European Foulbrood. European foulbrood (EFB), another bacterial brood disease, is caused by Streptococcus pluton. This disease is considered a stress disease and is most prevalent in spring and early summer. This is the period when brood rearing is at its height, although the earliest brood rarely is affected. European foulbrood frequently begins to disappear with a nectar flow and may disappear entirely for the balance of the year; or it may reappear during nectar dearths in the summer or fall. Occasionally the disease remains active throughout the entire foraging season. All castes of bees are susceptible, although various commercial strains differ in susceptibility.

The disease and its symptoms are highly variable, probably because several other types of bacteria are often present in dead and dying larvae. EFB usually does not kill the colony, but a heavy infection will seriously reduce population development. European foulbrood generally kllls larvae two to four days old while they are still coiled in the bottom of the cells. (See Figure 35.) Unilke American foulbrood, most of the larvae die before their cells are capped. A spotted pattern of capped and uncapped cells develops only when EFB reaches serious proportions. Occasionally, pupae die from the disease.

The most significant symptom of EFB is the color change of the larvae. They change from a normal pearly white to yellow, then brown, and finally grayish black. Larvae also lose their plump appearance and look undernourished. In such cases, larval remains appear twisted or melted to the bottom of the cell. (See Figure 36.) They form a thin brown or blacklsh-brown scale and show distinct lines where their breathing tubes are located. Recently dead larvae are rarely ropy. Scales can be removed easily from the cells because they are rubbery rather than brittle as in American foul-brood. Bees remove the scales under the stimulus of a nectar flow or feeding.

Streptococcus pluton does not form spores, but the organism often overwinters on combs. It gains entry into the larva in contaminated brood food and multiplies rapidly within the gut of the larva. Not all infected larvae die from the disease. Some develop normally and void the germ or regurgitate the bacteria onto the underside of the cappings, which then become sources of the disease.

Since the honey of infected colonies is contaminated, EFB can be spread by robber bees or by the interchange of contaminated equipment among colonies and drifting bees.

In some cases, European foulbrood can be eliminated by requeening colonies with a young queen. Requeening accomplishes two things: it gives the colony a more prolific queen and it permits a time lag between brood cycles that allows the house bees to remove diseased larvae from their cells.

The antibiotic Terramycin can be fed as a preventive measure against the disease. Follow the

Figure 35. Larvae infected with European foulbrood. (Photo by M. V. Smith, University of Guelph, Guelph, Ontario.)

same recommendations and precautions as described in the section on American foulbrood.

Figure 36. European foulbrood advanced - larva twisted in cell (Photo by M. V. Smith, University of Guelph, Guelph, Ontario.)