Those who have studied the Dutch elm disease have shown that several different insects under some conditions may transmit the disease. In the United States the principal vector is the smaller European elm bark beetle (Scolytus multistriatus). The native elm bark beetle (Hylurgopinus rufipes) may also transmit the disease, but is much less effective. The greater efficiency of the former is due to its feeding habits.

Both insects breed under the bark of infected trees and the fungus pathogen grows and forms spores in their breeding tunnels so that the new broods of each species have equal opportunities for becoming contaminated with the fungus spores. But when the new crops of beetles emerge, their behavior is quite different. Those of the European species fly to healthy trees and feed on the young branches and in the process inoculate the tree with the disease. Those of the native species, however,do not feed on young twigs. They may bore into the trunk of healthy trees but they rarely penetrate deep enough to reach the sapwood and inoculate the tree. Neither insect is able to establish breeding tunnels in healthy trees, but those trees which were inoculated by the European species when it fed on the small branches are weakened so that they are subject to the attack of both species. These diseased and beetle-infested trees supply a new brood of contaminated beetles. Thus a slight difference in feeding habit may greatly influence the efficiency of an insect vector.

In a study of insect transmission of plant diseases, consideration must therefore be given to the feeding and breeding habits of all the insects that are potential vectors. Many other insects develop in infected trees and have abundant opportunities to become contaminated with the pathogens but are of no significance as vectors. For example, the wood-boring beetles (Buprestidae and Cerambycidae) breed in infected trees and come in contact with the pathogenic fungus, but when they attack the new tree the adult female of the wood-boring beetle does not bore into the tree but deposits her eggs on the surface or in niches in the bark. When the eggs hatch the young larvae bore into the tree, and because they have had little or no opportunity to become contaminated with the fungus they rarely serve as vectors.

More than 40 species of insects visit rye blossoms and feed on the spores of the ergot fungus, but not all have equal importance as carriers of ergot. Among those that feed on the spores are certain species of pollen-eating flies. They also regularly visit healthy flowers and feed on the pollen. In doing so they transmit the disease. Thus they are more effective vectors than those that do not eat pollen and visit healthy flowers only by chance or not at all.

An important aspect of plant pathology is the study of the influence of environment on disease. When the diseases are caused by fungi or bacteria the study . is complicated by the effect of the environment on two different organisms, the micro-organism and the crop plant, and also on the interaction of the two. If the disease is transmitted by an insect, one has to study three different organisms and their interactions.

The prevalence of a plant disease can be determined more by the influence of the environment on the insect vector than by its influence on the disease itself. That is obvious in such diseases as bacterial wilt of cucurbits, for which insects are the only known means of transmission. Any weather condition that influences the abundance of the cucumber beetles must also influence the prevalence of the disease.

Bacterial wilt of sweet corn is more prevalent following mild winters than it is following cold winters. It is generally agreed that this is because of the survival in mild winters of a greater number of contaminated flea beetles.

Unexpected situations often happen when environment modifies strikingly the activities of insect vectors. The soft rot of vegetables, caused by bacteria, is usually favored by wet weather, but when the disease occurs as a heart rot of celery it is most destructive in dry weather. It was hard to understand why until it was discovered that, on celery, the disease is transmitted by insects of the fruit fly group. The flies deposit their eggs on the celery leaves. When the eggs hatch, the young maggots burrow into the leaves and inoculate the plant with the bacteria. The insects will deposit their eggs only on moist leaves. In wet weather the eggs are deposited on the outer leaves of the celery plant where the decay causes little damage to the plant because the Outer leaves are resistant to decay and are removed and discarded at harvest. In dry weather, however, when the outer leaves are not wet, the insects seek the moist heart leaves on which to lay their eggs. When the larvae inoculate the heart leaves with the bacteria the growing point is killed and stem elongates, so that the plant has no market value. Thus a disease that normally is favored by moist weather is, in this case, more destructive in dry weather.

Insects also may indirectly influence the spread of diseases by birds. Chestnut blight is spread over short distances by wind and water, but spot infections have appeared 100 miles or more from known infected areas. They were started by the woodpeckers and sapsuckers whose beaks had been contaminated with spores while feeding on insects that were breeding in cankers on infected trees. Since the birds feed on both insects and healthy cambium, they are effective vectors. After feeding on insects in a canker and contaminating their beaks with spores, they would continue their migrations and fly many miles before pecking holes in the bark of healthy chestnut trees to feed on the cambium. But in so doing they would effectively inoculate the trees with the spores adhering to their beaks.

It is evident from this brief review that insects play an important role in the spread and development of plant diseases. Successful control of many plant diseases depends upon the control of the insects that spread them or otherwise influence their development. Sometimes the insect that transmits a plant disease is not directly injurious and would be of no economic importance if it did not transmit the disease. Frequently there is a symbiotic relationship between the insect and the disease that it transmits in which both the insect and the micro-organism derive mutual benefits.

J. G. LEACH has been head of the department of plant pathology and bacteriology in West Virginia University since 1938. Earlier he was professor of plant pathology in the University of Minnesota. He has done extensive research with insects in relation to plant diseases. In 1940 he published a textbook, Insect Transmission of Plant Diseases. He is a former president of the American Phytopathological Society.