The southern pine beetle, Dendroctonus frontalis, is a small insect that has a devastating impact on pine trees. The scientific name Dendroctonus means "tree killer"— a well-deserved name since this species is the most destructive forest insect pest in the United States. Like other bark beetles, the southern pine beetle spends most of its life beneath the tree bark. Adults deposit eggs beneath the bark, leaving the developing larvae to chew through the tree’s essential cambium layer. In addition to the damage caused by the insects, the southern pine beetle transmits blue stain fungi which can hasten the tree’s decline.
Although the southern pine beetle will attack most pine trees, it has been particularly deadly for shortleaf and loblolly pines. Its range has expanded from the southern United States into areas as far north as Pennsylvania and Connecticut. New Mexico and Arizona also experience beetle infestations. The devastation from southern pine beetle can be so comprehensive that swathes of dead pine trees are visible in aerial photographs.
The southern pine beetle lifecycle is 35 to 60 days, and there may be as many as six generations each year. Attacks may occur from spring through autumn. Adult females emit a pheromone that signals males and additional females to converge. Within a few days, thousands of beetles may colonize a tree and overwhelm its defenses, often expanding the devastation to nearby trees.
Adults chew through the bark and excavate galleries where they lay their eggs. The eggs hatch in two to nine days and the larvae enlarge the galleries as they eat and grow, causing disruption of the tree’s ability to transport water and nutrients. When mature, larvae bore to the outer dead bark, create a cell, and pupate. Emerging adults then bore directly through the outer bark, leaving a clear-cut, open hole.
When southern pine beetle attempts to bore into a tree, it encounters the tree’s natural defense – resin. It often takes the combined effort of a male and female pair to cut through the sticky resin into the tree cambium. The result of these skirmishes are visible pitch tubes, popcorn-like blobs that dot the bark of the tree. Sometimes frass (similar to fine sawdust) collects on the ground.
Beneath the bark, s-shaped galleries are carved by the larvae. The bark of an infested tree will also appear peppered with holes from the emerging adults. After substantial damage has occurred, the crown of the tree will brown and die.
Although southern pine beetle will attack healthy trees, it prefers easier hosts, such as trees already damaged or stressed. Keeping pine trees healthy will lower the risk of infestation. Regular inspections are important, as well as fertilisation, watering and mulching.
For valued trees that are at risk, especially in areas of known infestation, preventive bark treatment is often the most practical and economical solution. The cost of application is often small when weighed against the value of the tree, the cost of felling, and potential expanded beetle infestations.