Hi there Darlene!
Unfortunately almost every beekeeper will deal with Varroa mites at some point with their colonies. Since their introduction in the late 1980s they spread quickly across North America and are now found in every state.
The best approach we have found when dealing with these mites is to develop strong, large, healthy colonies of bees. We have found treatments such as antibiotics, used to support bees, and arachnicides, used to kill mites, in the long run can have more detrimental effects to our colonies' health. Often treatments like these create pesticide resistant mite populations that are more difficult to deal with and will also weaken your workers and your queen.
You can still treat with powdered sugar in your top bar even without a screen bottom board. The mites will fall to the bottom of the hive and usually will not be able to climb back up onto the combs. Allowing your bees to swarm is another way to treat for mites. As this allows the queen to go through a brood break often stopping the cycle of mites temporarily. Bees will also use drone comb as a mite trap. Often segregating their drone comb to on section of the hive, and killing developing drone larvae with the most developing mites. Beyond that continuing to look for and collect colonies that are very hygienic and good groomers. Splitting these colonies and culling colonies that do not cope with mites well is often the best way to deal with Varroa mites.
Hope that helps Darlene! Please let us know if you have any additional questions about mites or other pests. You can also reach our customer service team by phone at 877-325-2221 or by email at firstname.lastname@example.org.