Ent. exp. & appl. 5 (1962): 249--254. North-Holland Publishing Co., Amsterdam

T H E UPPER LETHAL TEMPERATURES OF H O N E Y B E E S BY

J. B. FREE & YVETTE SPENCER-BOOTH

Rothamsted Experimental Station, Harpenden, Hefts., England

Survival of bees at high temperatures depends on the duration of exposure and relative humidity. At the higher temperatures bees survive short periods only and ,do best at low relative humidities because they can cool themselves by evaporation more. At somewhat lower temperatures bees can survive longer at high relative humidities because desiccation is the limiting factor.

PIRSCH (1923) concluded that honeybees die at 46 ° to 48°C and ALLEN (1959) found that they die after 1 hour at 48 °. This paper investigates the upper lethal temperatures of bees kept singly at different relative humidities.

METHOD

MELLANBY'S (in litt.) method was used. Bees were put singly in wire-mesh cages, 4.5 cm long and 2.0 cm diameter, suspended near the tops of closed 60 ml test tubes for experiments lasting 1 or 2 hours, and at the tops of closed 1,000 ml flasks for those lasting 20 hours. The relative humidity in each vessel was kept at 15%, 50% or 100% by the appropriate mixture (SOLOMON 1951) of water and potassium hydroxide, or water alone. The vessels were immersed in a con- stant-temperature water bath. Unless otherwise stated, 16 bees were tested at each temperature and relative humidity. Equal numbers of bees were tested simultane- ously at the different relative humidities.

Before most experiments, bees were acclimatised to 35 o by keeping them singly in small cages, supplied with sugar syrup, in an incubator for 24 hours. All bees were given this treatment after each experiment, and the number alive after 1 and 24 hours was recorded. Bees were weighed individually immediately before and after each experiment.

EXPERIMENTS AND RESULTS

Effect of 1 hour's exposure (a) Bees accl#natised to 35 °

Bees were acdimatised to 35 ° before being exposed to the three relative humidi- ties at temperatures of 40 ° to 50 ° for 1 hour (Fig. 1). The results are shown in Figure 1, and those for the critical temperatures of 44 to 46 ° are summarised in Table I. Statistical tests showed significant differences between the numbers surviv- ing after exposure at the different humidities, the results being assessed over the range of experimental temperatures. At the end of the exposure, 1 hour later and 24 hours later, survival was lowest among bees exposed at 100% relative humidity,

250

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ALIVE I HOUR LATER ALIVE 24 HOURS LATER

X'-~,

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I I 1 4 0 42 44 46 4 8 , SO

TEMPERATURE [°C)

~ll, X ;~, / / ,,\ 1~ / : / :I ~:..'~,',.~,, " 1 ,% l i

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\ k , \ ~ '~,. J<--X "ill I k.- i - I I I I ~ - i - I

, 0 42 44 46 48 so ~o 4 , 4 , 46 4,, so TEMPERATURE (°C) TEMPERATURE (,°C)

RELATIVE HUMIDITY: I00% = x x S O ° / o = o - - - o iS% -0-- . . . , ,o

Figure 1. Effect of exposing bees, acclimatised at 35°i to various temperatures and humidities for 1 hour.

Relative humidity

T A B L E I

Expected percentage survival (mean for temperatures 44, 45 and 46 ° ) End of exposure After 1 hour After 24 hours

15% 99 95 35 30% 96 85 40

100% 85 72 21

the only difference which yeas not significant at:least at 5 % level occurr ing at the

t hour inspection in the comparison with those exposed at 5 0 % r.h. At the imme-

diate and 1 hour inspections survivals were highest among bees kept at 15% r.h.,

all differences between these and the bees exposed at 50% r.h. and 100% r.h.

being significant at least at 5% level. At the 24 hours inspection bees which had

been e•posed at 50% r.h. had most survivors, the difference being significant at

T A BL E II

Mean % loss of weight, of bees previously acclimatised at 35 °, after 1 hours exposure at various temperatures and humidities

Alive at the end of experiment Dead at the end of experiment Relative humidity %: 100 50 15 100 50 15 Temperature (°C)

40 0.7 2.2 3.2 . . . .

41 1.5 2.8 3.5 2.3 -- --

42 2.0 1.8 3.1 - - 1.3 - - 43 3.4 5.5 5.8 -- -- --

44 6.9 5.3 6.8 -- 0.6 4.8 45 12.7 11.8 20,1 3.7 5.6 - - 46 10.0 11.0 15.1 8.9 10.9 - - 47 8.8 14.2 20.0 11.8 9.0 7.5 48 - - 33.7 21.7 9.5 12.5 ~14.0 49 - - 5.5 12.1 10.8 14.7 10.7 50 - - - - - - 18.8 14.9 17.1

UPPER LETHAL TEMPERATURES OF HONEYBEES 251

the 1% level in comparison with bees kept at 100%, r.h., but not significant in comparison with those kept at 15% r.h.

Live bees tended to lose more weight than dead ones at lower relative humidities at all temperatures, but especially between 45 ° to 47 ° (Table I I ) . At 15% and 50% r.h. the mean weight lost by live bees was greater in ten of the eleven tem- perature and humidity combinations in which comparisons are possible.

( b ) Bees acclimatised to 20 °

Bees were acclimatised to 20 ° for 24 hours, before being exposed to various relative humidities at 45 ° , 46 ° , and 47 ° . Fewer survived at each temperature and humidity than of bees acclimatised to 35 ° (Table I l l ) .

TABLE I n

Number o[ bees (out of 16 at each temperature and relative humidity), acclimatised for 24 hrs to 20 ° or 35 °, surviving at various temperatures and relative humidities

Relative humidity %: Acclimatisation temperature (°C) Temperature of exposure (°C)

45 Alive at end of experiment Alive 1 hour later Alive 24 hours later

46 Alive at end of experiment Alive 1 hour later Alive 24 hours later

47 Alive at end of experiment Alive 1 hour later Alive 24 hours later

100 50 15 20 35 20 35 20 35

10 13 14 15 12 16 3 10 12 13 7 15 0 2 0 7 1 5 1 13 6 15 15 16 1 12 3 13 13 16 0 3 0 5 1 2 0 5 1 12 7 13 0 5 1 10 4 12 0 2 0 3 0 2

(c) Bees of various ages

bees that had emerged within a period of 24 hours from were marked and introduced to a colony, a different colour

E]fect of 2 hours' exposure

Bees acclimatised at 35 ° were exposed to 46 ° and 47 ° for 2 hours. None survived at 100% r.h. At 46 °, 6 survived at 50% and 8 at 15% r.h. and at 47 ° one survived at each r.h.

A( ,~,eekly intervals combo m an incubator, being used each week. When the first group was 3 to 4 weeks old, the survival of bees of different ages was compared (Table IV) . At 46 ° age did not appear to affect survival, but bees younger than 2 weeks taken directly from the hive survi- ved better than older ones at 47 ° and 100% and 50% r.h. (P < 0.01 and P < 0.05). Bees of all ages behaved similarly after they were acclimatised at 35 ° for 24 hours.

Weight loss during the experiment did not vary consistently with age, but those acclimatised at 35 ° lost more than bees taken direct from the hive (mean loss of weight 21.9% and 11.1% respectively; P < 0.001), although their mean weights immediately before the experiment were similar (138.5 mg and 141.4 mg respectively).

252 j. B. FREE & YVETTE SPENCER-BOOTH

Ei]ect o I 20 hours' exposure Bees acclimatised to 35 ° were exposed to 36 °, 38 °, 40 ° and 42 ° for 20 hours.

At 38 ° to 42 ° survival increased with increased relative humidity (Fig. 2).

TABLE IV

Number of bees, of known ages, alive after exposure/or 1 hour to various temperatures and relative humidities (When other than 16 bees were used the numbers are given in brackets)

Temperature of exposure

(°c) 46 Bees direct /tom hive

Bees direct from hive 47

Bees acclimatised to 47 35 ° before experiment

Age of Percentage relative humidity bees

(weeks) 100 50 15

~ 1 4 11 14 1 - -2 4 11 13 2 - - 3 4 13 15 3--4 2(15) 11(15) 13(15)

< 1 - 4 12 i l 1--2 5 9 9 2 - -3 0 6 10 3--4 0(10) 3(9) 7(9) < 1 10 12(14) 15(15)

1 - -2 15 16 15(15) 2 - - 3 9 14(14) 15(15) 3--4 5(6) 5(6) 6(6)

ALIVE AT END OF EXPERIMENT

14

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4 ", ~ L.. \ ' \ 2 .... 0... •

I I I 3 6 3 8 4 0

TEMPERATURE (Oc) 42

RELATIVE HUMIDITY IO0°/o = ×

36

x

ALIVE I HOUR LATER ALIVE 24 HOURS LATER

IO

in

~ 6

4

16

~.. \ \ x 2 - - ""a . . ' ...... '~X'O

38 4 0 . . 42 TEMPERATURE(-"°C)

16

,o

36 38 4 0 42 TEMPERATURE(OC)

s o % = , - - - ~ i s % - - o . . . . . . .

Figure 2. Effect of exposing bees, acclimatised at 35 °, to various temperatures and humidities for 20 hours.

The weight loss of bees during the experiment increased with decrease in humidity, but at 50% and 15% r.h., at each temperature, the mean weight loss of bees alive at the end of the experiment was less than that of those that died (Table V).

UPPER LETHAL TEMPERATURES OF HONEYBEES 253

TABLE V

Mean % weight loss of bees previously aeelimatised at 35 ° after 20 hours' exposure at

Relative humidity %: Temperature (°C)

36 38 40 42

various temperatures and humidities

Alive at the end of experiment Dead at the end of experiment

100 50 15 i00 50 15

7.4 19.0 20.8 1.2 24.8 39.3 6.2 23.6 22.0 9.2 39.6 52.3 6.1 22.8 20.6 9.7 44.6 49.2

18.6 34.6 34.9 11.4 48.2 44.2

DISCUSSION AND CONCLUSIONS

Acclimatisation temperature influenced the upper lethal temperature of adult bees as found by MELLANBY (1954) for Tenebrio molitor and Aedes aegypti larvae. The greater tolerance of heat by young than by old bees presumably reflects the tenden- cy of young bees to keep more to the brood nests of their colonies (FREE 1960) where the temperature is highest (34 ° to 35 °) (e.g. GATES 1914, HIMMER 1932); after acclimatisation at 35°C, age did not appear to influence the upper lethal death temperature, whereas young bees then had a higher chill coma temperature than older bees (FREE & SPENCER-BoOTH 1960).

Bees probably survive high temperatures for 1 hour better when the relative humidity is low, because they can cool themselves by evaporation better, as shown by their greater loss of weight. Less obvious kinds of thermal injury were not decreased by low humidity, however, as many bees alive at the end of the experi- ments died soon afterwards. For long exposures, the lethal temperatures were lower and survival below them increased with increase in relative humidity, probably because desiccation was a limiting factor. Similar effects of exposure on survival of other insects at different relative humidities have been found by MELLANBY (1932) and GUNN & NOTLEY (1936) .

The mean weight loss of live bees per hour at 15% r.h. was about 14 times greater at 45--49 ° for 1 hour than at 40--42 ° for 20 hours. At the higher temperatures ventilation and evaporation from the tracheae was probably increased by the increased respiration (ALLEN 1959) and the epicuticular waxes had probably become more permeable to water (WIGGLESWORTH 1953); the amount of water bees consume increases with increasing temperature (FREE & SPENCER- BOOTH 1958). However, this does not explain why live bees lost more weight than dead bees after 1 hour's exposure, but less after 20 hours' exposure, which sug- gests that water was being actively secreted and retained respectively by the live bees.

At an outside temperature of 50 ° bees can keep the temperature inside their hives down to 38 ° (WOHLGEMUTH 1957). Recommendations (e.g. TOWNSEND &

ADIE 1952, HAMILTON 1960) to spray water on colonies and hives likely to be exposed to hot dry conditions while being moved to another site are probably sound, as bees can cool the colony by evaporating the water on the combs and on their mouthparts (DuNHAM 1931, LINDAUER 1954) and the higher relative humidity produced by the evaporation, and the replacement of body water reserves by drinking, make death by desiccation unlikely. If the bees cannot keep their

254 J . B . FREE & YVETTE SPENCER-BOOTH

colony temperature below 45 °, wi thhold ing water to keep the humidity down

might enable them to survive for an hour or two, but not for longer, and it would

be better to maintain the water supply and increase ventilation.

W e are most grateful to Dr. Marjory Morris for statistical help.

ZUSAMMENFASSUNG

DIE OBERE LETHALTEMPERATUR BEI DER HONIGBIENE 1. Die meisten Bienen iiberlebten einsttindige Einwirkung yon 44°C unabh~ngig yon der relativen Luftfeuchtigkeit, jedoch iibezstanden mehr Bienen 45-49 ° bei niedriger rel. Luft- feuchtigkeit, wahrscheinlich well sie sich dann sfiirker dutch Verdunstung abkiihlen. Ihr Gewichtsverlust w~ihrend der Behandlung stieg mit abnehmender rel. Luftfeuchtigkeit an Bienen, die bei mittlerer oder geringer Luftfeuchtigkeit iiberlebten, verloren mehr an Ge- wicht als diejenigen, die starbeni 2. Die l~berlebensrate yon Bienen, die 20 Stunden lang bei 38-42 ° gehalten wurden, nahm mit ansteigender rel. Luftfeuchtigkeit zu, wahrscheinlich infolge abnehmender Aus- trocknung; bei mittlerer oder niederer tel. Imftfeuchtigkeit iiberlebende Bienen verloren weniger Gewicht als die, welche starben. 3. Die Akklimatisationstemperatur beeinfluflte die Lage Aes thermalen Todespunktes, und das erkl~irt vermutlich, warum junge Bienen, die sich in den W~irmeren Teilen ihrer Kolonie aufzuhalten pflegen, einen htiheren thermalen Todespunkt haben als iiltere.

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different temperatures. ]. exp. Biol. 36: 92--101. DUNHAM, W. E. (1931). A colony of bees exposed to high external temperatures. ]. econ.

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Received for publication : July 30, 1962 .