The most problematic pest beekeepers encounter in the United States today is the varroa mite. The varroa mite (Varroa destructor) is an ectoparasite associated with spreading disease, pathogens and reducing the lifespan of male and female honey bees. The mites accomplish this by creating wounds in honey bees with piercing/sucking mouthparts, then feeding on the hemolymph within. Research suggests that these mites transfer single-stranded RNA virus between bees, along with infections of bacteria, including Melissococcus pluton (EFB). This type of bacterial infection of larvae or pupa is considered a secondary infection, since the mite initiated the process and the bacteria followed.
EFB is normally transmitted when the bacterium becomes mixed with the bee bread, nectar or diluted honey, and then fed to young larvae. The bacteria then replicate in the larvae mid-gut, killing the larvae within 4-5 days from when the egg hatches. With “normal” EFB, beekeepers generally see open brood infected with the bacteria, though you may see some dead larvae under perforated sealed brood. This is because when a larva is fed the bacteria, they can either live or die, depending on the bacteria concentration, and also the size of the food mass in the mid-gut. I believe that these bacteria can also gradually starve out a larva, causing death in the pupa stage. There are many images below of different stages of EFB and two compound microscope pictures from I. B. Smith, Jr. at the USDA in Beltsville, MD.
Now I will discuss what I think is happening with a secondary infection due to mites. Mites can transfer EFB in a few ways, the first being when they enter the royal jelly before the cell is sealed. If the mite had the bacteria on its body, it could transfer bacteria to the brood food in the cell. The second method of EFB transfer is if the mite contained the bacteria inside the body, it could spread EFB through feeding. In this case, the mite would have to feed on the larvae once it’s free from the royal jelly. This would mean the larvae, now between 7½ and 8 ½ days old, has been infected with the bacteria. Normally larvae are infected as soon as they start consuming contaminated brood food. This infection with the bacteria at this late a stage in the life cycle may be detrimental to its lifespan and health because the limited food mass left in the bee. These symptoms are likely what are happening when you see signs of EFB in the open brood, and a lot of perforations in the sealed brood with different staged larvae/pupa. While the symptoms could look like mite damage, you clearly can see EFB infected larvae under perforated sealed brood (refer to images below). I have included images of what I believe is a colony secondarily infected with EFB by varroa mite. I also think these symptoms may be a delayed response to EFB or secondary bacteria. I have also included some EFB images above for reference. For more information on EFB see EFB Blog.
If you see this contagious brood disease in your colonies, the recommended treatment is three dustings of terramycin, 5 to 7 days apart. If you want to be sure you have EFB you can use a cotton swab and a ziplock bag and send samples to the USDA to be tested. If the symptoms return, repeat the steps above to clear up the bacteria. Tylosin is another broad spectrum antibiotic used to treat gram positive organisms, EFB is a gram positive bacteria but I have heard rumors from beekeepers that this antibiotic does treat EFB.