In short, Chewed Down Brood (CDB) is a visual indicator of a honey bee colony’s fight against Varroa mites. It occurs when hygienic bees detect mites reproducing within a sealed brood cell, uncap that cell, and begin to remove the developing pupa to interrupt the mite’s life cycle. This behavior is both a positive sign of the colony's genetic defenses and a clear warning of an active mite infestation.
Chewed Down Brood is a double-edged sword for the beekeeper. It reveals your colony possesses valuable mite-resistant genetics, but it also confirms an active and potentially serious Varroa mite infestation that requires your immediate assessment.
What CDB Reveals About Your Colony
Chewed Down Brood is a specific type of hygienic behavior, often linked to a trait known as Varroa Sensitive Hygiene (VSH). Understanding what you are seeing is critical to making the right management decision.
The Mechanism: A Targeted Defense
Bees with this genetic trait can somehow sense the presence of reproducing Varroa mites on a pupa, even through the wax capping. In response, they chew a hole in the capping and remove the pupa, an act often referred to as cannibalism.
This behavior is a highly effective defense. It not only removes the individual mites on that specific pupa but, more importantly, it disrupts the mite's reproductive cycle, preventing several new mites from emerging.
The Appearance: What to Look For
You can identify CDB by observing a patch of sealed brood and looking for cells that are uncapped or have sunken, perforated cappings. You may see parts of a white pupa being removed by the bees.
These uncapped cells will often be scattered among otherwise healthy-looking capped brood, creating a slightly mottled or "shotgun" brood pattern. It is distinct from disease-related patterns, as the removed pupae appear white and healthy, not discolored or melted as with brood diseases.
Interpreting the Signs Correctly
Observing CDB is not a simple "good" or "bad" sign. It is a data point that signals two competing realities happening within your hive.
The Positive: Your Bees Are Fighting Back
Seeing CDB confirms your colony has a degree of natural mite resistance. This is a highly desirable trait. Colonies that actively perform this behavior are better equipped to manage their mite loads than colonies that do not.
The Negative: You Have a Mite Problem
This behavior is only triggered when Varroa mites are present and actively reproducing within the brood. The bees are not performing a drill; they are responding to a real threat.
If you see a significant amount of CDB, it means the mite infestation is substantial enough to trigger a widespread defensive response.
Understanding the Trade-offs: Behavior vs. Infestation Level
The most common mistake a beekeeper can make is to see CDB, assume the bees have the problem under control, and do nothing.
The Danger of Complacency
While hygienic behavior helps, it is often not enough to keep mite populations below the damaging thresholds that lead to colony collapse, especially in the late summer and fall. The rate of mite reproduction can easily outpace the bees' ability to remove them.
CDB is a Clue, Not a Conclusion
CDB is a qualitative sign of a mite problem. It tells you that mites are present in the brood. It does not tell you the overall infestation level in the hive.
To get a true picture of the threat, you must follow up a CDB observation with a quantitative measurement, such as an alcohol wash or a sugar roll mite count. This gives you a specific number (e.g., mites per 100 bees) to guide your treatment decisions.
How to Respond When You See CDB
Your observation of Chewed Down Brood is a call to action, not a reason to panic. Use this information to guide your next steps in mite management.
- If your primary focus is simple colony survival: Treat the presence of any CDB as a clear signal to perform a mite count immediately. If your count is above the recommended treatment threshold for the time of year, you must intervene with an appropriate mite treatment.
- If you are seeing large, widespread patches of CDB: This indicates a severe mite infestation that is likely already harming the colony. Your bees are in a desperate fight. Perform a mite count to confirm, but be prepared to apply a treatment right away.
- If you are a breeder selecting for resistant stock: Documenting CDB is a valuable data point. A truly superior colony is one that exhibits strong CDB behavior while also consistently maintaining a very low mite count without treatment.
Ultimately, viewing Chewed Down Brood as a critical piece of data empowers you to work with your bees' natural defenses to ensure their long-term health and survival.
Summary Table:
| Aspect | What CDB Indicates |
|---|---|
| Primary Cause | Active Varroa mite infestation within sealed brood cells. |
| Colony Behavior | Expression of Varroa Sensitive Hygiene (VSH), a natural defense trait. |
| Implication | A positive sign of genetic resistance but a serious warning of mite pressure. |
| Required Action | Perform a quantitative mite count (e.g., alcohol wash) to assess infestation level. |
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