IN MOST of my examples of insect transmission, the insects make the wounds through which the fungi or bacteria penetrate the plant and also transport the micro-organisms from plant to plant. In some instances the insect may not be so important in transporting the spores of a fungus but may provide wounds through which windblown spores may enter.

That appears to be the case in the association between the brown rot of peaches and plums and the plum curculio. A relationship between the curculio and brown rot of peaches and plums has been observed for a long time, but the importance of the relationship was not fully realized until organic insecticides like benzene hexachloride and parathion became available and effective control of the curculio became possible.

In orchards where the curculio is effectively controlled, much less brown rot occurs than where the curculio is not controlled. There is no evidence that the curculio is a major factor in disseminating the spores of brown rot, which are readily wind-blown. But the insects influence the development of brown rot by making wounds in immature plums and peaches through which wind-blown spores are able to infect. The fungus has difficulty in infecting immature fruits if the skin is uninjured, but the fungus grows readily in the punctures made by the curculio. Spores formed on the injured green fruits provide an abundant source of infection for the ripening fruit later in the season.

The logical method of control for insect-borne diseases is to control the insect vectors. But that has not always worked, because our best methods for controlling the insects were not good enough. Many insect-control measures have reduced losses from direct-feeding injuries but have permitted enough insects to survive to transmit the disease effectively. DDT, lindane, parathion, methoxychlor, and other new organic insecticides have given more complete control good enough to give us the idea that the possible control of all insect-transmitted diseases should be reconsidered from the standpoint of better control of the insect vector.

Moreover, when it has not been possible to control an insect effectively, it has been difficult to determine accurately to what extent the insect is responsible for transmitting a disease. By using the more effective insecticides to get more nearly complete control of known or suspected insect vectors, a more accurate measure of the importance of insect transmission of many plant diseases can be had.

A relationship also exists between insects and the rust fungi. Many of the rust fungi, such as the destructive black stem rust of cereals, reproduce sexually and produce structures that have functions comparable to the male and female organs of the flowering plants. To complete the life cycle, a male cell must enter the female organ so fertilization can take place. The rust fungi depend largely on insects for this process of "pollination."

In the black stem rust of wheat the process occurs on the leaves of the barberry bush (Berberis vulgaris, B. canadensis, and B. fendleri), which is the alternate host of the fungus. The spores come from the grass host and infect the barberry leaf. They are of two sexes usually designated as+ and , because there are no morphological differences that would identify them as male and female. On the barberry leaf each spore produces a spot in which are formed numerous flask-shaped structures, the pycnia. Each pycnium produces thousands of small spores (pycniospores) and numerous short hyphae. If the pycnium originated from a + spore, the nuclei in the pycniospores and hyphae are of the + sex. Those arising from spores have nuclei of the sex.

The spores function as gametes comparable to the pollen of higher plants. The fungus hypha correspond in function to the stigmata and are called receptive hyphae. If a pycniospore comes in contact with a receptive hypha of the opposite sex, it germinates and fuses with a cell of the receptive hypha. The nucleus of the spore passes into the cell of the receptive hypha and becomes associated with the nucleus of the opposite sex and eventually fuses with it, thus effecting fertilization. The pycnia are self-sterile the spores produced in a + pycnium will not fuse with the receptive hyphae of the same pycnium or of other pycnia of the same sex. They must be transported to a receptive hypha of the opposite sex if fertilization is to take place.

The pycnia are produced on the upper side of the barberry leaf in a bright yellow spot, and the spores and receptive hyphae are covered with a drop of sugary, fragrant solution. Flies and other insects are attracted to infected barberry leaves by the bright color of the spots and the solution, on which they feed. In feeding on the solution and moving from one spot to another, the insects transfer spores from + to pycnia and vice versa, thus insuring "pollination" of the fungus.

Sexual reproduction often results in hybridization between different races of rust and results in the production of new races, some of which will attack the new varieties of wheat that have been bred for rust resistance. Thus the insects, with the aid of the barberry bushes, are breeding new varieties of rust almost as fast as the plant breeders can breed new varieties of wheat.

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