Bark beetle.

INSECTS ARE also responsible for spreading the destructive fungi that cause blue staining of standing coniferous trees, logs, and lumber. The stains do not weaken the wood, but they often cause a reduction in the value of wood products.

The relationship of insects to the spread of blue stain fungi resembles that of Dutch elm disease. The reason may be that the fungus causing that disease is closely allied to those causing blue stain. Again, as was true of the Dutch elm disease fungus, the insects that carry blue stain fungi from tree to tree are bark beetles. The habits of this group of beetles make them effective in transporting the spores of fungi from one tree and placing them in another.

Broods of the bark beetles develop between the bark and wood. That happens because adult beetles emerge from one tree and seek out another, alight on it, and bore through the bark, where they excavate tunnels and lay their eggs. Thus they carry the fungi into the bark with them. Once the fungi are implanted in the bark they begin to grow and produce spores in the tunnels and chambers occupied by the developing insects. When the beetles complete their development they bore their way out through the bark and, loaded both externally and internally with fungi, they seek out new trees to attack, thus starting the cycle over again.

All trees in a stand are not attacked equally by the bark beetles. We do not know why. We do know that fire-damaged trees are heavily attacked and that trees weakened by drought and root rot are more likely to be attacked than vigorous ones.

In logs and lumber, the beetles have a minor part in the spread of blue stain fungi. That is because wind and rain easily dislodge the spores of the fungi from the surface of the wood, where they are produced in abundance, and scatter them widely. Damage to logs and lumber can be prevented by the application of chemical sprays. When timber values are high, losses among standing trees can be offset considerably by the salvage and sale of the stained timber.

OTHER FOREST DISEASES are also carried by insects. It has been shown by J. G. Leach and his coworkers at the University of Minnesota that a definite relationship exists between the amount and rapidity of decay in Norway pine logs and the extent that two species of wood-boring insects tunnel into the wood.

The fungus causing persimmon wilt has been isolated from the wood immediately adjacent to insect-feeding injuries in otherwise healthy trees, indicating that some of the spread of this wind-borne fungus to healthy trees is related to the abundance of such insect-caused wounds.

A vascular disease of willow, caused by the bacterium Pseudomonas saliciperda is transmitted by the poplar and willow borer. The beetles become contaminated with the bacteria while feeding upon diseased willows. When they fly to healthy trees and resume their feeding, disease transmission takes place.

The fungus Polyporus volvatus is common in dead or dying trees of several species of conifers killed by bark beetles. It causes decay of the sapwood and is probably introduced into the trees by the beetles.

The pattern of occurrence of oak wilt suggests that the fungus causing it may be transmitted from tree to tree by insects, but experimental proof is lacking.

CURTIS MAY is a principal pathologist in the division of forest pathology, Bureau of Plant Industry, Soils, and Agricultural Engineering. He has been engaged in research on the diseases of forest and shade trees for more than 25 years. He is a graduate of Ohio State University and was on the staff of the Ohio Agricultural Experiment Station before he joined the Department of Agriculture in 1933.

WHITEFORD L. BAKER is an assistant leader of the division of forest insect investigations, Bureau of Entomology and Plant Quarantine. He was graduated from Clemson Agricultural College in 1927 and, after graduate study at the University of Minnesota, joined the Department of Agriculture in 1929.

The western hemlock stainer is a very small beetle that constructs individual cells for rearing its young in timber trees.