This external parasitic mite attacks and feeds on two varieties of honey bees, Apis millilfera and the Eastern honey bee, Apis cerana, which is native to my area of the country. The resulting disease is known as invarroosis. These mites can only reproduce in a honey bee colony where they attach to the bees' bodies and weaken them by sucking fat storage cells. The species is also a vector for at least five debilitating bee viruses.
Varroa destructor mite is the most damaging pest known to honey bees in the United States and most of the world. Since its discovery in the U.S. in 1987, it has spread rapidly throughout the country, aided by the movement of infested commercial honey bee colonies. Varroa is so widespread and its effects on bee colonies so serious that beekeepers must routinely treat or their colonies will likely perish.
In recent years, resistance to traditional chemical miticides has become a serious problem. Fortunately, the number of management tools and strategies for control has increased as well, providing beekeepers with a wider variety of options for more sustainable mite management.
Varroa is an external parasite that feeds on the blood of both immature (brood) and adult bees. It lives either on the adults or within brood cells. A mated adult female mite, called a foundress, enters a worker brood cell 15 to 20 hours prior to capping (40 to 50 hours pre-capping for a drone brood) and feeds on the larva after the cell is capped. She lays her first egg about 60 hours later and may lay as many as six eggs at 30 hour intervals. Her young feed on the bee prepupa and pupa, taking 7 to 8 days (females) or 5 to 6 days (males) to mature. Mating occurs within the capped cell. Although several eggs are laid, the average number of mature, viable female mites produced per foundress in a cell is less than two.
When the bee emerges from the cell, the new female may stay attached to the bee or may attach to another bee. Depending on the time of year, she will stay on the bee for a few days to a few months. Eventually she will enter a brood cell to begin the reproductive cycle over again. The original foundress may survive to infest another cell.
Symptoms of Varroa destructor infestation are not always obious before mite numbers have reached damaging levels. Adult bees parasitized by mites as brood may exhibit deformed legs and twisted wings, which are thought to be caused by a virus. Infested colonies will appear weak and the brood pattern may be spotty. Bees may be overly defensive and are often seen discarding larvae and pupa. The colony may desert a heavily infested hive.
Colonies entering winter with a Varroa infestation may not survive. A colony dying from Varroa in winter will usually display a small number of dead bees and sometimes only a moderate amount of honey. There could also be no remaining bees.
Monitoring (sampling) and detection should be a component of any management program. Beekeepers need to know if their bees are infested and to what extent. Preventative treatments of legal Varroa miticides are used by most beekeepers since prevention insures losses are kept to a minimum. Treatments may not be needed if Varroa isn’t present or if mite populations are low.
During routine casual inspections of colonies, the mites or symptoms of an infestation such as twisted wings may be noticed. If Varroa are easily seen on bees or damaged bees are numerous, a heavy infestation is already likely and treatment should begin immediately. Several methods for determining the extent of infestation are described below.
Ether roll and powdered sugar methods
Collect approximately 300 bees from the hive into a jar. Spray them with two squirts of ether starting fluid, sealing and rolling the jar, and counting the mites that have fallen off. Bees should be collected from the brood cluster by raking the mouth of the jar through the bees so they fall into the jar. Be sure not to collect the queen.
Quickly cap the jar, knock the bees to the bottom, remove the lid, add the starting fluid, and replace the lid. This will quickly kill the bees and cause the mites to dislodge. Vigorously roll the jar for 30 seconds to collect the dead mites on the side of the jar and count them. If eight or more mites are found, treatment is recommended.
A modification of this method uses powdered sugar to dislodge the mites without harming as many bees. The same method described above is used to collect the bees. However, a canning jar should be used so that the solid lid can be replaced with 1/8" screen mesh. After collecting bees and capping the jar, 1 tablespoon of powdered sugar is poured through the screen lid onto the bees. The jar is gently rolled without spilling any of the sugar, then left to sit for a several minutes.
Gently shake the sugar and mites from the jar. Brief shaking will recover about 70% of the mites and longer shaking will recover 90%. The sugar and mites can be sieved to make detection easier.
Capped brood inspection (pupae pull)
The inspection of older capped brood (pupae) requires the uncapping of about 100 cells per hive and inspecting them for mites. Older capped brood can be distinguished by the darker color of the capping. A drone brood should be used since Varroa prefer to infest them rather than worker broods. A worker brood can be used if a drone brood is unavailable. The brood can be uncapped individually with tweezers or forceps; a hive tool can be used to uncap several cells at once. Try to inspect brood from different parts of the hive. If 10% or more of the inspected cells contain Varroa, treatment is recommended.
Bottom sticky traps
This method involves placing sticky traps onto hive bottom boards to collect mites which fall from the colony. Before inserting the sticky trap, the bottom board should be cleared of propolis and burr comb. A sheet of sticky material, set in a frame, is placed within the hive on the bottom board. Mites drop from above onto the sticky material where they are trapped and can be counted later by the beekeeper. This method is less time consuming and does not harm the bees or require disturbing the brood cluster. It does require the purchase or construction of the trap and a return trip to the apiary. Traps should be left in place for 3 days. Because mites are collected from the entire colony, colony size must be taken into consideration when interpreting results. Treatment is recommended if 25 or more mites are collected in 24 hours from a medium-sized colony during mid-August to mid-September.
The above photo shows a bee coated with oxalic acid crystals, an organic acid that is found naturally in honey. This method is extremely effective, very quick and inexpensive. For Varroa control, the oxalic acid is used in a very weak solution (3.2%).
The photo below shows a hive being treated with common powdered sugar, another inexpensive, non-toxic way to control the mites.
(photo credits: varroa mites on pupae by Waugsberg [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]; honeybee coated with oxalic crystals by Chamblis [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)