Forest landowners are often more familiar than homeowners about the pests that can attack their pine trees. Because of this, the phrase “bark beetle outbreak” can strike dread as they envision entire stands of pines being wiped out by aggressive, fast-moving insect infestations. Fortunately, appropriate forest management practices such as thinning can reduce the risk of these outbreaks. Unfortunately, pine bark beetle attacks in urban settings can be just as upsetting to homeowners as they watch trees in their yard die one by one. Early signs of infestations in crowns can progress quickly from yellow needles to orange and then red ones. Often homeowners notice these signs too late to save valued yard trees. What can homeowners do if pine bark beetles are attacking the trees in their yard, and how can they prevent future infestations?
Identify the Species and Understand their Impact
Southern pine beetles, Dendroctonus frontalis (SPB): These insects are extremely destructive bark beetles, despite being native to South Carolina. These beetles are smaller than a grain of rice and can destroy millions of dollars’ worth of timber during periods of a severe outbreak in pine forests. They are also costly pests in developed areas and can impact pine trees in urban settings. Activity normally begins in the spring after individuals overwinter in or under the bark of previously infested trees. Pairs of SPB will most successfully attack trees that have been weakened or stressed by overcrowding, drought, disease, human impacts such as soil compaction or equipment damage to stems, or have suffered other damage like a lightning strike. As the beetles bore into a stem between the bark plates, a tree’s first response is to attempt to push them out boring tunnels, or galleries by producing sap, or pitch. This will result in a small (< ½” diameter) popcorn-looking pitch tube, which can be covered in brown sawdust and seen emerging from the beetles’ entrance holes. If the tree is not able to ward off the attack, the smell of this resin, as well as pheromones released by female SPB will attract other individuals. Initial attacks usually occur on the middle to upper part of a tree’s stem. Trees are killed by the girdling effect of the S-shaped galleries made by the breeding pairs between the bark and the wood, and by the blue stain fungus introduced by the beetles, which clogs the xylem (water transport system). The next generation results from eggs that are laid in niches off of these galleries. The larvae eat their way through the inner and outer bark, pupate, and exit the tree through small, round holes as an adult to infest other trees. All stages of the life cycle can be found in a single tree, and 4 to 6 generations can occur in one year in South Carolina. When beetle numbers are high enough, even healthy pine trees will not be able to survive an attack.
Ips engraver beetles: These beetles are 4 distinct species found in the southeastern United States, which vary in size and location of where they attack a tree along its stem. The six-spined engraver (Ips calligraphus), the five-spined engraver (Ips grandicollis), and the four-spined engraver (Ips avulsus) occur region wide, while the pine engraver (Ips pini) occurs in the Appalachian Mountain region. Like SPB, the Ips beetles are attracted to stressed or injured trees, but can also infest logging slash. Unlike SPB, their infestations are normally limited to clumps of trees, and they can attack up and down the stem and branches. Ips often attack stems already infested by SPB, but are not always associated with a SPB outbreak. Their life cycle begins with the male boring into the face of the bark and creating a nuptial (mating) chamber in the inner bark. Pitch tubes are less common in Ips infestations, but brown sawdust can sometimes be present. After releasing a pheromone to attract females, each of the male’s mates constructs an egg gallery off of the nuptial chamber. Depending on the number of females, these galleries can form an H, I, or Y shape and run vertically up and down the stem. Larvae create perpendicular feeding galleries off of the parent galleries and feed on the inner bark until they pupate, mature, and exit the tree. As with SPB, the tree is killed by a combination of the girdling effect of the galleries and the blue stain that is introduced by the beetles. Ips usually have 6 to 8 generations per year. Since Ips can infest slash and downed trees, these materials should be removed from around adjacent healthy trees.
Black turpentine beetles, Dendroctonus terebrans: These beetles are much larger than the SPB or Ips. They get their name from their attraction to terpenes produced by wounded pine trees, such as those being worked for turpentine. Infestation by these beetles is often secondary to some other insect attack, disease, wound, or condition which has weakened or stressed a tree. The large pitch tubes created by the insect are usually located on the lower 10 to 12 feet of the pine stem. Their egg galleries are found running down the stem from the point of entry. Because larvae do not form their own feeding galleries, they feed randomly in groups on the inner bark and create large patches of excavated areas. They also mature more slowly than the other bark beetle species, resulting in 2 to 4 generations per year. Unlike other pine bark beetles, the black turpentine beetle attacks scattered trees in very low numbers, and light attacks may kill only localized sections of phloem tissue. However, if the tree is already stressed from other species of bark beetles, or there are numerous attacks per stem, tree mortality may occur.
Reducing the Risk of Pine Bark Beetles to Urban Trees
As with many tree health concerns, preventative measures are your best defense against pest infestation. There is no guarantee that homeowners will be able to protect all of their yard trees in periods of severe pine bark beetle outbreaks. However, steps can be taken to improve the overall health of these trees to increase their chances of surviving an attack. The first step is to limit the amount of impact a tree suffers from day to day activities in the yard. This includes protecting tree trunks and roots from getting damaged by lawn maintenance equipment like lawnmower decks and weed eater string, and ensuring soil around trees is not compacted by vehicle traffic. Also, increasing watering during times of dry weather and improving the moisture holding capacity of soil by mulching around the base of the tree instead of maintaining it in grass will improve tree health under dry conditions. Removing injured or diseased trees will decrease the risk of pine bark beetles being attracted to an area. However, care should be taken not to damage neighboring trees when trees are taken down since fresh wounds can also attract a wave of pine bark beetle activity.
Several chemical treatments are labeled to prevent the spread of pine bark beetles from an infested tree to healthy neighboring trees. However, since all areas of the pine stems prone to attack must be treated, this can be very expensive when full tree heights must be sprayed with contact insecticides. Systemic treatments may have some positive affect, but they can also be costly and require the insecticide to be injected into the tree. Choosing the right insecticide and application method should only be done by consulting a trained professional. Contact your local state forester or university extension office if you think you have a pine bark beetle infestation. It is important to remember that any of these treatments will only be effective on healthy trees. Stems that are already showing signs of infestation, such as changing needle color, pitch tubes, or exit holes are already infested and will not benefit from treatment.
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