Techniques for Bark Beetle Control

Salvage – This technique is usually feasible only when a relatively large volume of wood is available. Prompt action in salvaging large infestations is often necessary to prevent beetle expansion and to realize the best possible return from the sale of infested trees. Any trees containing beetle broods in salvage operations should be removed from the site before beetles emerge.

Cut and Leave – This course of action is best suited for large timber stands because the resulting downed trees prevent or inhibit further immediate utilization of the land. Attacked trees and a border of healthy trees are felled toward the center of the spot. High temperatures on the top of the trees, increased humidity on the bottom, and increased predation from other forest insects and animals, may combine with disruption of emergence and attack patterns (time and distance) to make this an alternative for beetle control. Results of recent research are ambiguous as to the efficacy of this technique.

Pile and Burn – This may be a viable option for small infestations in rural areas. The trees are felled in the same manner as for the cut and leave or salvage techniques. Heavy equipment is often necessary to pile trees together so they can be burned. These direct control techniques are described in detail by Swain and Remion (1981).

Chemical Application – Chemicals will protect healthy trees and/or kill broods of beetles within infested trees if they are used properly. The cost of spraying is often small when weighed against the value placed on the tree, the cost of tree removal, or potential damage of expanded beetle infestations. If chemical control is selected, the most cost-effective control can be realized only by correct identification of the beetle species involved, careful selection of trees to be treated, and correct application of the insecticides.

The following procedural guide will help you select chemicals, techniques and equipment to maximize benefits from chemical control while minimizing detrimental effects.

Selection of Chemicals – There are currently 3 insecticides registered for control of bark beetles on southern pines. These are lindane, chlorpyrifos (Dursban 4E) and fenitrothion (Sumithion 8E) . Landowners may purchase lindane for their own use without an applicator's license. Although Dursban 4E and Sumithion 8E are not restricted use pesticides and a pesticide applicator's license is not required to purchase or use them; they bear labeling that limits their use to professional applicators. There is no such limitation on the use of lindane. The selection of chemicals will usually be dictated by their effectiveness, safety and relative costs for each application. Chemicals selected for preventive control may be less desirable for remedial control than other compounds.

- Preventive Control -

For economic preventive control a chemical should be selected which will prevent attacks for relatively long periods (3-12 months). If prevention is deemed necessary for a relatively short period, for instance 2-3 months in the fall before low temperatures terminate beetle activity, then selection of a chemical with a shorter residual life may save money and reduce nontarget contamination. For preventive control, lindane will protect pines for up to 12 months from SPB and Ips beetles (Berisford and Brady, 1976; Brady et al., 1980; Berisford et al., 1981a). Chlorpyrifos and fenitrothion can provide protection from SPB for about 3 months (Brady et al., 1980; Berisford, et al., 1981a) and presumably can provide similar protection from Ips spp. Lindane can usually prevent BTB infestation for one growing season whereas such information on Dursban and Sumithion is not yet available.

- Remedial Control -

Chemicals for remedial control are not required to have long residual. Since they are designed to kill beetles and brood developing within a tree, 2-3 weeks residual is usually all that is necessary. Lindane, chlorpyrifos and fenitrothion all provide good remedial control of SPB and Ips spp. although chlorpyrifos and fenitrothion appear to be slightly better (Berisford et al., 1981b). These compounds also break down faster in the environment than lindane. The BTB is difficult to control in a remedial situation because it attacks at the base of the trees, frequently very large ones, where they are protected by very thick bark. Lindane appears to be somewhat better than either of the other compounds in this case. Efficacy of lindane is increased by using a fuel oil carrier instead of water. However, stressed trees or thin-barked trees may be damaged by the oil.

- Cost of Control -

Costs of bark beetle control obviously are an important factor in deciding what chemical to use. In fact, costs may determine if any chemicals will be used. When determining relative costs, several factors should be kept in mind.

1. Costs of different chemical concentrates per gallon etc. may be deceiving since concentrations may vary substantially. Be sure you know what you are buying.
   
2. The recommended concentrations for chemicals are often different for remedial vs. preventive control. Actual costs per tree vary according to the beetle species involved, whether trees are standing or felled, and obviously the size of the trees. Costs for BTB control are lowest since only the basal portion of trees need be treated for either remedial or preventive control. Remedial control for SPB and Ips beetles is most economical if trees are felled first since there is little overspray or drift. However, felling and turning costs may represent additional expense unless trees are to be cut anyway. Spraying standing trees for SPB and/or Ips spp. is the most expensive operation. Trees must be treated well into the live crown resulting in some overspray and drift due to the high pressures required and the difficulty in keeping the spray stream on the tree at all times.

- Relative Risks to Applicators and Non-Target Areas -

All of the chemicals registered for bark beetles of southern pines are toxic to other organisms including humans. Some are more toxic than others, depending on how and when they are contacted, i.e. whether they are swallowed (oral toxicity) or are only in contact with the skin (dermal toxicity). The toxicity of all compounds must be established before they can be registered for use. The terminology for determining toxicity is standardized so one may easily compare relative toxicities of selected chemicals. Table 1 shows the relative oral and dermal toxicities of lindane, chlorpyrifos and fenitrothion. Toxicity is reported as the LD₅₀ which is the amount of a compound that constitutes a lethal dose for 50 percent of a test population. The LD₅₀ is expressed in milligrams (mg) of compound per kilogram (kg) of body weight of the animal receiving the dose. Therefore, chemicals with the lowest LD₅₀s are the most toxic. These figures vary with the species of test animal and are based on tests with pure samples and not the diluted formulations available for purchase.

Once insecticide sprays have dried on trees, they represent little risk to humans or other mammals (Berisford et. al., 1981c). However, in the process of spraying for bark beetles, nontarget organisms may be adversely affected. Care must be taken to minimize nontarget contamination to insure that pets, beneficial insects such as bees, water supplies and fish are not adversely affected. Do not spray under windy conditions or near lakes or streams. When near edible crops, do not spray more than necessary to wet the bark as excessive spray only runs off.

Certain ornamental plants may be damaged by insecticides so you should minimize drift.

Table 1. Oral and dermal toxicity of chlorpyrifos, fenitrothion and lindane to laboratory animals.

¹ Each figure represents the LD₅₀ of oral and dermal treatments to laboratory animals (white rats) (Kenaga and Morgan, 1978).