Thanks for reaching out with these beekeeping questions! Please see my comments and responses below in bold.

I have a few questions about combining hives.

Background:

Queenless hive occupies two medium hive bodies and has plenty of resources - pollen, honey, bees. Are you sure that it is queenless? Sometimes colonies appear queenless (no open brood, etc.) when they are in the process of requeening or when they have a new queen that hasn't begun laying. If you're not sure, you can give the hive suspected to be queenless a frame with eggs and young larvae from one of the nucs and check the frame a few days later. If the frame has emergency queen cells, then the hive is likely queenless.

Nuc #1 – Was a 3-stack of nuc boxes until recently. Now graduated to two mediums and occupying about 75% of the space. Queen hatch date approx.. 6/15/2025. This small colony is doing well.

Nuc #2 – Still a very small colony but showing great promise. Of the five frames, two have nicely capped brood. A third has young larvae and eggs. Queen hatch date approx. 7/23/25.

If I combine one box from the queenless hive with Nuc #1 – am thinking I will have trouble with losing foragers from either one or the other, since both hives have established foragers. (Does the queenless/broodless hive have anything other than foragers? Honey bee workers can take on the jobs of younger workers if younger generations are missing.) The hives are about ten feet apart from one another and face different directions. Have read about closing up the combined hive for several days (well-ventilated and when cooler), then putting a distraction (leafy branch) in front of the hive when they are opened up, so they will re-orient. Have also heard that it doesn’t always work. I have not seen any evidence supporting this method, and I'm also under the impression that it doesn't work. Should I move this combine to a new location for a bit, then move them back? (If so, how long do they need to be gone in order to reorient when they come “home”?) I'd prioritize combining the queenless hive soon before you get laying workers. Even though foragers often return to the location of the original hive, I've often found that bees often find a home/hive when combining hives in the same bee yard.

When combining colonies, I've read that the “stronger colony” should be on the bottom. In these cases, is the stronger colony the one with more bees or the one that is complete (has a queen, brood, etc.). I’d like to put the queen-right colonies (the "nucs") on the bottom in both cases. Thoughts? I think it can often work either way, but I like keeping the queenright hive in its location and putting the queenless boxes on top.

Finally…. Once combined and settled in – after a week or two - I believe both hives will be populated enough for a treatment of Formic Pro. Any concerns with using Formic on these very new queens? (Used according to all instructions, of course, paying attention to temps, etc.) Perhaps single-pad treatment vs. double-pad? (They should all have low mite counts, given brood breaks or lack of brood, but “it’s that time of year”; colony decrease, mites still on the rise.) I don't have strong concerns using Formic Pro with new queens as long as you are following all the label instructions as you said. There is always a risk of losing queens during a Formic Pro treatment, but in my experience, I tend to not lose queens during treatment very often. 

Frequently Asked Questions - NOD Apiary Products Ltd. says (italicized text):

Will Formic Pro kill my queen?

During dearth periods, when ambient temperatures are above 29.5°C (85°F), there is an elevated risk of queen loss, supercedure, or delay in egg laying. Treatment should be postponed until temperatures drop or nectar flow resumes. Formic acid will initially disturb colony activities and may, within one day of application, result in queen rejection or slight increase in adult bee mortality. Some brood mortality may occur in the initial stage of treatment.

Natural honey bee emergence and mortality rate is approximately 1,500 bees per day. A one-day equivalent of natural mortality (i.e. 1,200 to 1,500 bees or up to 2 cups) may be observed at the hive entrance during the treatment. Treatment may trigger supercedure of older or poorly mated queens, regardless of age.

Do not destroy queen cells that may be observed prior to, or post treatment. Supercedure, even if thought to be set in motion by treatment, is a natural process, and should be allowed to proceed for the health of the colony. Verify colonies are queen-right one month after treatment. Mother and daughter queens present post treatment is not uncommon."

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Here are Labels for FORMIC PRO (75710-3) | US EPA. You should follow whichever labels came with your products. The most recently posted Formic Pro label, US EPA, Pesticide Product Label, FORMIC PRO,12/01/2023, states: "The colony should be a minimum of 10,000 bees, covering approximately six 9” deep frames." So, I'd be sure to check that your colonies meet the minimum size requirement after you do the combinations.

Happy beekeeping!