Sunday, 23 May 2010

Monitoring for Varroa & Nosema

by Nick Delaney

Notes from a talk by Nick Delaney given at our 15th May 2010 meeting:

A. Varroa Destructor
Follow this link for images of varroa  destructor.


  • Varroa is an external parasitic mite, distributed particularly in Asia (origin site), the US, the UK and continental Europe.
  • Varroa infestation provides a context for significant weakening of a colony through infection, disease and shortened lifespan.
  • Varroa control is therefore viewed as a primary aim of much beekeeping activity, although methodologies vary greatly.

Photo © Wikipedia
Adult females are reddish brown in colour, have eight legs and a flattened ovoid body of 1 – 2 mm in diameter. They attack both brood and adult bees, sucking blood and leaving open wounds, which weakens the bees and spreads disease/infection.

Adult males are about half the size of females and yellowish in colour. They only attack sealed brood.
Mites have a preference for drone brood, probably because the pupation period is longer.

As noted above, the feeding habits of the mite results in parasitic weakening of the host and, most importantly, cross-contamination of disease. The major disease indicators of varroa infestation include deformed wing virus, acute bee paralysis virus and slow paralysis virus.

Bees can tolerate varroa at low levels and are believed to have co-existed with the mite for decades in Asia, where hive mite populations do not increase exponentially throughout the season as is the case in Europe.
However, once a critical mite population threshold is reached hives will be affected by a combination of:

  • Decreased adult bee weight
  • Decreased adult bee lifespan
  • Virus spread
  • Deformed wings
  • Reduction in drone numbers
  • Increase in drone infertility

Photograph © Wikipedia
It is probable that a fatal mite threshold is reached in European (mellifera) honeybees because the pupation period of the European honeybee is two to three days longer than that of the Asian (cerana) honeybee. In Asia, therefore varroa mites, which breed in capped brood, only have time to produce one offspring. In Europe between 5 and 6 offspring are produced on average.

It is this hyperbolic increase in mite population throughout the brood season in
Europe which creates the critical infestation threshold and typically overcomes
a colony during the cold season due to a fatal weakening of the winter bees.

Due to the destructive nature of varroa infestation it is essential to monitor mite population throughout the season and take action if levels breach a safe threshold.

The standard monitoring methodology is to count mites falling to the floor of a hive using a sticky board, preferably in conjunction with a mesh varroa floor.

The sticky board is left on the hive floor, ideally for seven days, and the number of varroa stuck to the board is then divided by the number of days the board has been in place.

This calculation gives an average daily mite count which is then multiplied by a season-dependant constant to provide an estimate of mite numbers in the hive. In the UK, typical seasonal multipliers are as follows:

  • May to August 30 times average daily mite drop
  • September to October 100 times average daily mite drop
  • Rest of year 400 times average daily mite drop

Thus, if one counts 28 mites on a sticky board left in place for 7 days during May, this gives an average daily drop rate of 4 which, using the May to August multiplier estimates the mite population to be 120. The same drop rate in March would suggest a mite population of 1,600.

The National Bee Unit (NBU) recommends mite control intervention when the estimate infestation level based on such a calculation exceeds 1,000 mites (although the old MAFF guidance was 2,500 mites, so some leeway).

There is little disagreement about any of the above discussion. Where practices begin to diverge is in the treatment used to bring mite levels down and/or maintain them below a critical level. Possible treatments fall into 4 broad categories:

1. Hard chemicals
Varroa has traditionally been treated with Apistan (fluvalinate) or Bayvarol (flumethrin) which was initially successful in radically reducing mite populations.

However, eventually the blanket application of these chemicals meant that mites were effectively selected for resistance and these treatments are no longer effective.

The use of chemicals is also likely to contaminate honey.

Whilst new chemical treatments are being developed and, notably in China and the US, illegal chemicals are widely used inevitably such practices will simply create further resistant strains of mites.

Current guidance even in forward thinking traditional environments such as the NBU is therefore to avoid hard chemical treatments.

2. Soft chemicals
There have been attempts to develop chemical treatments which are not systemic and which will not therefore build up resistance. A key group of such treatments are naturally occurring acids such as formic (fumigant) and oxylic (liquid) acid.

Whilst effective in controlling varroa, dosage is extremely difficult to control and both acids are harmful to bees in the wrong dose.

Even at the right dose, the acids appear to shorten bees’ lifespan and whilst a colony can cope with this during the summer, a shortening of the lifespan of winter bees can cause colony collapse.

3. Natural
(a) Powder dusting
Usually powdered sugar, dusted over bees and combs, which results in a 30% drop of phoretic mites in first hour after treatment. Also thought to stimulate grooming which can improve mite fall.

Whilst this simple mechanical treatment undoubtedly reduces the number of mites in the short term, the 30% efficacy rate means that frequent and repeated treatments would be necessary to control a hive suffering true
hyperbolic mite population growth.

This treatment also does not attack the breeding and juvenile mites still in capped brood at the time of treatment.

This treatment is probably therefore only suitable to maintain mite populations in colonies which already display signs of natural mite resistance.

(b) Essential oils/plant extracts
There are a number of products based on essential oils and plant extracts, predominantly thymol, which have been used with varying degrees of reported success.

Whilst a natural extract, bees dislike the smell of thymol and may be disturbed by its use. It will also taint honey if used before harvesting.

For an essential oil recipe and methodology visit this link.

(c) Integrated Pest Management (IPM)
IPM is an attempt to bring together various methods of pest control that, in combination, contain mite levels more effectively than any single treatment.

The following combination of activities would be in line with a generally ‘natural’ approach, and should be successful without the need to resort to chemicals.

  • Free cell size
  • Powder dusting
  • Drone trapping in traditional hives or culling in horizontal Top Bar Hives
  • Interruption of brood cycle through hive splits
  • Varroa floors
  • Essential oils

(d) Behavioural breeding
This isn’t really a treatment but it is probably the long-term solution to the varroa problem.

In nature, parasites co-evolve with their hosts. There is no advantage to the varroa in destroying its host colony, quite the contrary, and it is likely therefore that over time it would evolve to moderate it population growth in European bee colonies. Bees also would naturally select for mite-resistant traits.

The blanket use of hard chemicals when varroa first appeared in managed European bee apiaries means that such co-evolution has not yet properly begun. However, some European bee strains, notably Russian, have a natural resistance to varroa built from a propensity to groom aggressively to remove mites from adult bees, and to detect the mites in capped brood and eject the infested pupae from the hive.

Several projects are now underway to breed from these strains and in time it is likely that naturally resistant bees will be made available to commercial beekeepers and hobbyists.

In the meantime we should all monitor our own colonies and, where possible, actively look to split and expand our more robust populations.

B. Nosema
Follow this link for images of Nosema

A common, host specific unicellular parasite of the class microsporidian, which affects all insects. Nosema apis is the species which affects the European honeybee.

Under a microscope the parasite looks like a grain of rice. It develops and multiplies in the cells of the epithelium in the mid-gut of adult bees and is mainly transmitted through the transfer of spores when young bees clean up faecal material on contaminated comb.

Reduces the lifespan of infected bees, increasing winter mortality and causing poor spring build up.
There are no specific symptoms but the disease is linked with dysentery and virus diseases indicated by bees crawling outside the hive.

Infected colonies can appear to recover during summer when bees defecate away from the hive and infected bees die without transmitting the disease.

However, the spores persist on contaminated comb and often trigger a more severe infection the following winter.

Due to the lack of direct symptoms there is no specific monitoring programme but where bees display any of the linked indicators noted above a simple diagnosis test should be performed.

Kill 30 to 35 bees and mash the bodies in a pestle and mortar with a few drops of water. Observe a drop of the resulting soup under a x400 microscope to identify the distinctive rice-shaped spores per the photo above (the NBU will do this test for you).

The only effective treatment for nosema apis is Fumidil © B, an antibiotic fed to the bees through treated syrup.

It is difficult to use correctly, and combs will also need fumigating with acetic acid or replacing using a shook swarm technique as the spores persist in the comb even if reduced in the bee population.

by Nick Delaney
May 2010

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