**2. Types and use of preventative applications of insecticides**

quality and quantity, fish and wildlife populations, aesthetics, recreation, grazing capacity, real estate values, biodiversity, carbon storage, endangered species and cultural resources.

**Common name Scientific name Primary host(s)** Arizona fivespined ips *Ips lecontei Pinus ponderosa*

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California fivespined ips *I*. *paraconfusus P*. *contorta*, *P. jeffreyi*, *P*.

Douglas-fir beetle *Dendroctonus pseudotsugae Pseudotsuga menziesii*

fir engraver *Scolytus ventralis Abies concolor*, *A*. *grandis*, *A*.

mountain pine beetle\* *D*. *ponderosae P*. *albicaulis*, *P*. *contorta*, *P*.

northern spruce engraver *I*. *perturbatus Picea glauca*, *Pi. x lutzii* pine engraver *I*. *pini P*. *contorta*, *P. jeffreyi*, *P*.

pinyon ips *I*. *confusus P*. *edulis*, *P*. *monophylla* roundheaded pine beetle *D*. *adjunctus P*. *arizonica*, *P. engelmannii*, *P.*

southern pine beetle *D*. *frontalis P*. *engelmannii*, *P. leiophylla*, *P.*

spruce beetle\* *D*. *rufipennis Pi. engelmannii*, *Pi. glauca*, *Pi*.

western pine beetle\* *D*. *brevicomis P*. *coulteri*, *P*. *ponderosa*

**Table 1.** Bark beetle species that cause significant amounts of tree mortality in coniferous forests of the western U.S.

While native bark beetles are a natural part of the ecology of forests, the economic and social impacts of outbreaks can be substantial. Several tactics are available to manage bark beetle infestations and to reduce associated levels of tree mortality. While these vary by bark beetle species, current tactics include tree removals that reduce stand density (thinning) and pre‐ sumably host susceptibility [1]; sanitation harvests [1]; applications of semiochemicals (i.e., chemicals produced by one organism that elicit a response, usually behavioral, in another

western balsam bark beetle *Dryocoetes confusus A*. *lasiocarpa*

\*Species for which preventative insecticide treatments have been well studied.

eastern larch beetle *D*. *simplex Larix laricina*

Jeffrey pine beetle *D*. *jeffreyi P*. *jeffreyi*

*lambertiana*, *P*. *ponderosa*

*flexilis*, *P*. *lambertiana*, *P*. *monticola*, *P*. *ponderosa*

*flexilis*, *P. leiophylla*, *P. ponderosa*, *P. strobiformis*

*pungens*, *Pi. sitchensis*

*magnifica*

*lambertiana*

*ponderosa*

Preventative applications of insecticides involve topical sprays to the tree bole (bole sprays) or systemic insecticides injected directly into the tree (tree injections) [3]. Systemic insecti‐ cides applied to the soil are generally ineffective. In an operational context, only high-value, individual trees growing in unique environments or under unique circumstances are treat‐ ed. These may include trees in residential (Fig. 1), recreational (e.g., campgrounds) (Fig. 2) or administrative sites. Tree losses in these environments result in undesirable impacts such as reduced shade, screening, aesthetics, and increased fire risk. Dead trees also pose poten‐ tial hazards to public safety requiring routine inspection, maintenance and eventual remov‐ al [4], and property values may be negatively impacted [5]. In addition, trees growing in progeny tests, seed orchards, or those genetically resistant to forest diseases may be consid‐ ered for preventative treatments, especially if epidemic populations of bark beetles exist in the area. During large-scale outbreaks, hundreds of thousands of trees may be treated annu‐ ally in the western U.S., however once an outbreak subsides (i.e., generally after one to sev‐ eral years) preventative treatments are often no longer necessary.

**Figure 1.** Tree mortality attributed to western pine beetle in San Bernardino County, California, U.S. In the wildland urban interface, tree losses pose potential hazards to public safety and costs associated with hazard tree removals can be substantial. Furthermore, property values may be significantly reduced. The value of these trees, cost of removal and loss of aesthetic value often justify the use of insecticides to protect trees from bark beetle attack during an out‐ break. Photos: C.J. Fettig, Pacific Southwest Research Station, USDA Forest Service.

draws, non-payment of annual registration maintenance fees, and registration of new prod‐ ucts at federal and state levels. Several studies have been published on the efficacy of various classes, active ingredients, and formulations that are no longer registered [e.g., ben‐ zene hexachloride (Lindane®)]. Therefore, we limit much of our discussion to the most com‐ monly used and/or extensively-studied products (Fig. 3). A list of products registered for protecting trees from bark beetle attack can be obtained online from state regulatory agen‐ cies and/or cooperative extension offices, and should be consulted prior to implementing any treatment. Furthermore, all insecticides registered and sold in the U.S. must carry a la‐ bel. It is a violation of federal law to use any product inconsistent with its labeling. The label contains abundant information concerning the safe and appropriate use of insecticides (e.g., signal words, first aid and precautionary statements, proper mixing, etc.). For tree protec‐ tion, it is important to note whether the product is registered for ornamental and/or forest

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settings, and to limit applications to appropriate sites using suitable application rates.

**Figure 3.** The carbamate carbaryl and pyrethroids bifenthrin and permethrin are commonly used to protect trees from bark beetle attack in the western U.S. Several formulations are available and effective if properly applied. Residual ac‐ tivity varies with active ingredient, bark beetle species, tree species, geographic location, and associated climatic con‐

When evaluating preventative treatments one of three experimental designs is generally used. Each has its own advantages and disadvantages. Laboratory assays require trap‐ ping and/or rearing of live bark beetles for inclusion in experiments. Captured individu‐ als are immediately transported to the laboratory, identified and sorted. Damaged (e.g., loss of any appendages), weakened, or beetles not assayed within 48 h after collection should be discarded. Generally, serial dilutions of each insecticide are prepared, and tox‐ icity is determined in filter paper or topic assays [7]. The life-table method is used to es‐ timate the survival probability of test subjects to different doses of each insecticide [7]. Filter paper assays more closely approximate conditions under which toxicants are en‐ countered by bark beetles during host colonization, especially for products other than contact insecticides [7], but both methods ignore important environmental factors (e.g., temperature, humidity and sunlight) and host tree factors (e.g., architecture) that influ‐

ditions. Photos: C.J. Fettig, Pacific Southwest Research Station, USDA Forest Service.

**4. Experimental designs for evaluating preventative treatments**

**Figure 2.** Conditions before (left) and after (right) a spruce beetle outbreak impacted the Navajo Lake Campground on the Dixie National Forest, Utah, U.S. Daily use decreased substantially due to reductions in shade, screening and aesthetics associated with mortality and removal of large diameter overstory trees. Photos: A.S. Munson, Forest Health Protection, USDA Forest Service.

Although once common, insecticides are rarely used today for direct or remedial control (i.e., subsequent treatment of previously infested trees or logs to kill developing and/or emerging brood). While remedial applications have been demonstrated to increase mortality of brood in treated hosts, there is limited evidence of any impact to adjacent levels of tree mortality. Furthermore, there are concerns about the effects of remedial treatments on nontarget invertebrates, specifically natural enemy communities. Many of these species respond kairomonally to bark beetle pheromones and host volatiles, and their richness increases over time [6], suggesting that the later remedial treatments are applied the more likely non-target organisms will be negatively impacted.
