Adult bark beetles emerge in the spring from dead or dying elms and begin feeding on the twigs of healthy elms. The female then excavates an egg gallery in the bark of dead or weakened elms. Eggs are deposited along the walls of the gallery. The larvae, upon hatching, burrow into the bark at right angles to the egg gallery. Pupation occurs at the end of the larval tunnel. New adults tunnel to the bark surface and leave the tree through circular emergence holes. There are usually two generations a year. The beetles overwinter in the larval stage.
Chemical control and good tree maintenance are the two methods used to reduce bark beetle populations. Insecticides are used to prevent feeding by the adults in the spring. Tree sanitation involves removal and disposal of dead elms and elm limbs which eliminate breeding and larval development sites.
COLUMBIAN TIMBER BEETLE, Corthylus columbianus (Hopkins)
All hardwood trees in the eastern half of the United States are subject to attack by the Columbian timber beetle, but oaks, maples, birch, basswood, yellow-poplar, and elm are the preferred hosts in the South. The beetle causes two major types of damage: 1) physical damage caused by the 1/32″ to 1/16″ hole excavated by the adult into the sapwood, and 2) degrade caused by stain which may extend for a considerable distance above and below the gallery. Large diameter trees are preferred as hosts, but trees as small as one and one-fourth inches in diameter may be attacked.
Callow adult and pupae of the Columbian timber beetle.
There are two and sometimes three generations of this insect each year. Adults from the first generation emerge from late May through June and those from the second in October. The first evidence of attack is the white dust which collects at the entrance hole. Later, depending on the tree’s physiological condition, a sap-soaked area may develop around the entrance hole. The adult bores a horizontal hole into the sapwood of a healthy tree for a few inches and later constructs two or three shorter lateral branches. “Cradles” (or egg chambers) are then constructed for a short distance perpendicular to these galleries. The female deposits a single egg in each chamber. The offspring spend their entire developmental period within the cradles feeding on fungi which grow on the sapwood. It is this fungus which causes the extensive staining characteristic of Columbian timber beetle attack. Winter is spent in both the pupal and adult stages in the brood galleries.
Columbian timber beetle entrance holes in yellow-poplar.
Chemical control is not practical for forest trees.