Cause

American foulbrood, the most widespread and most destructive of the brood diseases in the United States, is caused by a spore-forming germ known as Bacillus larvae. Adult bees are not affected by this disease.

Only the spore stage of Bacillus larvae is infectious to honey bees. All castes of honey bees are susceptible to the disease, but worker larvae are particularly susceptible. However, larvae become immune to the disease about 3 days after the egg hatch.

Effect

Only a few dead larvae or pupae will be seen when the colony is first infected by the disease. Occasionally, enough larvae become infected to weaken or kill the colony the first season. On the other hand, the disease may not develop to the critical stage until the following year. If left unchecked, American foulbrood quickly spreads to healthy colonies in nearby apiaries.

Symptoms

If American foulbrood is infecting your colony, the cells of your brood comb will have a scattered and irregular pattern of capped and uncapped cells, and cells with sunken and punctured cappings. This "pepperbox" appearance will contrast with the entirely sealed cells of a healthy brood comb. (See fig. 1. )

A larval color change is one of the signs of infected larvae. Dying larvae gradually change from pearly white to dark brown.

The "pepperbox" pattern begins to form as the larvae shrink; the capping is drawn down into the cell so that the normally convex capping becomes concave. In advanced stages of the disease many of the cappings are punctured. (Cappings over dead brood are often removed by adult bees. )

The decay and drying of dead brood takes a month or more. The larval remains dry and later form a brittle scale that adheres to the lower wall of the cell. (See fig 2. )

One way to verify that American foulbrood is causing larval death is to remove and examine some of the decayed larvae from the brood cells. During the early stages of decay, about 3 weeks after death, the body wall of the cell can be easily ruptured. Using a toothpick or matchstick, thrust into the decayed larvae and withdraw the decaying mass. If the disease is present, an inch or more of brown, gluelike thread can be withdrawn. This condition is known as the "ropy stage. " (See fig. 3. )

When the remains begin to turn brown and become ropy, an odor can be detected that is typical of the advanced stages of this disease. This same odor persists even when the scales are formed.

Larvae often develop into pupae before death occurs. Pupae undergo the same changes in color and consistency as larvae.

Figure 1. Brood combs showing (A) healthy brood necessary for high honey production, and (B) diseased brood, which results in weakened colonies and low honey production.

When a pupa dies from American foulbrood its tongue generally protrudes from the scale to the center of the cell. (See fig. 4. ) This symptom is characteristic only of pupae infected by American foulbrood. It should not be confused with the short blunt protrusion-or "false tongue"- associated with European foul-brood.

Spread

American foulbrood can spread in a colony when. Nurse bees transmit bacillus spores to young larvae.

. Honey is stored in cells that once contained diseased brood.

. Bees are exposed to contaminated honey.

. Equipment is used for both diseased and healthy colonies.

Nurse bees can inadvertently feed bacillus spores to young larvae. Soon after the larva has been sealed in its cell, or just after it changes to a pupa, the spores will germinate in the gut of the larva and multiply rapidly, causing death. New spores will form by the time the larvae dies. When the house bees clean out the cell containing the dead larva, spores will be distributed throughout the hive, thus infecting more larvae.

Figure 2. Honey bee larvae killed by American foulbrood, as seen in cells: (A) Healthy larva at age when most of brood dies of American foulbrood; (B-F) dead larvae in progressive stages of decomposition (remains shown in F are scale); (G) longitudinal view of scale.

[missing image]

Figure 3. One way to determine whether or not American foulbrood is causing larval death is to remove and examine decayed larvae from the brood cells. Here, removal of a gluelike thread, known as the "ropy stage," proves the disease is present.

Honey stored in cells that once contained diseased brood becomes contaminated and may be fed to susceptible larvae. As the infection weakens a colony, the colony cannot defend itself from robber bees from strong colonies. The robber bees take the contaminated honey to their own colony and repeat the cycle of infection and robbing.

When bees are exposed to contaminated honey, or the same equipment is used for diseased and healthy colonies, there is a danger of disease spread. Therefore, it is extremely important that diseases are detected in their early stages, and that equipment is free from disease organisms.

Control

No commonly used strains of honey bees are immune to American foulbrood.

Burning. - When American foulbrood is discovered in your apiary the diseased colonies should be destroyed by burning. Before you burn diseased colonies, however, you must dig a pit to hold the burned material.

Dig a pit 18 inches or more in depth and wide enough to hold all the material to be burned. Situate the pit in a place not likely to be disturbed.

Immediately after all the bees have been killed, place the hive on pieces of burlap or strong paper; this will make it easier to gather up and burn the bits of comb, honey, or dead bees. Do this quickly to reduce the possibility of robber bees spreading the disease to healthy colonies.

When burning the colony, kindle a fire beneath it with cross members strong enough to support the weight of the frames. Allow plenty of ventilation. A brisk, hot fire is necessary to quickly burn the brood and honey.

Do not burn the bottom boards, hive bodies, inner covers, or outer covers. These items should be scraped to remove all bee glue (propolis) and wax, and then scrubbed with a stiff brush and hot soap solution. Afterward, dispose of the wash water and burn the scrapings so they are not accessible to bees.

The scraping and scrubbing procedures above will not sterilize the bee equipment. To do this, completely immerse your equipment for 20 minutes in a boiling lye solution (sodium hydroxide) containing 1 pound of lye to 10 gallons of water. Wooden parts can be damaged by longer exposure. Weaker solutions may not remove all the wax and propolis from the hive equipment.

Remember that lye solutions are caustic and can cause severe burns. Before using the lye, read the label carefully and observe all precautions.

Treatment. -The burning of diseased colonies in the apiary gets rid of only those colonies in which American foulbrood is present in an active form. To prevent the spreading of the disease throughout the apiary, and to control it, oxytetracycline (Terramycin) or sodium sulfathiazole has been used. Labels for drugs carry specific instructions for their application, subject to State laws and regulations. Consult with State apiary inspectors, extension apiculturists, or State entomologists before using any chemicals.

Figure 4. Pupal tongue from honey bee pupae killed by American foulbrood protrudes from the scale to the center of the cell.