Sacbrood disease targets honey bee larvae, causing their death before pupation. While adult bees can carry the virus without outward signs, their lifespan is shortened when infected.
The disease is caused by the Sacbrood virus (SBV), with multiple viral genotypes identified. This article aims to provide insight into the disease and guidance on how to treat and manage it in honey bee colonies.
Origins of Sacbrood Disease
Sacbrood disease arises from infection by the Sacbrood virus, which can enter hives through wandering bees, robbing behaviors, or actions by beekeepers moving infected equipment or bees. Preventative measures, such as queen replacement, help block the virus from invading a colony.
Ways Sacbrood Spreads
The virus remains infectious within larvae for up to a month and has spread worldwide through swarming, making it a major concern in the US. Varroa mites also facilitate transmission. Nurse bees play a key role by acquiring the virus from diseased larvae or brood remnants and passing it on to healthy larvae during feeding.
Infected adult bees do not show symptoms but exhibit altered behaviors and live shorter lives. Worker bees affected by the virus begin foraging earlier and soon reduce foraging activity, eventually ceasing to collect resources. Despite this, contaminated pollen inside the hive can still infect young bees consuming it.
Identifying Sacbrood Symptoms
Effective management depends on accurately diagnosing honey bee diseases like Sacbrood by assessing visible signs and conducting field or laboratory analyses. Evaluating features such as colony health, brood condition, and comb characteristics provides reliable diagnosis, surpassing symptom-only assessments.
Examine the brood comb face closely, noting the condition of brood cells. Infected larvae may be found in both capped and uncapped cells. Capped cells with Sacbrood sometimes show perforations created by bees attempting to remove dead larvae, a sign also seen in American Foulbrood, which can cause diagnostic confusion.
When bees clean out infected cells, they contact the virus present on dead larvae and risk spreading it to healthy brood during feeding or contact.
Conversely, this hygienic behavior also helps reduce infection levels in the hive by removing sources of contagion, allowing the disease to persist only at low levels.
Key Signs of Sacbrood Disease Include:
1. Dead Larvae Shape
Larvae infected with the Sacbrood virus die in their cells, typically before pupating, retaining a rounded shape that distinguishes them from larvae affected by other diseases. Their skin becomes firm and flexible, showing visible segmentation, unlike American Foulbrood where segmentation disappears in dead larvae.
2. Color Changes in Dead Brood
As the infected brood dies, its color shifts from pearly white through yellow to brown, then gray and ultimately black. These gradual changes can be seen across areas of infection on the brood comb.
This progression contrasts with American Foulbrood, where dead brood moves from light to dark coffee brown shades before turning black, without the intermediate gray stage.
3. Larval Posture and Coloration
Dead larvae from Sacbrood display a distinct two-tone appearance, with their heads darker than their lighter-colored bodies. The head is also positioned raised, unlike larvae dying from American Foulbrood.
Honey bee larvae afflicted by Sacbrood disease typically twist within their brood cells and perish with their heads pointed upwards. In contrast, larvae dying from American Foulbrood do not show such two-tone coloring; instead, their bodies display a uniform coloration. Moreover, the heads of larvae affected by American Foulbrood remain lowered rather than being raised.
4. Inspecting the Scale of Sacbrood-Affected Bee Brood
When Sacbrood infects honey bee larvae, worker bees actively remove the deceased larvae from their cells. If these larvae are left unremoved, they desiccate and eventually form a black scale that clings inside the brood cell. This scale might be mistaken for remnants left by other bee diseases. However, the scale produced by Sacbrood-infected larvae can be easily lifted from the cell in one intact piece.
This contrasts with diseases like American Foulbrood, where the scale adheres firmly to the cell wall and resists removal as a single piece. Locating Sacbrood-affected brood that has hardened into scales within a hive is challenging and typically only noticeable when the disease has extensively spread across many brood cells. Such widespread Sacbrood outbreaks are relatively uncommon in beekeeping.
5. Puncturing Dead Brood to Confirm Sacbrood
In the early stages of decomposition, dead larvae from Sacbrood disease can be carefully extracted from their cells. Using a tool like a matchstick or twig, these larvae can be lifted and will dangle in a sac-like manner, a characteristic that lends the disease its name. To further diagnose, puncturing these larvae reveals watery contents that easily flow out if Sacbrood is indeed the cause of death.
Ropiness Test for Diagnosing Sacbrood Disease
Various factors can cause brood mortality in a hive, beyond just specific diseases. When encountering dead brood, beekeepers often apply the Ropiness test. However, this test is unreliable for Sacbrood because larvae infected with this disease liquefy and lose their stickiness after death, thus failing to produce the characteristic ‘rope’ during testing. Any observed ropiness in Sacbrood cases is usually due to secondary bacterial activity on the decomposing larvae.
Ropiness can be an indicator of several bee diseases, including American and European Foulbrood. Consequently, results from the Ropiness test should be interpreted alongside other clinical signs and observations within the hive.
If uncertainty persists after conducting the Ropiness test, it is advisable to consult a laboratory for accurate disease identification. Samples can be collected with simple materials following proper protocols before submitting them for laboratory analysis.
Is There a Cure for Sacbrood Disease?
While chemical treatments are often favored by beekeepers for managing bee diseases, Sacbrood disease lacks any effective chemical cure. Therefore, management focuses mainly on preventive measures and alleviating the stress on infected colonies. By reducing disease pressure, the affected colony becomes stronger and better equipped to cope with control strategies. One highly dependable method is requeening the colony with a new queen.
Introducing a vigorous queen along with maintaining strict apiary hygiene plays a critical role in preventing Sacbrood outbreaks. The disease tends to establish more readily in unclean hives. Good hive sanitation results from the combined efforts of beekeepers and bees cleaning the hive. Removing debris such as broken comb, spills, and dead bees promptly helps, as bees may struggle to clear large debris efficiently, thus allowing them to concentrate on maintaining clean brood cells.
When Sacbrood has taken hold in a hive, assisting hive hygiene can help save the colony. By removing debris and dead bees, you enable the colony to better focus on eliminating infected larvae. Installing a screened bottom board can enhance hygiene by letting dead bees and larvae fall through to the ground for easier removal and hive cleanliness.
Apiary Practices for Managing Sacbrood Disease in Honey Bees
Additional strategies to combat Sacbrood include spacing hives to prevent drifting between colonies, providing supplementary feeding to reduce colony stress, and merging weakened colonies. Maintaining a robust bee population makes it harder for Sacbrood disease to overwhelm the hive.
1. Hygienic Behavior in Honey Bee Colonies
The genetic makeup of bees strongly influences their hygienic behavior, a trait that beekeepers can exploit to control Sacbrood disease. An afflicted colony often signals suboptimal genetics. Replacing the existing queen with one from superior stock can improve overall hygiene, despite temporary disruptions in the colony.
Over time, the offspring of the new queen will inherit better hygienic traits, enhancing the colony’s disease-fighting capabilities. When requeening after Sacbrood infection, ensure you select a queen of higher genetic quality, possibly sourcing one from neighboring beekeepers.
Supporting the colony with feed during requeening helps mitigate stress since the new queen requires time before starting to lay eggs. Interruptions in the brood cycle caused by this process may also contribute to better control and recovery from Sacbrood disease.
2. Employing Requeening as a Treatment for Sacbrood
Honey bees naturally combat Sacbrood virus infections through their hygienic behaviors, particularly by detecting and removing dead larvae beneath brood cell caps. However, excessive stress can weaken this defense, making colonies vulnerable to the disease.
Factors such as insufficient nectar and pollen, a failing queen, adverse weather, pest infestations, and other diseases diminish colony strength, reducing its ability to ward off Sacbrood. Addressing these issues by boosting colony population is essential. Providing sugar and pollen supplements encourages the production of more worker bees.
Beekeepers managing multiple hives can transfer frames containing brood to bolster weaker colonies. Once the brood matures into adult bees, the colony’s workforce increases. These frames can either replace infected brood frames or be inserted into the super box above the brood chamber in the affected hive.
If your beehive has a queen excluder, you’ll need to remove it before placing the brood frame inside the super box. This enables the queen to enter that section, accompanied by nurse bees who recognize the new brood and start tending to it. Providing more space for egg-laying can be crucial to preventing the hive from collapsing entirely.
How to Requeen a Beehive to Treat Sacbrood Disease in Honeybees
To introduce a new queen, use a queen cage equipped with a candy plug. Remove the coverings from the candy plugs and position the caged queen centrally within the hive so that her pheromones reach all colony members evenly. The worker bees will gradually chew through the candy plug to release the queen.
Withholding honey and sugar feed from the colony accelerates how quickly the bees free the queen from her cage. After the queen is freed, honey-filled frames should be returned to the hive, and feeding should be resumed to ensure the colony receives adequate nourishment.
Once the new queen is introduced, regularly check the hive for any developing queen cells and remove or destroy them. This prevents competition, helping the new queen establish herself during the stress caused by the Sacbrood disease.
3. Early Detection Practices for Sacbrood Disease
Consistent hive inspections help identify indications of Sacbrood disease before it spreads widely. If you notice that more than 5% of the brood is infected, requeening the colony is recommended.
Comb sections infected with Sacbrood should be extracted from the hive and either melted down or stored for at least two months. Since the Sacbrood virus does not persist long outside of larval transmission within the colony, this reduces the risk of reinfection.
Combining the removal of infected frames with requeening allows the new queen to replenish the brood with healthy larvae. It’s also advisable for beekeepers to replace brood frames every 2 to 3 years, either gradually or by replacing all at once, as a preventative measure against disease.
Preventing the Spread of Sacbrood Disease
All beekeeping tools and equipment must be thoroughly sanitized before being used on different hives. Regularly cleaning these items with solutions such as washing soda or bleach is essential for eliminating infectious agents from their surfaces.
Using a cloth dampened with disinfectant to wipe equipment can be effective, and scorching tools that can tolerate heat also sterilizes them. Whenever hive parts are moved between colonies, they should be carefully cleaned beforehand to avoid transferring pathogens.
It’s critical to recognize that Sacbrood disease may exist at undetectable levels within strong colonies without causing harm. However, transferring contaminated materials to other colonies, especially weaker ones or those recently requeened, can lead to serious outbreaks. Therefore, even seemingly healthy hive parts must be sanitized before reuse elsewhere.
Maintaining strict hygiene during bee transport and pollination activities is vital. Never relax your sanitation practices while moving hives, and upon arrival, continue with rigorous hive management to minimize the risk of disease transmission.
Conclusion
Sacbrood disease causes honey bee larvae to die before pupation, leading to an accumulation of ecdysial fluid beneath their skin that forms a sac, giving the disease its name. Infected larvae undergo color changes and eventually dry into scalelike structures. This viral disease is most common from spring through early summer, demanding vigilant attention to prevent costly losses. Effective management strategies exist to combat it.
Proactive prevention measures are essential to protect your honey bee colonies from Sacbrood disease. Utilize the outlined treatment and management approaches to control and safely manage this condition in your apiary.