First, there are many types of borers these could be. Here are a few groups:
Since most wood-boring insects are considered secondary invaders, the first line of defense against infestation is to keep plants healthy. Proper care of trees and shrubs discourages many borer pests and helps infested plants survive. Good sap f low from healthy, vigorously growing trees, for example, defends the plant from damage by many borer pests. Good horticultural practices include:
- Selecting well adapted species of trees and shrubs that are not commonly attacked by wood borers in your area. Arizona ash, birch, cottonwood, locust, soft maple, flowering stone fruits (such as peaches and plums), slash pines (in west Texas), willow and poplar are especially prone to borer attack.
- Choosing and preparing a good planting site to avoid plant stress, freeze damage, sun scald and wind burn.
- Minimizing plant stress and stimulating growth by using proper watering and fertilization practices.
- Avoiding injury to tree trunks from lawn mowers, weed trimmers or construction.
- Properly thinning and pruning during [proper] months.
- Removing and destroying infested, dying or dead plants or plant parts, including fallen limbs.
Basically, the borers will go for the weakest trees around, so the more ideal of conditions you can provide, like plenty of organic matter, water, proper fertilization, the better your trees chances are against borers (or almost every other pest problem, actually).
Now, for control, that can be hard. Again, from the same source, here are some methods of control:
Non-chemical control for infested plants
Once trees and shrubs are infested, non chemical options for borer control are limited. One option is to remove and destroy heavily infested or injured plants. Damage sites also can be inspected closely to determine if the larvae stages can be extracted from the plant with a pocket knife, wire or other suitable tool.
It is important to remember that stressed, unhealthy trees can be attacked repeatedly and will need repeated applications of insecticide indefinitely. In most cases this is neither economical nor environmentally justified. When chemical treatments are used, efforts always should be made to improve overall tree health.
Most of these products are applied as sprays to the trunks and branches, and are non-systemic, residual insecticides (e.g., bendiocarb, carbaryl, chlorpyrifos, endosulfan, es-fenvalerate, f luvalinate, lin-dane, methoxychlor, sumithion). While these products do not kill larvae that have already penetrated the sap-wood or heartwood, they will kill adult and larval stages tunneling through the treated bark layer. These are primarily a preventive treatment. Some products (those containing paradichlorobenzene and ethylene dichloride) act as fumigants to repel egg-laying adults or kill accessible larvae.
Unfortunately, chemical sprays are ineffective against borers once they are inside the tree. They will be effective against adult around and laying eggs on the tree at the time of spraying, and also newly emerged larvae before they enter the bark. Some adult borers, such as metallic wood borers, and longhorned beetles, will feed on the leaves of host trees, so canopy sprays can be effective where these are noticed.
This table will also be of use, although localized to Colorado. The flight periods may differ by location: