February 21, 2015

1 - Tree borers may create hidden infestations in stressed trees; the best control is providing good tree care.

Yard and Garden February 21, 2015


We just uncovered this giant worm (photo sent with e-mail) about 3" in length in the roots of our very, very large old live oak tree. This is the only one we found. Is it something to be concerned about that we should spray? Thank you for your advice and information. I should mention that we live in Mesilla Park, not in the desert part of Las Cruces.


I know I have seen this grub before in presentations, but cannot remember exactly what it is. I thought it wise to send the question and photo to Dr. Carol Sutherland, NMSU Extension Entomologist and NMDA State Entomologist. She did indeed know this insect and was able to provide much useful advice.

Here is Carol's information: This big fat larva is that of a 'long horned beetle' in the Order Coleoptera (beetles), Family Cerambycidae (long-horned beetles), Genus is very likely Derobrachus. Derobrachus is the big (about 2.5-3" long) reddish-brown beetle that you might find dead or dying around a security light in the summer. They have some very obvious mandibles---that look like toe-nail nippers. This is a common native beetle that takes advantage of all kinds of broadleaf trees and shrubs---anything with a diameter large enough and a condition that is weak enough to interest the female---anything from mesquite, desert willow and larger desert shrubs to any shade tree I can think of---but not conifers. That would be a different genus but with similar habits. They are excellent fliers and amazing at tracking down opportunities for reproduction; remember that their lives depend on it.

The life cycle of this insect involves several years of feeding, molting, tunneling and maturing before emergence as an adult. It began several years ago as a female beetle mated and flew around looking for a place to lay some eggs on a weak or struggling tree. The big freeze we had in February 2011 could have caused the stress needed to cause her to target this tree. Other stresses that could have caused her to target this tree include drought, dramatic temperature fluctuations, water quality changes, freeze injury in addition to Feb. 2011, parking cars on the roots, using plastic over the roots to keep out the weeds instead of weed barrier, and other such events.

Dr. Sutherland stated that spraying for this pest is not a recommended course of action. She stated that the pest is hidden and protected from spray in the wood of the tree for approximately 4 years before it emerges as a beetle to mate and potentially infest other plants. You would not be aware of its presence until it created its exit hole or, as in your case, you managed to find the larva. She stated that there is no way to know if there are other larvae in this tree. She said, "We do not have tree X-rays and no 'sap test' for beetle infestation exists---like a blood test might work for certain diseases or conditions people have." She points out that systemic insecticides are not likely to help because wherever they are in the wood, that particular area of vascular tissue (sapwood or heartwood) is likely not very active in moving the insecticide up or down in the tree. And we have no idea what the strength of the product is inside the tree.

She states further, "If the tree is still standing and you are trying to keep it, do the best job of tree care you can. This discovery of one grub may not be a death sentence, although that may be what is coming eventually. Are you seeing die-back in the branches and limbs? Any bark peeling on the trunk? Oaks have strong wood, but if wood borer damage is severe enough, larger limbs could break or fall in a wind storm."

Her last warnings are relevant and you should indeed watch for hazards that may form to protect yourself, structures, and property. Removal of damaged branches may be necessary while you try to prolong the life of the tree.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


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