This image shows a section of a frame with a wax moth tunneling below sealed brood. This symptom is called Bald Brood.
An interesting brood symptom you may come across in a weak hive in the spring is Bald Brood. Bald brood is caused by the Lesser wax moth (Achroia gresella) or the Greater wax moth (Galleria mellonella.) You can see in the image below the linear pattern of uncapping that occurs due to the wax moths tunneling behavior. These symptoms can sometimes be confused as hygienic behavior; I have included an image showing some hygienic behavior of uncapping. One visible difference between hygienic behavior vs. bald brood is that there is no linear pattern of uncapping sealed brood with hygienic behavior. Colonies with bald brood often have raised sidewalls that are slightly above the other sealed brood, this symptom is not usually present when bees are uncapping larva/pupa associated with varroa, a lethal gene or another brood disease. I have seen these symptoms a handful of times over the past several years but in most cases the colonies were weak ranging from 3-6 Frames of bees. The best defense against wax moth is a strong colony. If you investigate cells near the ends of the linear pattern you can probably find the wax moth larva, look for perforations in the sealed cells. For more information on wax moth please see my previous wax moth blog here “Wax Moth“or “Jennie’s blog on Wax moth damage”.
This image is cropped from the image above to give you a close up of what bald brood looks like. You can see the raised cell walls around the uncapped pupa. Also, note the darker brown convex sealed cells in between the open cells (convex but not normal looking.) This was the path of the wax moth larva. There is a perforation in the cell to the right of the last uncapped cell. I found the wax moth in that perforated cell. There is a photo of the wax moth larva below.
Wax moth larva that was removed from the perforated cell pictured above.
Bald brood is caused by the wax moth tunneling below the sealed brood. Bees can detect something is wrong when the wax moth is tunneling below and start to uncap cells.
This image shows the results of a hygienic test. You can compare the edges of the open cells to the open cells in the bald brood image. This image also shows how the hygienic bees will start to chew down and remove pupae and larvae.
This is an image of hygienic behavior. The bees are uncapping and chewing down larvae because they detected varroa mite, lethal gene or some other brood disease. Note there is no real pattern to the uncapping. Also note the pepperbox brood pattern.
One may confuse the linear uncapping for early stages of chalkbrood so I have included this image to compare to bald brood.
In this image you can see hygienic behavior displayed in circled uncapped brood. I also circled some other problems related to varroa mite. You will see a few bees circled with Deformed Wing Virus (DWV.)