Identification of Bark Beetles
Often the first most obvious indication of bark beetle attacks in pines are yellowing or red crowns. These symptoms usually are evident long after initial attacks have occurred. Since fire, mechanical damage, disease, herbicides, etc. may cause similar symptoms, the presence of bark beetles should be verified before any control action is contemplated.
On closer examination of a tree, early evidence of bark beetle damage is usually readily visible and easy to identify. This evidence includes pitch tubes or brown boring dust on the outside of the bark, characteristic galleries beneath the bark, and beetle adults and larvae in the inner bark.
Techniques for the control of bark beetles may differ according to the beetle species. Therefore, correct identification is essential before using insecticides for bark beetle control. There are three important groups of bark beetles in the south. They are: (1) the southern pine beetle, Dendroctonus frontalis Zimmerman (SPB), (2) the black turpentine beetle, Dendroctonus terebrans Olivier (BTB), and (3) three species of Ips beetles, Ips avulsus (Eichhoff), I. grandicollis (Eichhoff) and I. calligraphus (Germar), which are common in most areas, and I. pini (Say), which frequently attacks, white pines in the mountains. Since they have similar habits, Ips spp. beetles will be considered as a group.
Black Turpentine Beetle
The BTB is about ¼ inch long and is the largest of the pine bark beetles. Its rounded abdomen is typical of the genus Dendroctonus and its large size easily separates it from the otherwise similar SPB (Fig. 1).
Attacks by the BTB are characterized by large pitch tubes an inch or more in diameter concentrated around the basal portion of a tree up to a height of 8-10 feet (Fig. 2). The characteristic egg galleries made by the adults extend from the entry point (pitch tube) downward and parallel with the grain for 12-18 inches. Eggs are deposited along one side of this gallery and the larvae feed "shoulder to shoulder" in large groups during their development. This group feeding habit makes a characteristic "feeding patch" (Fig. 3). If the feeding patches are evident but no beetles are present, they have vacated the tree and no chemical treatment is warranted. Trees attacked by BTBs often survive because the BTB does not carry blue stain fungi as do the SPB and Ips beetles. BTB larvae must girdle a tree before it will die. However, sublethal infestations may weaken trees, thereby predisposing them to Ips beetle attack. The publication on chemical control of the BTB by Merkel (1981) contains more detailed information.
The southern Ips spp. vary considerably in size but they all have the same general appearance and their habits are similar. Ips spp. beetles are easily separated from Dendroctonus spp. by the shape of the posterior part of the abdomen. Whereas beetles of the genus Dendroctonus have rounded abdomens, Ips spp. have concave abdomens with 4-6 spines on each side of the concavity (Fig. 4). To the unaided eye, the abdomen appears to be "chopped off".
|Fig. 1. Southern pine beetle (left), rice
grain and Black turpentine beetle (right).
|Fig. 2. Black turpentine beetle pitch tubes|
|Fig. 3. Black turpentine beetle feeding patch.|
Attacks by Ips spp. often cause small pitch tubes to form at the point of attack. These are usually less than ½ inch in diameter but generally cannot be distinguished from those associated with SPB attacks. On severely weakened trees, the only evidence of early stages of attack is brown sawdust accumulated in bark crevices and on spider webs on the trees.
Ips spp. beetles make characteristic egg galleries that are easy to recognize. These beetles are polygamous with 2 to 5 females associated with each male and gallery construction reflects this habit. Male beetles usually initiate attacks and form a cavity called a nuptial chamber in the inner bark at the point of attack. Females join the male and construct egg galleries which radiate out from the nuptial chamber and are generally parallel with the grain of sapwood (Fig. 5). These galleries usually groove the sapwood and are generally free from boring dust. Eggs are deposited along the sides of these galleries and then fan out to utilize the available food (Fig. 6). The larvae of some Ips spp. make round or oval feeding chambers at the end of their feeding gallery.
Larvae, pupae and adults are readily visible if the bark is removed down to the sapwood (Fig. 7). If Ips galleries are present but no life stages are visible, the bark should contain numerous exit holes and the tree will not require treatment. Successful Ips attacks almost always kill the host tree. Although attacks by Ips avulsus (the smallest Ips) will sometimes kill tops or limbs but not the entire tree. Details of Ips spp. habits may be found in Wilkinson and Foltz (1982).
|Fig. 4. Ips spp. abdomen.||Fig. 7. Ips spp. under bark|
|Fig. 5. Ips spp. egg galleries.||Fig. 6. Feeding galleries of Ips spp. larvae|
Southern Pine Beetles
Evidence of SPB attack is similar to that of Ips beetles, i.e. small pitch tubes and/or brown boring dust (Fig. 8). The bark should be removed to make positive identification. The SPB makes characteristic "S-shaped" galleries that wind through the inner bark of the host trees (Fig. 9). These galleries are normally packed with brown dust called frass. Eggs are laid along both walls of the galleries and larvae tunnel a short distance (usually less than ½ inch) from the oviposition site. Larval galleries end in a "feeding chamber" where the larvae complete their development and pupate. The larval habits of southern pine beetles differ from Ips beetles. The larvae are frequently not visible in the bark when it is removed from the sapwood since they normally tunnel into the outer bark during later larval stages. Larval galleries and feeding chambers may be visible, but the presence or absence of larvae, pupae or brood adults often may be confirmed only by removing frass from feeding chambers on the inside of the bark or by removal of outer bark down to the area infested by the beetles (Fig. 10). Trees which have been vacated by SPBs should not be sprayed. This only increases costs and may kill natural enemies of the beetles which have not yet left the trees.
Fig. 8. Pitch tubes of southern pine beetles
Fig. 9. Characteristic southern pine beetle galleries (winding S-shaped)
Fig. 10. Stripping outer bark to expose beetle brood
Fig. 11. Blue stain
If SPBs or Ips beetles have successfully constructed egg galleries, they usually have introduced blue stain fungi into the wood (Fig. 11) and the trees will die even if they are sprayed. Therefore, even though treatment of recently attacked green trees will prevent additional attacks and kill most of the beetle broods which develop, the trees cannot be saved. If other trees are being removed from an infestation, these green infested trees should be removed also. Beetle infestations that occur in the fall frequently die out during the winter and do not require any action except monitoring. You may wish to contact your county forester or extension agent regarding a decision on whether or not to apply controls and what method is most appropriate.
Developed by the University of Georgia Bugwood Network in cooperation with USDA Forest Service - Forest Health Protection, USDA APHIS PPQ, Georgia Forestry Commission, Texas Forest Service
and the Pests and Diseases Image Library - Australia
Last updated on Friday, July 21, 2006 at 04:22 PM
www.barkbeetles.org version 2.0