In the past several weeks, there have been an increasing number of areas where shortleaf pine trees, Pinus echinata, have turned brown and died. The pockets of dead trees are usually small, ranging from just a few trees to less than an acre in size. Trees have died both in the forests and in residential settings. The foliage in the upper crowns starts to fade turning a greenish-yellow, which quickly progresses to all the needles turning reddish-brown and the tree dying. These areas are most visible in contrast to the surrounding normally dark green foliage of healthy trees.
The most probable cause of the present shortleaf pine mortality is a common tiny bark-boring beetle of the Ips genus, called the southern pine engraver. Three different species are present and they often differ in the part of the tree they attack. The fourspined Ips (small southern pine engraver, Ips avulsus) attacks branches and upper bole; the eastern fivespined Ips (I. grandicollis) prefers the mid-bole; and the sixspined Ips (I. calligraphus) prefers the lower half of the bole. One or more species may be found in a single tree. The adult beetles are dark red to brown to almost black and are small, only 1/8- to 1/5-inch long. They are distinguished from other bark beetles by their scooped-out posterior with four to six spines on each side. Larvae are white bodied with orange-brown heads and are legless. Pupae are waxy-white and similar to adults in size.
Ips adults tunnel in the phloem just under the bark, where they lay eggs, and their larvae mature to a new generation of adults. Several generations develop within a pine stand in a single year.
Signs of bark beetle attacks are a birdshot-hole pattern of exit holes on the bark, small white to red-brown pitch tubes, and galleries (tunnels) under the bark. Main galleries created by Ips parent beetles are either Y- or H-shaped (fivespined and sixspined Ips), or mostly straight with the grain (fourspined Ips). Larval galleries extend laterally from parent galleries. The southern pine beetle (Dendroctonus frontalis), uncommon in Missouri, has S-shaped galleries, a rounded posterior, and is smaller than the Ips beetle.
Ips beetles also carry spores of a bluestain fungus that infects beetle-attacked pines. These fungal colonies grow into the outer sapwood, stopping upward water flow and causing needles to wilt and die. Foliage can change from green to reddish brown in 2 to 4 weeks during the heat of summer. Most Ips beetles have left a tree by the time needles are reddish brown. Trees with active Ips infestations will have either green or fading foliage. During cool or moist weather, a tree may still be green when beetles depart.
Although the Ips is a native insect and population cycles are normal, trees that are stressed or in poor condition are more susceptible to attack. Climatic events such as drought or windstorms can also trigger Ips infestations. The increased number of infestations occurring in 2007 is likely due to stresses on trees from prolonged drought in July and August, which followed unusual spring weather of an early warm-up in March and an exceptionally severe freeze in April.
Forest stands: The duration of Ips outbreaks in forest stands is often short-lived, lasting only one season. If infested spots are isolated and widely scattered, the best course of action often is to let the infestation die out on its own. Cutting and removing trees to stop the outbreak, may actually accelerate it, because harvesting produces fresh host odors (attractants), logging slash (potential breeding sites), and additional stress or injury to the residual stand.
Residential trees: The best strategy for landscape trees is to prevent infestations by increasing tree vigor through good tree care and providing supplemental water during droughts. There is no way to save a tree once it has been successfully colonized. Care should be taken when removing dead trees so that residual trees are not injured. Insecticide applications on nearby green trees are usually not practical, because the bark must be drenched for the entire length of the bole, and spray drift can be considerable. Such treatments require professional applicator equipment and are best reserved for high-value trees. Injections and soil drenches of systemic herbicides like imidacloprid are sometimes used for tree borer control; however, their effectiveness against Ips beetles has been disappointing.
A trees best defense against both disease and insects is to be in a healthy, vigorously growing condition. This is true whether the tree is in the forest or your front yard. For assistance with your tree needs, please contact the local Missouri Department of Conservation office. Phone 573-226-3616.