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Chapter 6 Bee pests and diseases
A.
Bee mites
B.
Hornets
C.
Microbial diseases
In many parts of Asia, the success or failure of beekeeping with the common honeybee A. mellifera depends largely on the ability of the beekeeper to take suitable measures to control diseases and pests affecting bees. Apis mellifera being the only introduced honeybee species in a continent possessing several native species of Apis, colonies of this bee are subject to infestation and attack by all the natural enemies of the native bees in addition to their own. Perhaps the most important are bee mites, hornets and microbial diseases, although passing mention must also be made here of the danger to bees from wax moths on the one hand, and from birds and mammals on the other.
Two species of bee mites are of economic importance: Varroa jacobsoni and Tropilaelaps clareae. Whereas the natural range of distribution of the Varroa mite covers the entire Asian continent, T. clareae is found only in or near the distribution range of its native host, Apis dorsata, in tropical and sub-tropical Asia.
The mode of parasitism of both mites is roughly similar. Fecunded adult female mites enter bee-brood cells before the latter are capped, and deposit eggs which hatch rapidly. The developing mites use their feeding apparatus to pierce the skin of the developing brood, and feed on their haemolymph ("blood"), which flows from the open wounds. The development cycles of the mites coincide with those of the host bees. When the hosts emerge from their cells, the mites also emerge, mate, and seek for other bee-brood cells. Some mated adult female mites attach themselves to the bodies of workers or drones; drifting of bees thus infested accounts for the spread of the mites to other colonies. Infestation is also spread by mites attached to foragers dropping off the bees when they are visiting flowers, and later clinging to another bee which subsequently visits the same flower.
(1) Symptoms and Diagnostic Procedure
For some beekeepers, the appearance of adult bees with deformed wings, shortened abdomens or missing legs is the first noticeable symptom of mite infestation. In fact, such symptoms indicate a late stage of severe infestation, which may lead to the loss of the forthcoming honey crop or even the entire colony.
While it is of course possible for beekeepers to make random inspection of brood cells for mite infestation and to assess its level, a more practical approach is to study the pattern of brood combs, which can give an early warning. A scattered pattern of sealed and unsealed brood cells normally taken as a sign of poor egg-laying queens, is often an indication of mite infestation. When this is the case, the housecleaning bees have eliminated the parasitized brood, leaving an irregular pattern of capped and uncapped cells.
(2) Control
There is no one best way to control bee mites. Many beekeepers resort to chemotherapeutic measures, although this approach requires that the risk of contaminating the honey and other hive products be restricted to a minimum.
Hive fumigation can control not only V. jacobsoni and T. clareae but also the tracheal mite Acarapis woodi (see page 53). The simplest fumigation method involves placing fumigating strips into the hives, setting them alight, and letting them smoulder. Commercially available strips include Folbex and Folbex VA, whose active ingredients are the acaricides chlorobenzilate and bromopropylate respectively; the strips are impregnated with other compounds that make them smoulder when lit. Beekeepers can make similar strips themselves, by soaking 2.5 x 9 cm strips of filtre paper in a saturated solution of sodium or potassium nitrate also containing an acaricide relatively non-toxic to bees (e.g. chlorobenzilate, bromopropylate, dicofol, tedion, amitraz, etc,) and allowing them to dry. Bromopropylate and amitraz are especially popular for this use.
Fumigation is best administered when all the bees are inside the hive, usually in late afternoon, and when the temperature is not below 10° C. All cracks and openings of the hive must be sealed with masking tape, rags or wet cloths. The strip is attached securely to an empty frame or placed on a sheet of flat metal that can be introduced into the hive through the entrance. The beekeeper lights the strip, and when it is smouldering well he introduces it into the hive, where the bees will he fumigated with acaricidal smoke for about 30 minutes.
Since the fumigant cannot penetrate the cappings of the brood cells, it can reach only the mites on the bees present in the hive. It should therefore be applied for a total of three or four times, at intervals of four days, in order to reach the mites which at the time of the first fumigation were still sealed within the capped cells.
Some beekeepers prefer spraying to fumigation. The acaricide is diluted with water and sprayed over the bees, the brood combs and the hive walls; three or four applications at four-day intervals usually yield good results.
In tropical Asia, where T. clareae is often a more serious danger than V. jacobsoni, beekeepers who do not wish to use acaricides turn to a biological control method which involves brood management. Since adults of T. clareae can survive without bee brood as food for only two days, most of the mite population of the hive will starve to death if deprived of the brood for three days. Taking advantage of this fact, the beekeeper confines the queen in a small egg-laying area and removes the brood combs to an empty hive box or forms new colonies with them. Some beekeepers combine this approach with a single fumigation.
In many parts of Asia, hornets (genus Vespa) are serious pests of honeybees, whose colonies can be significantly weakened by hornet predation. If the insects' nests can be found, destroying them is the best way to control hornets, but since they have a long flight range, finding their nests is usually difficult.
Where hornet predation is not too intense, reducing the hive entrance and making efforts to catch hornets that come to forage in the vicinity can often prevent serious destruction. Where predation is particularly intense, however, the surest way of dealing with the problem is to move the trees out of the area completely until the hornet population is much reduced.
(1) American Foulbrood (AFB)
AFB is the most serious disease affecting bee brood. It is caused by a bacterium, Bacillus larvae, whose spores can remain in the hive and on contaminated equipment for long periods of time. Under favourable conditions (e.g. when the colony is under stress From other factors), the spores germinate sod infect the older honeybee larvae.
(a) Symptoms and Diagnostic Procedure
The presence in the comb of dead older Larvae or pupae lying flat in their cells is the first noticeable symptom of AFB. The cappings of the cells concerned are discoloured, slightly sunken, and punctured. The dead brood's colour is initially dull white, eventually turning dark brown.
When bee brood is found dead with these symptoms, a rapid way of determining whether the death was caused by AFB is the "stretch test". A small stick, about the size of a match-stick, Is inserted into the mass of the decayed larva or pupa, and gently withdrawn. If the dead brood adheres to the tip of the stick and can he stretched as much as 2.5 cm before it breaks and returns to its original position, this "ropiness" typifies the presence of AFB. The stretch test cannot be used when the dead brood has dried, hut dead brood that Is brittle and adheres closely to the lower cell wall suggets the presence of AFB.
(b) Control
Since only by burning can the beekeeper be certain that the spores of the pathogen are destroyed, the trees in colonies infected with American foulbrood should be killed and burned, along with the brood chambers, supers, frames and honey of the colonies.
Some beekeepers use antibiotics to control or prevent AFB, generally preferring Terramycin (oxytetracycline) or sodium sulfathiazole, fed to the colonies mixed with powdered sugar.
(2) European Foulbrood (EFB)
Generally speaking, EFB is not as serious a threat to honeybee colonies as AFB. The pathogenic bacterium is Streptococcus pluton, which does not form spores and cannot remain dormant for long periods. The disease is often found in colonies that are under stress conditions.
(a) Symptoms and Diagnostic Procedure
EFB affects only young honeybee larvae, about 4-5 days old, usually at the coiled stage. When the stretch test is used, the dead larvae cannot be pulled out in a thread, nor do they adhere to the cell walls. Their texture is rubbery rather than brittle, as in AFB.
(b) Control
Improving the colony's strength by requeening, supplemental feeding, and adding frames of emerging brood is often a sufficient control measure. In severe attacks, the beekeeper may wish to feed the colonies with an antibiotic such as Terramycin (oxytetracycline) or sodium sulfathiazole. The recommended dosages are 1 part Terramycin TM 50 (oxytetracycline) mixed with 20 parts of powdered sugar, or 0.2-0.6 g of the active ingredient of sodium sulfathiazole dissolved in 4 litres of sugar syrup.
(3) Sacbrood Disease
Sacbrood is the most common viral disease of the common honeybee; it is found in association with colonies under stress. The spread of the disease in the apiary can cause severe losses.
(a) Symptoms and Diagnostic Procedure
The most evident symptom of sacbrood is the presence in the comb of dead larvae which have failed to pupate. Affected larvae are sac-like in apearance, with relatively tough skins and a watery interior; their colour changes from white to pale yellow and finally turns dark brown and black, the first parts to blacken being the head and thorax.
(b) Control
No chemotherapeutic method of control of sacbrood disease is known; infected colonies can often recover by themselves. Requeening colonies, coupled with the manipulations necessary for increasing colony strength (e.g. adding frames of emerging brood and supplemental feeding) are often adequate as control measures.
(4) Nosema Disease
Nosema is a disease of adult trees, caused by the protozoon Nosema apis. Like most other bee diseases. it usually effects colonies under stress (poor laying queens, unhygienic hive conditions, inclement weather, etc.). Severe attacks can significantly weaken colonies.
(a) Symptoms and Diagnostic Procedure
There is no reliable method of identifying the disease by the naked eye. The laboratory method involves crushing the abdomens of suspected bees and examining the fluid obtained through a field microscope. The large bacilliform spores of Nosema apis, if present, can be easily identified by their fluorescent edges.
(b) Control
The disease is controlled by requeening, usually combined with feeding the bees on fumagillin (25 mg to 1 litre of sugar syrup). The drug must not be fed to the bees when there is danger of contaminating hive products.