupy their place. Unfortunately, trees in New Mexico landscapes, especially transplanted and non-native trees, are by definition trees growing under stress. They are automatic targets for insect attacks. For this reason, it is important to properly plant trees and irrigate them appropriately until they have become established. That is why native and non-native trees adapted to our climate do better than trees which are common in moist climates.

Yellow leaves in locust trees

Question: My locust tree has a lot of yellow leaves. I can see them when I look up into the tree, and now they are falling. It is June, not autumn - is my tree sick? What can I do to stop it?

Answer: While there are some diseases and insects which may cause leaf yellowing, I suspect your tree is doing what it is supposed to do. When the leaves first form in the spring, the temperatures are lower and the tree needs less water. It produces a lot of leaves so that it can produce as much food through photosynthesis as possible. As summer progresses, the temperatures go up and the tree needs more water to sustain its leaves. Even if the soil contains enough moisture, there is a limited ability of the xylem to convey water within the tree trunk from the roots to the leaves. Trees like the locust adapt to this increased water demand by disposing of excess leaves. Though this reduces the production of food through photosynthesis, by this time the tree has already produced a considerable amount of food to support its growth and begun to replace that withdrawn from storage in the trunk and roots. With the reduced number of leaves, it will continue to make food to supply current needs and to store for next spring when it will be needed to support the spring growth flush.

Don't worry about your tree. Unless other symptoms such as twig and branch die-back or cupping of leaves develop, it is just doing what it should. Continue to irrigate your tree as it needs water, every 10 to 20 days. Water deeply and enjoy the shade.