Pine Engraver Beetle
The pine engraver beetle (PI) belongs to a group of bark beetles called Ips, or engraver beetles. The PI is a native insect in Nevada’s woodlands and is typically present in low numbers. As with many forest insects often considered as “pests”, the PI plays an important role in the ecosystem. It kills weak and damaged trees, continuing the important process of nutrient cycling. Additionally, a variety of wildlife depend on the beetles as food. However, when the beetle populations increase to epidemic levels, or the insects infest particularly valuable trees in the home landscape, they are a problem.
The PI attacks small diameter ponderosa, Jeffrey and lodgepole pines throughout the trees range and may occasionally attack other pine species. Often it will attack tips of larger diameter trees (see Figure 1), especially in the vicinity of windfall or large amounts of slash (see figure 1). The insect occurs in one of four stages: egg, larvae, pupae and adult, and spends most of its life under a trees bark. Male beetles, dark brown to black and approximately 4mm long, initiate attacks in the spring upon emerging from material infested the previous fall. If the attack is successful, the male beetles release a chemical attractant (pheromone) which draws a mass attack of male and female beetles. The males bore through the bark and construct a nuptial chamber in which they mate with multiple females. The female beetles then bore tunnels (egg galleries) away from the nuptial chamber along which they lay eggs singly along both sides.
Unlike most other bark beetles, Ips egg galleries are free of sawdust and are etched into the wood (hence the name engraver beetles see; wishbone shaped galleries in Figure 2 ). The eggs hatch into small, white larvae, which begin feeding on the inner bark, moving out at right angles from the egg gallery. After a few weeks they have completed their growth and the larvae form a pupal chamber at the end of their feeding gallery. Here they transform into pupae, and then into adults. The adults bore out of the tree and fly to new material to begin another generation.
The beetles produce three to four generations per year. The first generation emerges in the spring, April to May depending on temperatures, and typically infests freshly killed material from the previous winter such as blown down trees and broken branches. The material is still green and provides a ready food source for the first generation, whereas live trees are most vigorous this time of year and are very resistant to attacks. As subsequent PI generations are produced, pines enter the drier summer months with declining vigor, and are therefore more susceptible to beetle attacks. The beetles can carry a fungus into the attacked host which grows and clogs the trees water conducting tissues. The combination of larval feeding in the inner bark and the fungus proves fatal to the tree.
Evidence of Attack
The most visible symptom of a successful beetle attack is a change in the trees crown color. Foliage turns from green to yellow and finally to a reddish-brown color. By the time the tree crown changes to a red color, the beetles have usually left the tree. Carefully inspect green trees nearby for signs of attack. Inspection of the branches and trunk will often reveal boring dust collected in the bark crevices and at the base of the tree. Pitch tubes may be present at the entrance holes. The characteristic egg galleries will be visible beneath the bark.
The best method of controlling PI problems is prevention. Efforts should be made to prevent accumulation of large amounts of slash or windfall trees, cordwood rounds or leave logs on the surface after thinning. Slash should be chipped or piled and burned before PI emerge or should be scattered in the sun where it will dry out and make it an unsuitable breeding medium by the time beetle flight occurs. Since beetle attacks on healthy trees are usually unsuccessful, measures to maintain healthy, vigorous trees will go a long towards controlling PI problems. Thinning overcrowded stands of trees and pruning out mistletoe infections will maintain tree vigor. Trees with severe mistletoe infections should be removed. Prevent construction-related damage to trees such as soil compaction, root destruction and “barking” the stem and branches. Avoid burying or paving over tree roots. All these activities will make trees more susceptible to PI attacks. Individual trees can be sprayed with an insecticide to protect them during periods of stress during drought, or following injury. The insecticide must be applied to the main trunk and all large branches as high up as possible. March through early April is the best time to spray depending on average temperatures. Beetles chewing through the trees bark will eat the poison and die. This will not save trees which are already infested. Contact the Nevada Division of Agriculture for a list of pesticides registered for use against bark beetles on pine. Read, understand, and follow instructions on pesticide labels.
Schedule tree cutting activity for the fall which will allow time for the slash to dry out. Material larger than 4 inches in diameter should be split to promote rapid drying. Cutting slash in short pieces and scattering it in sunny openings will speed its drying. Removing, chipping or burning slash are also effective methods to prevent PI problems. By eliminating slash, the first generation of beetles in the spring has very little host material to breed in, and beetles populations can be kept low. Avoid backfilling over root areas. Four or more inches of dirt over roots often stresses trees, making them attractive to beetles. Avoid stacking fresh firewood against standing green trees. Beetles may be attracted by the cut logs and broods that develop in the firewood tend to crawl directly to the standing trees to make new attacks.
Infested material should be removed from the forest before the insects mature. An alternative is to burn, chip, or bury the wood, or peel off the bark which exposes the insects to the weather and predators. If the material is desired for firewood, it should be piled in direct sunlight and covered with thick (= 4mil) clear plastic. The edges of the plastic should be buried underground. Avoid tearing the plastic. This will raise temperatures under the plastic and bake the insects under the bark. Keep the pile covered for 2 to 3 months during sunny weather.