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Varroa Destructor (Varroa Mite)

Varroa is an external parasitic mite that attaches to and feeds on the honey bee. The mite is a vector for at least five debilitating bee viruses, including Deformed Wing Virus (DWV). A bee with DWV cannot leave the hive to forage or defend. A significant infestation can lead to the death of the bee colony, typically between late autumn and early spring. The adult female mite is reddish-brown, 1-1.8mm long, 1.5-2mm wide and with a flat, button shape and 8 legs and resembles a tick. The male is white.

Reproduction

Varroa mites reproduce on a 10 day cycle.  The female enters a brood cell and submerges herself in brood food and lays eggs on the larvae just after the cell is capped.  Five to six eggs are  deposited while the adult female mite feeds on the bee brood.  The first egg is unfertilized and develops into a male mite, the remainder are fertilized and develop into female mites.   After the eggs hatch, the young mites feed on the developing pupa. The male mite mates with its female siblings and dies.  The original female mite and her mated female offspring leave the cell when the young bee emerges, attached to their bee host and spread to other bees and larvae or enter another cell.  Drone cells are preferred as the extra 3 days for drone development allows additional time for reproduction.

Feeding and Common Positioning

Adult mites suck on the fat body (hemolymph) of adult bees and larvae, leaving the bee in a weakened condition.  Most mites feed on the underside of the abdomen; those seen on the backs of bees are attempting to transfer to another bee, preferably a nurse bee.

How long can a mite survive without a bee or larvae host?

Varroa mites can survive for a few days without a host and have been seen to climb back up into the brood chamber if they have simply been dislodged - for this reason some beekeepers recommend a layer of petroleum jelly or similar on the varroa board. When a solid floor is used instead of a mesh floor - mite return to the hive is more likely.

Mites have been filmed climbing from a flower to a bee.

Varroosis and Deformed Wing Virus (DWV)

Open wounds become sites for disease and virus infection. Infected adult worker bees have a shorter life-span.  Newly emerged infected bees are often smaller in size and may have crumpled or disjointed wings and shortened abdomens.

Varroosis = varroa infestation + virus infection

Other symptoms of mite infestation are:

  • Rapid colony decline or collapse

  • Reduced adult bee population

  • Evacuation of the hive by crawling (unable to fly) bees

  • Spotty brood with brood partly removed

  • Queen supersedure

  • Abnormal brood with symptoms similar to European Foul Brood

 

Varroa mites reach other bee colonies through the drifting or robbing behaviour of bees.

Action is required to prevent eventual colony death.

Monitoring – Detection and Mite Count

Varroa detection should be a routine part of your regular inspection schedule.

 

Varroa floor debris (natural mite drop)

Four times a year, insert the removable slide under the open mesh floor and count the number of fallen mites / number of days inserted x 100 to estimate mite numbers in the colony.   

Treatments

An integrated pest management strategy is recommended.

Open Mesh Floor

10% of mites are lost through the use of an OMF as they cannot climb back up to the brood box. Removable slide drawer is excellent for monitoring mite drop and should be left open at other times.

Alcohol Wash

This method may not appeal to most beekeepers. Equipment needed:

  • rubbing alcohol

  • small plastic tote, preferably with rounded inside corners to assist with scooping up bees

  • small measuring cup

  • tea strainer

  • glass jar for straining/storing alcohol

Comb Trapping / Drone Brood Removal

This method takes advantage of the mite's preference for drone larvae, which have an additional 3 days in the cell allowing more reproductive cycles to complete for the mite. When drone cells are being produced, insert a shallow (super) frame next to the outer frame closest to the brood. Workers will build drone comb below the bottom bar.

 

When the cells are sealed, remove shallow frame, cut off and destroy the brood comb, which will have a concentration of mites. Up to 50% efficacy.

Icing Sugar Dusting

Dust icing sugar over the bees causing them to groom each other – this is a non-toxic method and can be carried out in the active season and is also quick.

 

An individual treatment is only 10% effective, but can be done at weekly intervals.

Queen Comb Trapping

Efficiency of 95% has been claimed for this non-chemical approach. The beekeeper must be able to find the queen and put her in a cage without harming her inadvertently. The window of opportunity is small - the procedure should be started after eggs have been laid which develop into forager bees for the main honey crop (end of April / beginning of May) and before eggs are laid for winter bees.

For a period, brood is restricted to trap combs. This requires a special cage to envelop the  brood comb and is covered in queen excluder material, trapping the queen on one comb, but allowing her to move between two sides of the frame. Click for a fuller explanation of the process.

Apiguard (Soft chemical)

Low release Thymol based gel placed in the hive that can be used any time except during a honey flow. Limited to 2 applications of 1 tray at 2 week intervals i.e. 2 trays to treat one hive. Treatment lasts 4 – 6 weeks and bees spread it as they move around the hive. Apply in spring or summer when temperatures are above 15 degrees C. Close mesh floors and vents in crown-board during treatment. 85-95% effective if correctly used. Supers must be off as Apiguard has a strong scent. It is recommended that feeders are not used during treatment. Some users report that the queen stops laying during the treatment period. Works by breaking down the Varroa mite's protein molecules. No resistance results.

Apistan (Hard chemical)

Effective on adult bees and spread by contact – plastic strips are inserted between frames when there is little or no brood (late Oct to Nov) and kept in place for 6 weeks. Remove after 6 weeks to reduce resistance in mites. Active chemical is Fluvinate. At least 95% effective if correctly applied. Supers must be off. Some users report that smaller drones, with less sperm may result. Apistan is fat soluable and is easily absorbed into wax.

Apilife Var (Soft chemical)

An organic treatment using 74% Thymol oils of camphor, eucalyptus and menthol.

 

Supers must be off. Some users report losses of bees and open brood.

 

Three treatments of 7 - 10 days duration are required. No resistance results.

Apivar and Apitraz (Hard chemical) 

Active ingredient is Amitraz, a synthetic pesticide that kills mites and ticks. Amitraz changes mite behaviour. Very efficient and does not accumulate in the wax. Strips must be left in for 6 - 10 weeks, with the supers off. Some amount of resistance can build up and it is recommended that the spring treatment is Oxalic Acid based. Some report reduced queen performanace and drones have shown reduced sperm viability in some instances.

Oxalic Acid

n organic acid found in several plants, including Rhubarb leaves. OA can be applied using a Trickle or Vapourisation method and is also effective against Acarine and will eliminate Wax Moths from the hive.

Trickle method: mix OA powder with a sugar solution and trickle between the brood frames, during winter when the colony is brood-less, onto the bees using a plastic syringe, the bees response will result in distribution. Use 5ml of Api-Bioxal per seam of bees. Up to 94.5% effective. May harm open brood and does not kill mites in sealed cells. Works by damaging the claspers on the proboscis of the mite so they cannot suck.

Rhubarb Leaves

Rhubarb leaves contain Oxalic Acid. Leaves can be crumpled a little to release moisture and placed directly on the top bars of the brood box and below the super.

 

Worker bees chew through the leaves and spread the oxalic acid around the hive. In addition, the leaves do not dry out and continue to release oxalic acid in gaseous form.

Vapourisation method

Place Oxalic Acid crystals on a heating tray into the bottom of the sealed hive. Vapour is released and circulates in the hive for a proscribed length of time.

 

Careful monitoring of dose and time is required, as well as a suitable mask to prevent inhalation of fumes.

Fogger method

If you need a varroa mite treatment for your multiple bee colonies, then a Varomorus Propane Insect Fogger is a good option.

An average of a hundred bee colonies can be treated in 25 minutes. A puff of vapour from this equipment will treat one bee colony. Effectiveness ranges from 95% to 99.9% after treatment.

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