Also known as screened floors, or varroa floors.

The use of OMFs was first properly researched by Dr Helmut Horn in his November 1987 publication in A.D.I.Z.Bee Craft in July 1990. Click here for a summary of this paper from APIS August 1999. Whilst some beekeepers still do not use OMFs, most modern beekeepers consider them an essential part of hive construction. Supported by further significant research, most Regional Bee Inspectors now advocate their exclusive use. They can be used in both National and WBC hives. "Observations on the Overwintering of Honeybee Colonies in Hives with Open and Solid Floorboards" which was subsequently translated from the original German by A. E. McArthur for the British journal

They allow better ventilation of the hive in summer, thus allowing smaller hive openings for the bees to defend, therefore reducing robbing and encouraging better tempered bees. They reduce the formation of condensation inside the hive in winter, thus reducing disease and pests. They are essential for monitoring natural mite mortality (daily mite drop) as part of Integrated Pest Management. They also allow the beekeeper to assess the efficacy of treatments for varroa, and they facilitate easy observation of the hive for health, for example looking at debris for signs of disease and assessing pollen on the solid floor slider. Their use also ensures there is no permanent debris on the hive floor thus discouraging wax moth infestation.

An OMF consists of either a fixed or removal (for ease of cleaning) floor constructed from varroa mesh. This is usually a galvanized, stainless or epoxy coated mesh, of 8 wires to the inch gauge (3mm), which is large enough for the mites to drop through and out of the hive, but small enough to ensure the security of the hive. Research shows that a significant % of live mites fall off the bees during the phoretic or transport stage of the mite life cycle, and Open Mesh Floors prevent these mites from climbing back into the colony. The complete floor includes a removable solid base or slider below the mesh, which can be slid in place when the beekeeper wants to monitor for mite drop, or to seal the hive such as when treating with thymol. A sticky backed plastic insert to trap the live mites (sticky side up) placed over the solid floor slider can allow accurate monitoring of natural mite mortality numbers in the hive during the season. The solid floor slider should be inserted for short periods only, to calculate the daily drop, and they should be left out over winter. The natural winter threat to bees is dampness, not cold, and they over-winter better with added the added ventilation of OMFs. However food consumption appears to increase over winter by some 10-15% due to thermoregulatory activity.

Monitoring daily mite drop

The slider should be inserted and left in place for a few days. If it is covered with sticky backed plastic under which is a drawn 3 inch grid, it makes the total number of mites easier to calculate. Research suggests that colony collapse is very likely before the end of the season if average daily mite drop for a typical colony exceeds the following numbers, and immediate action would need to be taken. The vigilant beekeeper would have been monitoring regularly and taken action before this situation arose:

French research has suggested that despite a mesh floor, some live mites may be able to climb back into the hive. This has led to various theories regarding the gap required below the floor and the consensus seems to be the floor should be 12 inches off the ground. Various more sophisticated varroa floors have been developed in Europe and Canada, but no evidence for their superiority exists over simple mesh floors.

Lancaster beekeepers have produced a 30 minute DVD (December 2006) demonstrating the preparation and practical use of Oxalic acid. This e-mail address is being protected from spambots. You need JavaScript enabled to view it if you would like to borrow a copy