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Index : Pests
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- 16. Boring Insects Attacking Mesquite Trees - Top
- Gardeners who grow fruit trees such as apples, peach, plum, and apricot are probably familiar with some very nasty insects known as flatheaded borers. As the name implies, their habit is to bore or tunnel into the trunks and larger branches of susceptible trees. They also have flat, wide heads; and white worm-like bodies about an inch or so long. Under the bark, they feed in the cambium area, the trees water and food transporting lifeline. Eventually, if left unchecked, the borers riddle the conductive tissue, destroying the tree.
Nothing is unusual about flatheaded borers in fruit trees. Apple, peach, and plum are somewhat out of there element in Tucson. They like cooler climes, and as a result, are often under stressful growing conditions here in the desert. Boring insects look for stressed trees to attack. What is unusual is the ever-increasing presence of flatheaded borers in mesquite trees. These are hardy desert trees, usually not suitable targets for borers.
The first signs of damage from flatheaded borers on mesquite trees are the appearance of amber colored globules of resin protruding from the trunk and branches. This resin oozes out wounds made by the insects into the trees cambium. Sometimes removing the resin will reveal the entrances into the galleries or tunnels made under the bark. Loose or pealing bark in the general area can also indicate the presence of borers. Pealing the bark back will reveal sawdust packed galleries 1/4 inch or wider cris-crossing through the wood. With only moderate damage to the cambium, branch die-back will also occur.
Flatheaded borer adults are dark-colored beetles about ½ inch long. They lay eggs in cracks and crevices of the bark. When the eggs hatch, the young larva tunnel directly from the egg into the bark. On healthy trees the water pressure exerted by the uptake of water in the cambium layer is usually so strong it literally blows these small, newly hatched worms right out of the tree. But if the tree is unhealthy or under stress, water uptake is reduced to the point that the small worms can penetrate and move into the cambium. There they begin feeding and growing. When nearly mature, they tunnel deep into the heartwood, where they pupate. As adults, they emerge from exit holes and eventually begin laying more eggs.
Trees likely to be attacked by borers are those under stress. Mesquites are adapted to our hot, dry conditions; however other factors can stress these desert dwellers. Trees may be planted too deep, or over time, sink in their planting holes. The result is a partially covered trunk base. When trunk tissue is covered with soil it slowly die, along with the tree. Death is usually gradual, but such trees are usually found by opportunistic borers. It's a good idea to check mesquites and other trees by digging at the base of the trunk. If you have to dig more than an inch to find the first set of roots, then the trunk is covered too deeply. The soil should be pushed back away from the trunk and kept back by using an edging material placed out from and around the trunk.
Other problems that can promote tree stress include; soil compaction, trenching or cutting roots, wounds from poorly made pruning cuts, and lack of water. Also the setback that trees receive in transplanting increases the possibility that borers may attack.
From a prevention standpoint, keep trees as healthy as possible. From a control standpoint, once signs of borer damage are noticed, spray the trunk and lower branches of the tree with the Lindane insecticide. Lindane has a vaporizing action that penetrates into the cracks and crevices of wood and into the borer tunnels. Spray large branches and trunk to the point of runoff. As with all chemical sprays, read and follow label directions carefully prior to use.
Because the egg laying period for adult flathead borers is from April through July, spray to kill the larva from May through September. The initial spray should be followed with a second application 4 to 6 weeks later.
Finally, to keep borers from returning, it will be necessary to find and eliminate the stress factors that brought about the borer attack in the first place.
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Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on September 13, 1998
- Updated: February 21, 2001
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