Re: Strange goings on
They could be either being killed in fights with robbers, or they could be dead larvae being removed killed by varroa mites, or the hive could be getting poisoned.
You have taken the right first step in reducing the entrance. You did not say if you are feeding the hive. You should also measure varroa mite numbers somehow, varroa weakened hives are often robbed.
44 years, been commercial, outfits up to 4000 hives, now 120 hives and 200 nucs as a hobby, selling bees. T (mostly).