Further tests using a polyhedrosis virus to control the alfalfa caterpillar

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Summary

The polyhedrosis of the alfalfa caterpillar, Colias philodice eurytheme Boisduval, is an important factor in the natural reduction of populations of the insect. Epizootics of the disease can be induced by artificial dissemination of the virus. This paper is a report of experiments and field tests conducted during 1947, 1948, and 1949, leading to the successful control of the caterpillar by artificial epizootics. The following general conclusions are the results of these experiments.

Natural epizootics of polyhedrosis cannot in general be depended upon to occur regularly enough to give satisfactory economic control of the caterpillar. Usually they occur only after serious damage to the alfalfa has already been done. Moreover, it is unlikely that natural epizootics can be induced at will through cultural practices.

Natural epizootics are apparently initiated by a complex of factors. One of the most significant of these seems to be population density. Climatic conditions are important primarily because of their effect upon the caterpillar population.

Virus infection and transmission: The virus may be transmitted by a number of mechanical factors including: contaminated adult Colias, insect parasites, carnivorous insects, wind, rain, and irrigation water. Passage of the virus via the egg apparently occurs by external contamination.

Temperature has seemingly little effect on the susceptibility of the insect to infection, but the length of the incubation period of the disease in the insect appears largely determined, within limits, by temperature. Within the temperature ranges tolerated by the caterpillar, the incubation period is shorter at higher temperatures and longer at lower temperatures. Temperature is also an important factor in determining population densities of the host.

Relative humidity does not appear to have much effect on the susceptibility of the host to the infection, or on the incubation period of the disease. It is important, however, in its effect on the population density of the caterpillar.

Application: Application of a virus suspension containing 5,000,000 polyhedra per milliliter at the rate of five gallons per acre appears adequate to insure infection of a field population and its reduction to below economic levels, at least under the conditions usually encountered in the northern San Joaquin Valley.

Application by airplane appears to be one of the most convenient means of treating most fields with a virus suspension. Small fields may be more suited to application by ground equipment.

Economics of control: Virus material can be prepared in large quantities at relatively little expense, and can be preserved at least two years. Cost of application itself is essentially the same as that of an insecticide application.