the Southern Pine Beetles:
Outbreaks of the southern pine beetle, Dendroctonus frontalis Zimmermann, occur periodically in the southern United States, killing millions of dollars worth of pine timber and landscape pines. Good forest management, or cultural care in the case of landscape pines, can minimize the risk of southern pine beelte attacks. However, knowledge of the beetle life cycle and symptoms of infested trees is necessary to properly deal with southern pine beetle attacks when they occur.
All pine species can be attacked, but some are more susceptible than others. In the Piedmont, shortleaf pine is more susceptible than loblolly or Virginia pines, and in the Coastal Plain loblolly is more susceptible than longleaf or slash pines. Ordinarily, unhealthy, weakened pines are the most susceptible to beetle attack, but as the beetle population increases, even healthy, fast-growing pines will be attacked.
The most obvious symptom of a southern pine beetle infestation is the discoloration of the needles in the tree crown. Needles fade from green to dull green, yellow, and finally reddish-brown before falling. In this image, the dead trees in the upper right were the first attacked in the spot. The most recently attacked trees are those with the light green crowns in the lower left. Crown discoloration is, however, only one symptom. Additional symptoms and evidence of the beetle are necessary for positive pest identification. The table below describes symptoms associated with various stages of SPB attack.
|Foliage||Pitch Tubes||Bark||Exit Holes||Ambrosia
|Freshly Infested||Green||Soft, white, light pink||Tight, hard to remove||None||None|
|Infested with Developing Brood||Green trees with larvae; fade to yellow before brood emerges||White, hardened||Loose, peels easily||Few, associated with attacking adult reemergence||White, localized areas around base of trees|
|Vacated, Dead Tree||Red, needles falling||Hard, yellow, crumbles easily||Very loose, easily removed||Numerous||Abundant at base of trees|
LIFE CYCLE STAGES
The southern pine beetle may have up to eight generations a year with four life stages during each generation: egg, larva, pupa and adult. Development of all stages continues throughout the year, slowing considerably in the winter and accelerating in the spring and summer. One life cycle or generation from egg to adult may take from 26 to 54 days depending on the season. Over a temperature range of approximately 60 to 85 degrees F, the length of time for each stage varies as indicated below . Adults reach an ultimate length of only 1/8 of an inch, similar in size to a grain of rice.
Host Attack - Initially, a few adult "pioneer" beetles attack the midtrunk of susceptible host trees. After the pioneer beetles successfully colonize a susceptible host tree, the beetles and the host tree produce chemicals that attract additional flying adult beetles. Attacks spread up and down the trunk over the course of three to five days.
Small, light yellow to white pitch tubes usually form where the beetles enter the tree along the entire trunk length. These masses of pitch are about the shape and color of popcorn, and can be the same size or slightly smaller. Especially weak trees may not produce pitch tubes, but reddish boring dust in bark crevices or cobwebs at the base of the tree is evidence of infestation.
Attacking adults construct winding S-shaped egg galleries in the cambium layer between the bark and wood, and lay eggs (see header). During gallery construction, the adult beetles introduce a blue-stain fungus. Along with the girdling effect of gallery excavation, the fungus contributes to the death of the tree by eventually plugging the tree’s water-conducting tissue.
Reemergence - Attacking adults begin to reemerge one to three days after the mass attack, mating and egg laying, and continue to do so for 10 to 14 days. Reemerged adults may attack additional pines and contribute to infestation growth.
Brood Development - Small C-shaped larvae hatch from the eggs and feed in the cambium and then in the inner bark. Near the end of the larval stage, the larvae move into the outer bark and pupate. The pupae develop into immature yellowish-white (callow) adults about one week before becoming dark in color. Upon full development, the adult constructs an exit or emergence hole through the outer bark.
New Adult Emergence - Emergence is temperature related, and the adults may remain under the bark for a period of time if conditions are not favorable. Usually colder air temperatures delay emergence. In addition, emergence does not take place all at once. A few beetles leave the tree initially, followed by a larger number, and then a declining number over an extended period of time.
The dispersal of emerging adult beetles is also affected by environmental conditions. During the winter, emerging beetles may not disperse, but re-attack the same tree instead. Usually, however, emerged adult beetles leave the host tree and, depending on the time of year, either aggregate on adjacent trees under attack or fly off to find a suitable new host tree elsewhere.
Keeping your pine timber stands healthy and vigorous, and having a good knowledge of the southern pine beetle habits and symptoms is essential to effectively deal with this destructive pest. Professional advice and assistance is available through the South Carolina Forestry Commission, Clemson University Cooperative Extension Service, U.S.D.A. Forest Service, forest industry personnel, and private consulting foresters. Acknowledgement: USDA Agriculture Handbook No. 575