Pest Management Strategy - Elimination of American Foulbrood Without the Use of Drugs
A Practical Manual for Beekeepers, by Mark Goodwin and Cliff Van Eaton

6 Spread of AFB

To be able to control the spread of AFB, it is important to understand how the disease is transferred between colonies. A number of possible means of spread have been suggested by beekeepers. These include:

extracted honey supers
transfer of brood frames
empty comb
other contaminated hive parts
robbing
drift
queens and package bees
swarms
foundation
beekeeping equipment (gloves, hive tools, honey extractors, etc.)
flowers and the ground in front of hives
feeding contaminated honey and pollen

In discussing the possible ways that colonies can become infected with AFB, it is important to remember that AFB infections do not occur because a single bacterial spore finds its way into a colony. In controlled experiments it has been shown that several million spores need to be fed to a honey bee colony in either sugar syrup or honey to infect one or more larvae.¹⁵

Obviously, large numbers of spores are usually required to initiate an AFB infection. So probably the best way to assess the importance of the various means of spread is to compare their relative ability to transfer large numbers of spores from one colony to another.

By far the most common causes of AFB spread are beekeeping management practices. The most significant of these are:

the movement of extracted honey supers between hives (often a year later), and
transferring brood frames between colonies

Most of the other causes beekeepers normally blame for the spread of AFB turn out not to be as significant or widespread as beekeeping management practices. Robbing can be an important cause, but is usually a result of inadequate levels of beehive inspection (ie., poor beekeeping management practice). Robbing of feral colonies is not a major source of AFB, at least in most situations and areas of the country.

Drift of bees from AFB hives to healthy hives is also not an important factor. Such things as hive tools, smokers and gloves, as well as the soil in front of hives, foundation, and queen bees, are of little consequence as sources of spread of the disease.

6.1 Extracted Honey Supers

Honey supers are the pieces of equipment that are most frequently swapped between hives. The colonies they come from are often not checked (or not checked thoroughly) for AFB when the honey is removed, especially if robbing is a threat. Care is generally not taken to ensure that the extracted (wet) frames are returned to either a) the box they were in, b) the hive the box was on, or c) the apiary where the honey was produced.

A trial was conducted to determine the importance of wet honey supers in the spread of AFB.¹⁶ Twenty supers of honey were collected from colonies with light AFB infections. Most of the supers came from colonies with less than 5 larvae/pupae exhibiting clinical AFB symptoms. These infections are of the type a beekeeper might miss.

The honey was extracted and the supers and frames put onto 20 AFB-free colonies the next spring. The 20 colonies were placed in an apiary with a further 20 colonies that did not receive such supers.

There were no obvious symptoms of robbing when the supers were placed on the colonies. However, samples of bees taken from each hive two days later all tested positive for AFB spores, including those from the colonies that did not receive AFB supers. Four (20%) of the colonies that didn't receive wet supers, and 9 (45%) of the colonies given infected honey supers, developed AFB infections.

Extracted honey supers are probably the major factor in the spread of AFB in New Zealand. Because this is the case, the proper inspection of hives when the honey is taken off, and the effective control of the subsequent movement of those supers within a beekeeping outfit, are essential parts of any programme to control the spread of the disease.

6.2 Transferring Frames of Brood

Beekeepers often transfer frames of brood from one hive to another, either to boost the populations of small hives, or to reduce the strength of hives in an effort to prevent swarming.

Unfortunately, transferring a frame of brood from an AFB-infected hive to a clean colony is a very effective way of spreading AFB. To put it in context, a colony may need to be fed 5 million AFB spores to become infected. However, one diseased larvae or pupae can contain 2,500 million spores, or 500 times the number required to initiate an infection.

If a frame of brood has 50 or more diseased larvae/pupae, and the house bees come into contact with large numbers of spores in removing the dead brood, the chances of sufficient spores being fed to a healthy larva to set off a new infection are greatly enhanced.

Next to extracted honey supers, the transferring of brood frames is probably the most common way AFB is spread between colonies in New Zealand.

6.3 Empty Comb

Empty, drawn comb is less likely to carry large numbers of AFB spores than comb containing brood or wet comb from honey supers. The only exception to this is frames from the brood chamber of an AFB infected colony that has died, either from the disease or other reasons. These frames can contain AFB scale (the dried remains of diseased larvae or pupae), and are therefore very likely to transfer the disease to a new colony.

6.4 Other Hive Parts

The importance of other hive parts (eg., empty supers, floor boards, hive mats, division boards, and lids) in the spread of AFB is unknown. However, they are likely to carry less spores than either frames of brood or wet honey frames, and so are probably less important in the spread of AFB.

6.5 Robbing

Robbing is generally defined as foraging honey bees from one hive taking honey from another. Since AFB can reduce the adult bee population of colonies to the point that they become susceptible to robbing, this is one of the natural mechanisms for the spread of AFB between colonies.

Researchers at Ruakura were presented with a graphic case of the spread of AFB through robbing several years ago. A truckload of 80 colonies was returned from kiwifruit pollination and put into a dump site. Twenty of the colonies were moved to another apiary site within a couple of days. A further 20 were removed from the dump site to another apiary two weeks later. Of the 40 colonies that remained at the dump site, 88% had to be destroyed over the following 3 months because they had contracted AFB. However, none of the first 20 hives that were removed developed the disease, while 80% of the 20 colonies that were removed two weeks later had to be destroyed.

At some time between the moving of the first group of hives and the second group two weeks later, 85% of the colonies at the dumpsite contracted AFB. The only reasonable explanation was that a large number of the colonies left on the site had robbed an infected hive or a supply of infected honey. Unfortunately, the source of the infection was never found.

This example illustrates the dramatic disease-producing effects that can result from robbing. There are also, however, a number of anecdotal reports of diseased colonies being robbed out without the remainder of the colonies in the apiary becoming infected. Possible reasons for this lack of disease spread include spore levels in the other colonies never getting high enough to create an infection, or the robbing bees coming from a different apiary.

6.5.1 Causes of Robbing

Robbing is often caused by the action or inaction of a beekeeper, although not necessarily the owner of the bees doing the robbing. The beekeeper concerned may not have disposed of honey or other material attractive to bees in an appropriate manner, may have allowed a colony to die out, or may not have protected hives well enough from stock so that the colony gets knocked over and becomes exposed to foraging bees.

In some cases, it may be a feral colony that is robbed, although it can only be guessed how frequently this occurs. A New Zealand survey indicates that although feral colonies occasionally have infections of AFB, generally they do not.¹⁷

6.6 Drift

Bees drift from one colony to another, particularly when the hives are set out in straight lines, when they are all painted the same colour, or when the entrances of hives are placed close together.

The drifting of bees between colonies is often mentioned as a major factor in the spread of AFB. Beekeepers cite examples where if one colony develops AFB, the one next to it will also develop AFB. However, there are many more examples where this does not happen, so coincidence cannot be ruled out.

The hive next to a colony infected with AFB will usually be the next one worked by the beekeeper, and if hive parts are intentionally or inadvertently moved between colonies, they are most likely to end up in the hive next to the one with AFB. It is therefore very difficult to be sure whether a colony developed AFB through drift or from other means.

Trials have been conducted to determine whether bees drifting from colonies with low level AFB infections (less than 50 larvae/pupae exhibiting disease symptoms) are likely to spread AFB.¹⁸ Colonies with low level infections are the kind that a beekeeper is most likely to miss.

A total of 24 pairs of colonies were set up, each pair consisting of one hive with a light AFB infection and one uninfected colony. The colonies in each pair were facing the same way and positioned as close together as possible in order to maximise the level of drift.

When the level of drift between the colonies was checked, it was estimated that the equivalent of 50% of the bees had swapped colonies over a 20 day period. However, this may have been due to a smaller number of bees moving backwards and forwards between hives, rather than a total of 50% of the bees swapping colonies.

The pairs of colonies were together for a total time of 7 years (an average of 103 days for each pair). Only 2 of the non-infected control colonies developed AFB. Both hives developed AFB at the same time that 12 other hives in 2 apiaries involved in another experiment close by also developed AFB. It is possible that the infection in these 2 non-infected colonies could have been due to robbing.

What is obvious, however, is that since only 8% of the control colonies developed AFB, bees drifting from colonies with light AFB infections cannot be considered to be a major factor in the spread of AFB. Whether the same can be said for drift from colonies with heavy infections is unknown.

Even if drift is not an important means of spread of AFB, care should still be taken to reduce it to a minimum, since excessive drifting can affect honey production. Three simple methods are effective in reducing drifting between hives.¹⁹ These are:

The use of irregular or non-repetitive layouts (patterns) of hives, and situating hives so they face different directions
The use of different coloured supers
The use of orientation marks near hives

6.7 Queens and Package Bees

It is theoretically possible for queens to transmit AFB from one colony to another. However, although queens have tested positive for AFB spores, it is unlikely that they would carry enough spores to create an infection. Where spore-carrying queens from AFB infected hives were intentionally placed in uninfected hives, they did not infect the new colonies.²⁰

Package bees are a broodless nucleus colony, comprised of a caged queen and an artificial swarm of bees. Package bees can be shipped long distances, and then installed into hives with empty comb in order to start new honey bee colonies. Although package bees contain no brood, they have been shown to be capable of carrying sufficient spores to produce an AFB infection once the bees begin producing brood in the new hive.²¹

6.8 Swarms

Swarms are a source of AFB. There are numerous reports of swarms developing AFB soon after they were placed in a hive. Swarms are less likely to develop AFB if they are placed on foundation, rather than drawn comb. When foundation is used, the bees must first draw out comb before beginning brood rearing, and they are therefore more likely to purge themselves of the AFB spores they are carrying before beginning to feed larvae again.

6.9 Foundation

At least some of the wax that is melted down for foundation must come from AFB infected colonies, and it is known that cappings wax from AFB hives can contain large numbers of spores (9 million/gm).²² However, most of the spores are removed by the initial melting and later processing of wax into foundation. Tests have been carried out on 8 lines of foundation produced in New Zealand, and no AFB spores were found.²³ Foundation is therefore no more than a very minor source of cross-infection.

6.10 Beekeeping Equipment

Unless a hive tool or the finger of a bee glove is inserted directly into AFB infected cells (eg., to perform the ropiness test), it is unlikely that these pieces of equipment will carry sufficiently large numbers of spores to be a major factor in the spread of AFB.

The honey extractor is also unlikely to be a major factor. Infected honey may be transferred between frames during the extracting process. However, the amount will be insignificant compared to the amount contained on a wet super coming directly from an AFB infected hive.

Nevertheless, beekeepers should still take precautions to ensure that their gloves, hive tools and extractors are not a factor at all, and this is preferable to a total lack of beekeeping equipment hygiene.

6.11 Flowers and Soil

Bees picking up chalkbrood spores from flowers has been suggested as a means of spread of chalkbrood disease, a fungal disease infecting bee brood. However, unlike AFB spores, relatively few chalkbrood spores need to be carried back to a hive to create an infection. Except where bee-collected pollen is applied to flowers in commercial quantities in artificial pollination, honey bees are very unlikely to pick up enough AFB spores from flowers to initiate the disease.

Many beekeepers have heard stories about a new hive developing AFB after it has been placed in the same position in an apiary as a previously removed AFB hive. The new infection is usually blamed on the contamination of soil in front of the hive with AFB spores.

Research has shown, however, that soil is unlikely to be a source of AFB infection. One study looked for spores in the front of AFB infected colonies, but was unable to find any at all. The study demonstrated that rain water quickly leached the spores from the soil surface.²⁴

6.12 Honey and Pollen Feeding

Honey is a well-recognised source of AFB infection. Retail packages of honey have been tested in a number of countries. Detectable levels of spores were found in 12 to 83% of samples tested.²⁵ A survey of 32 retail lines of New Zealand honey found spores in 25%.²⁶

Honey taken directly from an AFB hive was found to contain the highest concentration of spores of any hive product (24.3 million spores/g). Pollen pellets produced by AFB hives were also shown to contain large numbers of spores (4.5 million spores/g), although not as high a concentration as either honey or cappings wax.²⁷

If fed to a colony, both honey and pollen taken from a diseased hive can be a major source of AFB infection.

Pest Management Strategy - Elimination of American Foulbrood Without the Use of Drugs
A Practical Manual for Beekeepers, by Mark Goodwin and Cliff Van Eaton

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