News About Bee Diseases

by A. P. STURTEVANT

AMERICAN foulbrood is the most destructive and widespread disease of honeybees in the United States. For many years apiarists generally believed that once a colony became infected it was doomed to die if left alone. Various methods of treatment have been tried with incomplete success. Even attempts to eradicate the disease by destroying all infected colonies have not eliminated this threat to beekeeping.

Because of the emphasis previously placed on treatment or regulatory control measures, the few early recognized cases of apparent recovery from American foulbrood were more or less overlooked. The increasing frequency of such cases reported by beekeepers eventually brought to the attention of investigators the possibility of the discovery and development of strains of bees that might be resistant, if not actually immune, to American foulbrood.

Early work on this problem by investigators in Iowa demonstrated that a certain resistance to American foulbrood does exist in honeybees. It was demonstrated also that resistance could be inherited from one generation to another through the queen bees an indication that, through continued selection and breeding of queens from colonies of bees showing resistance to the disease, strains of bees might be developed that would be highly resistant and of value in combating the ravages of American foulbrood.

Extensive investigations concerned with the development of strains of bees resistant to American foulbrood have been carried on during the past decade by the Bureau of Entomology and Plant Quarantine in cooperation with several State agricultural experiment stations. The results have shown encouraging progress.

The methods used, in general, are as follows: Colonies of bees located in isolated apiaries to prevent accidental spread of the disease, and headed by queens selected for testing, are inoculated artificially in one way or another with American foulbrood material containing spores of Bacillus larvae, the cause of the disease. The colonies are examined regularly thereafter for manifestations of disease. Disease may occur in some colonies, which may then either show recovery or may develop disease beyond hope of recovery. Still other colonies may remain healthy, the bees never permitting disease to develop, at least so that it can be seen by the investigators. These "negative colonies" are considered to be the most highly resistant to the disease and are the ones from which queens are selected for further intensive breeding and subsequent testing the next year. During most of the time succeeding generations of test queens have been reared in isolated locations by natural mating methods, the parentage being controlled as much as possible under natural conditions.

Three strains of honeybees have been developed by this method. During the several years that these strains have been under observation and test, we have noted a definite improvement in resistance in two of the strains and a much slighter improvement in the third strain.

Improvement was rapid at first, but later the curve of improvement tended to flatten out at different levels somewhat below complete resistance. Recently some cross-breedings by natural coatings between two of the resistant strains have shown an increased resistance, with a possible development of hybrid vigor, as indicated by increased resistance and honey production.

Since 1943 the studies have been accelerated by the use of artificial insemination for queen bees. By this method of breeding, the parentage of the test queens can be completely controlled, either through intensive line-breeding or by known cross-breeding. A notably significant improvement in the level of resistance has been observed in all three of the line-bred resistant lines since use of the artificially bred queens was begun. The majority of such queens show as good colony performance as queens reared under natural conditions. Some of the intensively inbred queens produce brood of poor quality. These do not develop strong colonies. However, with most of the colonies headed by artificially inseminated queens, particularly those crossbred between two of the lines showing the highest resistance to disease, the results have been outstanding. This has been true not only as regards resistance but also as regards production of brood of high quality and increased production of honey. This again would indicate the development of hybrid vigor through cross-breeding.

A limited number of queens of strains bred for resistance to American foulbrood have been distributed throughout the United States. On the whole, favorable reports have been received concerning their use, largely in prevention rather than eradication of disease. Such strains have not yet been stabilized, however, and are not completely desirable for commercial beekeeping purposes. The greatest effort has been concentrated on improving their resistance to American foulbrood. Some of the experimental strains are inclined to be hot-tempered and not all of them are outstanding in honey production.

Unfortunately, the development of resistance to one disease does not confer protection against other diseases. With continued inbreeding of the naturally mated as well as the artificially inseminated queens, it has been noted that certain strains of bees resistant to American foulbrood are showing an undesirable susceptibility to European foulbrood. Therefore it will be necessary to continue more intensive breeding and selection work before completely desirable strains of bees can be developed that are resistant to both American and European foulbrood and have other desirable characteristics.

Our studies regarding the behavior of bees toward American foulbrood have given new information concerning factors associated with the development of the disease and the nature of resistance.

The behavior of bees toward diseased brood was observed by placing combs from experimental colonies in a cell-locating frame, noting the exact location of individual diseased cells, describing them, and examining the same cells at regular intervals.

We discovered that bees of all colonies removed some or all of the diseased brood remains. The time required for removing the material and the chances of recovery by its complete removal were found to be closely related to the number of cells of diseased brood present in the colonies. Light infections sometimes were overcome whereas heavy infections were not. No close relationship was found between colony populations, queen vigor, or strain of bees and the rapidity of cleaning the cells, or the spread of disease within the colonies. The diseased material usually was removed without disturbing the comb.