Euthanasia of honey-bee colonies: Proposal of a standard method

Abstract

Honey bees are most often kept for production purposes. Sanitary, regulatory, or zootechnical circumstances may lead the beekeeper or the veterinarian to dispose of a honey-bee colony. Unfortunately and at present, no standard method of euthanasia exists, leaving the door open to many more or less acceptable practices. Based on a short survey of current practices in 8 countries, we list and rank these methods. Although imperfect, the sulfur dioxide technique appears to be the most efficient. We suggest that it should become the reference method to be taught and incorporated into veterinary and regulatory guidelines.

In Europe and North America, veterinarians are increasingly involved in honey-bee health. As part of their practice, they may be required to recommend or carry out euthanasia of honey-bee colonies. There are many reasons for euthanasia: i) for sanitary reasons when a colony of bees is sick or moribund and no cure can be envisioned; ii) for regulatory reasons in the fight against certain contagious diseases (e.g., American foulbrood) or if the colony is affected by hive pests (e.g., small hive beatle); and iii) for zootechnical reasons such as queenless colonies with working bees that lay eggs (laying workers), colonies of low economic value, aggressive honey bees that are dangerous, or honey bees with undesirable genetic traits (e.g., Africanized bees). Unfortunately, the veterinarian and the beekeeper are at a loss when faced with the euthanasia of a bee colony because there is no euthanasia medication authorized in any country, nor is there a reference method. Moreover, this procedure is not generally taught to beekeepers or veterinarians. The aim of our paper is to give a short overview of methods usually used in the field and to compare them and propose a simple and reliable standard technique.

The OIE (World Organisation for Animal Health) defines euthanasia of an animal as “an act of inducing death by a method causing rapid and irreversible loss of consciousness, with minimal pain and distress to the animal.” It is difficult to clinically assess the pain, distress, and even consciousness of a honey bee. However, scientific publications report how immense the cognitive capacities of bees are (1) and that bees can feel emotions (2–4); although there is some controversy, particularly about pain (5,6).

The honey bee (Apis mellifera) is a social individual that can only survive when it belongs to a colony, which constitutes the animal sensu stricto. In addition to obvious sanitary reasons (risk of dispersal of pathogens) euthanasia, when it takes place, must therefore be exhaustive by killing all the individuals making up a colony. In other words, the qualities expected of a bee colony euthanasia are the speed of the killing, the irreversibility of this action, and the care of all the individuals making up the superorganism. This problem is similar to that of the mass culling of certain species such as aquatic invertebrates, but for honey bees there are, to our knowledge, no studies relating to their killing.

Furthermore, the concept of welfare for this species is very rarely mentioned. However, the honey bee is a domestic animal, bred and used for its production (pollination, animal products). Garrido and Nanetti (7) have recently proposed that, although welfare currently applies only to vertebrates, it would be wise to include the honey bee. Beyond this concept, euthanasia and the way in which it is justified and carried out, when necessary, must incorporate respect for the animal, ethics, and veterinary deontology. In a recent publication, Kyle and Applegate Jr (8) rightly stated that veterinarians are expected to approach euthanasia of honey-bee colonies as professionally and ethically as for any other species. However, Mutinelli (9) raises the difficulties inherent in this procedure from the point of view of the safety of operators, the regulations relating to the active substances used, and the need to respect animal welfare.

To understand the techniques traditionally used to euthanize honey-bee colonies in various countries, we surveyed some professionals and scientists during the spring of 2020. We obtained 16 responses from 8 countries (Canada and the USA for North America; France, Belgium, Italy, Spain, Switzerland, and Germany for Europe). Our survey revealed that the methods are very diverse and very rarely follow administrative directives or veterinary guidelines. Among the techniques proposed by the professionals interviewed, North American methods stand out from those used in Europe. In North America, hydrocarbon vapors, which are very toxic to bees, seem to be the most widely used [use of diesel is preferred to petrol because it is less flammable, at a rate of 300 mL to 1 L/hive, according to The Ontario Ministry of Agriculture, Food and Rural Affairs protocol (10)]. Other methods are also described by respondents in the USA and Canada, such as cooling or freezing and finally death of the bees with the replacement of the oxygen in the hive by CO₂ using dry ice (40 L/hive), suffocation (colony reduced to the state of a naked swarm enclosed in a watertight plastic bag and exposed to the sun for several hours) or poisoning with cyanide vapors. Finally, a method often described consists of spraying the bees with rubbing alcohol or soapy water. In Europe, the interviewees all mentioned the method of using sulfur dioxide (SO₂), but never hydrocarbon vapors as euthanasia. The few other techniques described for Europe (but much less frequently) were cooling the bees or immersing them in hot water to drown them (in the case of very small bee populations). Surprisingly, none of the interviewees mentioned the use of neurotoxic insecticides to euthanize bee colonies. In agreement with the answers we obtained, 2 recent reviews on this topic (8,9) cite the same methods, sometimes with more original anecdotes, such as the use of CO₂ from pressurized paintball canisters or spraying the inside hive with diatomaceous earth. Furthermore, according to the interviewees, the choice of the method used is generally not based on specific technical arguments but rather on traditional uses or following website suggestions.

The methods used to euthanize bee colonies are therefore very diverse, each with its advantages and disadvantages. The first part of our study consisted of comparing 10 of these different techniques among the most frequently cited and therefore used, considering criteria that contribute to the expected quality of euthanasia of an animal population such as a honey-bee colony: speed of action, cost, complexity of the method, potential risks for the operator and other possible disadvantages. The main disadvantages identified in the evaluation of this last criterion were the possible escape of bees during handling, the unsuitability of the method for certain colony sizes or materials making up the hive, the impossibility of reusing the hive after euthanasia due to possible toxic residues, or if the method could pose a risk to the environment such as fires. The 10 methods evaluated, all from our survey, were as follows:

• diesel poisoning (“diesel”);

• petrol poisoning (“petroleum”);

• hydrogen cyanide poisoning (“cyanide”);

• SO₂ poisoning (“sulfur dioxide”);

• long-term exposure to negative cold (“freezing”);

• suffocation of the swarm exposed to the sun (“suffocation”);

• exposure to cold and CO₂ (“dry ice”);

• suffocation by topical surfactant solution (“soapy water”);

• intoxication by topical alcohol solution (“alcohol”); and

• suffocation by sustained immersion in water (“drowning”).

Each criterion was scored on a scale of 1 to 3, depending on whether it was good (score of 1), average (score of 2), or poor (score of 3). Thus, for each method, a total score was obtained by adding up the scores for each of the rated criteria (Table 1). For obvious animal welfare reasons, we did not want to reproduce each of these methods to evaluate and compare them objectively according to predefined criteria. We preferred to compare the methods by assigning them scores based on the feedback of the respondents. Based on of this scoring, no method appeared to be truly satisfactory by obtaining an optimal score of 5 (corresponding to a score of 1 for each of the 5 criteria evaluated). The worst method, according to our scoring, is the use of hydrogen cyanide vapor. In contrast, the most acceptable method (with the lowest score) is the use of SO₂. It has the advantage of being quick to act, low in cost, and is relatively simple. The results of this scoring are in line with the conclusions of Mutinelli (9), who considers this method to be the fastest, most effective, and safest to euthanize honey-bee colonies.

Table 1.

Comparison of different methods used for euthanasia of honey-bee colonies, based on our survey in 8 different countries.

Method	Quicknessa	Costb	Complexityc	Risks for the operatord	Other disadvantagese	Scoref
Diesel	1	1	1	2	3α,γ,δ	8
Petrol	1	1	1	3	3α,γ,δ	9
Cyanide	1	2	2	3	2α,γ	10
Sulfur dioxide	1	1	1	2	1α	6
Freezing	3	1	2	1	2α,β	9
Suffocation	3	1	2	1	2α,β	9
Dry ice	2	3	2	1	1α	9
Soapy water	3	1	1	1	2α,β	8
Alcohol	3	1	1	2	2α,β	9
Drowning	2	1	1	1	2α,β	7

^aSpeed of action: (0 to 10 min) = 1; (10 to 30 min) = 2; (30 to 90 min) = 3.

^bCost: low = 1; medium = 2; high = 3.

^cComplexity: easy = 1; moderately easy = 2; complex = 3.

^dRisks for the operator: none = 1; burns or inhalation of toxic substance = 2; > 2 hazards = 3.

^eOther disadvantages: none = 1; 2 disadvantages = 2; > 2 disadvantages = 3; α = risk of honey-bee leakage; β = unsuitability of the method for certain colony sizes or materials making up the hive; γ = impossibility of reusing the hive after euthanasia due to possible toxic residues; δ = the method could pose a risk to the environment such as fires.

^fScore: sum of the scores obtained for each criterion.

Sulfur dioxide is a gas that is released by the combustion of sulfur. It is used in various industrial productions and in wine production. Sulfur dioxide is readily available (most commercial beekeeping establishments sell it in Europe), either in the form of strips of cotton cloth impregnated with sulfur, or in the form of liquefied compressed gas bottles. It is considered toxic to humans by the respiratory route and by contact (corrosive, it can cause burns on the skin and corneas). It should therefore only be used outdoors, by trained personnel and protected from inhalation and splashes. For honey bees, the acute toxicity of SO₂ by inhalation has not been documented, but it has long been used in fumigation to control undesirable insects. The regulations do not specifically allow its use in beekeeping, like all other methods described and used in the field. However, SO₂ is widely used in Europe by sanitary authorities, for example to control the spread of the small hive beetle Aethina tumida (11,12). For serial euthanasia (e.g., whole apiaries), the use of liquefied compressed gas is preferable to sulfur strips.

In the second part of our study, we tested this method of using SO₂ for euthanasia in its current use in beekeeping medicine. As part of our professional practice in the field and under real conditions, a series of 30 bee colony euthanasias were carried out from spring to autumn 2020 using this method. For all these colonies, euthanasia was carried out for sanitary or regulatory reasons (European and/or American foulbrood) because no other alternative was possible and with the systematic agreement of their owner. The method used was as follows:

1. late in the evening or early in the morning when all individuals have returned, the hive is sealed;

2. the operator is equipped with a protective mask;

3. a sulfur strip is lit with a torch, with exposure for several seconds until white smoke is produced (SO₂ is released);

4. the hive door is quickly opened;

5. the burning sulfur wick is placed on the frame heads; and

6. the hive is closed.

The concentration of SO₂ in the hive will gradually increase, soon reaching a lethal threshold for the bees. It is possible to simplify the method by using a liquefied compressed gas canister of SO₂; the gas being injected directly into the hive. As the combustion of the sulfur strip is very exothermic, it is safer to place the strip on a non-flammable support to isolate it from the wooden frame heads. The parameters measured to evaluate the effectiveness of this technique were: the time it took to stop perceiving vital signs (i.e., the disappearance of sounds emitted inside the hive by the colony) and the number of bees surviving when the hive was opened after 15 min. In the first few moments, the swarm makes a piping-like sound in response to the presence of SO₂. It is likely that this is an attempt by the workers to ventilate the hive to get rid of the toxic gas. This sound, which is the only perceptible sign of survival, quickly diminishes and finally disappears completely. In 80% of the euthanasias (24 colonies), the colony did not emit any noise within 2 to 5 min. For the other colonies in the study (6 colonies), no sound was audible after 2 min, between 5 and 7 min and after 7 min for 1, 4, and 1 case, respectively. When the hive was opened 15 min after lighting the sulfur strip, all the bees were dead in most cases (n = 29/30): only 1 colony still had a few living individuals (crawling, unable to fly). This colony was also the one whose last sign of survival took more than 7 min to disappear. Opening the hive showed that this failure was due to poor combustion of the sulfur strip, only half of which was consumed. Based on the weight of the dead bees collected after the euthanasias, and estimating a bee weight of 100 mg, the colonies averaged 3450 (min = 900; max = 9880) individuals at the time of euthanasia. All the euthanized colonies were reared in standard Dadant hives, with 5 to 10 frames.

In this study, the so-called “sulfur dioxide method” was effective in euthanizing colonies of honey bees. The colonies euthanized were relatively small (< 10 000 individuals in all situations tested), which is quite logical given that they were sick. We did not find any relationship between the time of disappearance of the sounds emitted and the size of the bee population. This method, however, can be used with larger colonies without difficulties. In this case it is possible to increase the concentration of SO₂ or to adapt it to the volume of the hive by injecting more compressed gas or by lighting 2 sulfur strips instead of one. The time taken for detectable signs of survival to disappear is in most cases between 2 to 5 min after the release of SO₂ in the confined environment of the hive, confirming the rapidity of action of this method. The survival rate of individuals after 15 min is zero unless the operator makes a mistake. It should be noted that SO₂ has no effect on the brood: the disappearance of the workers will indirectly lead to the death of the brood by rapid cooling, but this does not exclude the birth of a few bees from the sealed brood in the hours that follow. The operator should therefore ensure that the brood frames are destroyed quickly after euthanasia. The gaseous residues of SO₂ in the hive can be easily removed by open ventilation, whereas the physical residues can be removed by normal cleaning and disinfection. In the absence of appropriate drugs and regulatory references, we suggest that this SO₂ method be considered as the standard method for euthanasia of bee colonies. Use of this method should be reserved for trained personnel and subject to strict application of appropriate personal protective measures. Veterinary guidelines for the euthanasia of animals generally forget the honey bee: a modern and respectful approach to bee colonies would require it to be included. CVJ