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Acclimation of different strains of the olive fly, Dacus oleae, to low temperatures
J. Itwct
Physir,/.. 1977. Vol. 23. pp. 649 to 653. Pergumon
Press. Printed irk Greut Brituin.
ACCLIMATION OF DIFFERENT STRAINS OF THE OLIVE FLY, DACUS OLEAE, TO LOW TEMPERATURES BRIAN S. FLETCHER* and GEORGE ZERVAS Demokritos Nuclear
Research
Centre,
(Received
Aghia
Paraskevi,
Atticus.
Athens,
Greece
28 Mrcs 1976)
Abstract-Male and female D. olrar have similar powers of acclimation when exposed to low temperatures. Their torpor thresholds depend upon the temperature to which they have been acclimatised. During slow cooling (i.e. less than 1°C per min) they are capable of some rapid acclimation which enables them to lower their torpor threshold by almost 1°C degree. as compared with when they are chilled quickly. After abrupt transfer from WC to a different temperature, acclimation takes some time to be accomplished. At 15°C and above it occurs within IO days but at temperatures below this, progressive acclimation lowers the torpor thresholds to the very low levels typical of flies overwintering under natural conditions. During this long term acclimation torpor thresholds may change by almost 0.5’C per 1’ C change of acclimation temperature. No differences were observed in the ability of either flies from northern and southern Greece. or normal and y-irradiated laboratory reared flies to acclimate to winter conditions in the field. In all cases. torpor thresholds were progressively lowered in advance of the decline in weekly minimum temperatures.
INTRODUCTION Ducrrs oleae (Gmel.) is an important pest of olive fruits in many countries of the Mediterranean region. It is able to overwinter successfully in the adult stage even though it has to cope with both large seasonal changes and wide daily fluctuations in temperature (e.g. 0 to 26°C in 24 hr). Acclimation to constant low temperatures in the laboratory has been demonstrated in a variety of insects including Pvriplaneta amrricana (DEHNEL and SE<;AL,1956). Tetwhrio ttzolitor (NUTTAL, 1970), Triholiuttt cot2fiwttt and Musca dotnestica (ANDERSONand M~IXHMOOR, 1971), and Dacus trpni (MEATS, 1973. 1976). With the exception of D. tryoni, however, very little work has been carried out on the long term acclimation that might be expected to accompany seasonal temperature changes in the field. We studied both short term and long term acclimation of adult D. o/rue to low temperatures in a variety of situations in the laboratory and in the field. Comparisons were made between flies collected from northern and southern Greece to see if there were any geographical differences in their ability to acclimate to winter temperatures. Also, because control by the sterile male release technique is being devel-
mine if rearing at constant temperatures for a number of generations. or sterilising doses of y-irradiation affected their ability to acclimate to winter temperatures under field conditions. MATERIALS
AND METHODS
The laboratory flies used during this study came from a culture at the ‘Demokritos’ centre which had been reared at 25’C for 15 generations on an artificial diet (TZANAKAKIS et cd. 1970; TSITSIPIS. 1975). The sterilised flies were irradiated with 11 K-rads of C06’ y-rays in nitrogen during the late pupal stage. The two wild strains were bred from olives collected in Halkidiki. Northern Greece and Chania, Crete in October 1974. On emergence, the flies were separated into batches of 200 (sex ratio 1 :I) and placed in cages (30 x 30 x 3Ocm) provisioned with sugar. protein and water. Control flies were kept at 25-C in a large constant temperature room. Flies maintained at other constant temperatures were transferred to environmental cabinets. which had been preset to the required temperature, and maintained under a 12: 12 light/dark regime. The flies to be kept under natural conditions were oped for this species. similar studies were made with transferred in cages, shortly after emergence. on to normal and irradiated laboratory reared flies to detera series of shelves in an open space near the laboratory. They were protected from rain. but otherwise *Present address-C.S.I.R.O., Division of Entomology. completely exposed to the elements. A daily record Zoology Building. University of Sydney, N.S.W. 2006. Auswas kept of the air temperatures at the site. tralia. 649
BRIAN S. FLETC.HE.H
650 I:\1 irmrtiorl qf’ torpor
tefnpewtuw
Acclimation to different temperatures was studied by observing the changes in the torpor temperature of the flies. The torpor temperature of individual flies was determined by observing the temperature at which they ceased to readopt a normal standing posture when disturbed during cooling and the temperature at which they took up a standing posture again on rewarming. On average. Aies became torpid during cooling at a temperature a few tenths of a degree less than that at which they became active again on re-warming. Normally there was a good correlation between the two values for individual flies and therefore the mean was taken as the torpor temperature. The measurements were carried out in an apparatus adapted from that of MEATS (1974). Ten flies could be accomodated at the same time. They were attached with adhesive to thin wooden rods (Zmm diam.) by the dorsal surface of the thorax and allowed to take up a normal standing position on a metal platform in the observation chamber. A stream of air (I I.:min) that had been equilibrated to the temperature of the water bath. and then humidified. was passed through the chamber to regulate the temperature. Heating of the water bath was regulated by a thermostatically controlled coil and lowering of the temperature was achieved by adding crushed ice. if desired. rates of cooling greater than I C down to 5 & 6 C and 0.3 to 0.5 C per minute below this could be obtained. The flies were introduced into the observation chamber at the appropriate temperature and chilled at the required rate. Their torpor temperatures were then detcrmincd by frequently raising and lowering them from the substratum by means of the rods to which thcv were attached, and observing if they moved th& legs. RESULTS Preliminary measurements carried out on normal and irradiated laboratory reared flies that were acclimatised at 25 C indicated that the torpor temperatures of males and females were not significantly different. Even so. in all subsequent experiments five males and five females were tested in each replicate in case sexual differences might occur in more extreme conditions. However, no sexual differences in the extent or rate at which flies acclimated were observed in any of the conditions tested. In other experiments. the torpor temperatures of batches of normal and irradiated laboratory reared flies acclimatised to 25 C were measured at intervals throughout life (i.e. from newly emerged until nine weeks). Except that some batches of young flies (0 to IO hr post-eclosion) had slightly higher torpor temperatures. there were no significant changes in torpor temperature as the flies aged. remaining around 1.5 ) 0.4 C.
P 4
/ ,,$f i”I
5
5
10
15
ACCLIMATIZATION
20
Fig. I. Torpor temperatures of flies after and
rapid
cooling
(01 in relation temperatures.
25
30
TEMPERATURE
to
slow
their
cooling
(0)
acclimation
To determine the torpor temperature of flies acclimatised to a series of different constant temperatures. batches of flies that had been reared at 25 C were placed when 2 weeks old. into a series of constant temperature regimes ranging from 5 to 30 C. The torpor temperatures of the flies were then measured lOdays later, as it had been established earlier that acclimation to new temperatures took several days at least. The torpor temperatures were measured under two sets of conditions. Firstly, when the flies were placed into the apparatus at their acclimatisation temperature and then chilled slowly (i.e. less than 1 C per min). Secondly when flies were transferred in to the apparatus at a temperature 2 to 3 C above their torpor temperature (as indicated by the first series of experiments) and then chilled further. Different flies from the same batches were used for the two series of experiments. The results are shown in Fig. I. There was a direct relationship between the acclimatisation temperature and the torpor temperature. The lower the acclimatisation temperature the lower the torpor temperature. except at the lower end of the range (i.e. with acclimatisation temperatures of lo‘ and 5’~‘C)when after slow chilling the torpor temperatures were not significantly different from each other. There was a marked difference in the torpor temperature of flies from the same acclimatisation temperature. depending upon whether they were chilled slowly or rapidly. Flies cooled slowly became torpid at a lower temperature than those chilled rapidly. which indicates that a certain amount of re-acclimation occurs while the flies are actually being chilled. Ilnltke D. rrymi (MEATS. 197?), however, where after slow chilling flies acclimatised to temperatures
651
Acclimation of D. olrur to low temperatures between 30 and 8’ C had the same torpor temperature. in D. olrcle the torpor temperature increased with increasing acclimatisation temperatures when these were above 1o’C. Rcltes qf’ ucclitmrtion
to new constant
temperatures
rates of re-acclimation which occurred when batches of flies were transferred straight from 2.5 C into a series of constant temperatures ranging between 30 and 5’C are shown in Fig. 2. The torpor temperatures were measured using the slow rate of chilling. The results clearly indicate that even after a relatively small abrupt change in the temperature at which they are maintained the flies take several days to fully re-acclimate. Another interesting feature is that when flies were transferred into temperatures at the lower end of the range (i.e. 10 and 5 C), the torpor temperatures of the different batches converged so that after the first 2 to 3 days there were no significant differences. Furthermore the torpor temperatures of these batches of flies (i.e. those maintained at 10’ and 5 C) continued to decline throughout the entire period measurements were made. Thus 15 days after the transfer from 25°C. the torpor temperatures of flies from both batches were around 4.8’C and 39 days after transfer they had dropped to 3 C. The
7‘1~’ trcclimution of’ Hakidiki o/elk, to winter temperatures
and Crete
strains
of’ D.
Batches of flies that emerged from fruit collected in Halkidiki. Northern Greece and Crete were exposed in cages to natural winter conditions 2 to 3 days after emergence (i.e. mid-November). Other cages of flies from the same sources were kept at 25 C. The torpor temperatures of the different batches of flies were then measured at intervals throughout the winter, using the slow method of cooling. The flies were placed into the apparatus at a temperature which corresponded to the outdoor temperature at
I
I
I
I
16
30
NOV
FROM
25°C
F.ig. 2. Changes in torpor thresholds of flies transferred from ‘5 C to a series of different temperatures. The torpor thresholds of flies transferred to 1O’C and 5 C continued to decline over a period of many weeks.
25
JAN
temperatures
minimum
8
22
8
MAR
FEB.
of flies from Crete and Halki-
temperatures.
the time of the experiment. The results, along with the average weekly minimum temperatures are shown in Fig. 3. There were no significant differences in the torpor temperatures of flies from Halkidiki and Crete even when the temperature was very low. In both cases. the torpor temperatures of the flies decreased as the temperature dropped at the beginning of winter so that they remained below the average minimum temperatures. Thus. although on certain occasions the temperature may drop below the torpor temperature (e.g. 5 and h Jan. when the minimum tempcraturc was close to 0 C) for most of the time even during the night the flies avoid becoming torpid because of their ability to re-acclimate in advance of the decreasing temperatures. The wild flies kept at 25 C during the winter had torpor temperatures around 7.4 C which were similar to those of the laboratory reared flies kept under the same conditions.
-.
Normal
O-Q
lrrodmted
'*w-?
r' : :
I 30
NOV TRANSFER
11
diki kept under natural conditions during the winter months. The dotted line indicates the trend in weekly mean
16
AFTER
28 DEC
Fig. 3. Torpor
\
DAYS
14
I 28
14
DEC
11
25
JAN.
L 22
8 FEB
L 8 MAR
Fig. 4. Torpor temperatures of normal and y-irradiated flies that had been reared in the laboratory at 25 C for 15 generations when kept under natural conditions during the winter months. The mean weekly minimum temperatures are also shown.
652
BRIAN
S.
FLt’KHER
Batches of recently emerged laboratory reared flies (I to 1 days old). half of which had been irradiated during the pupal stage were placed outside in small cages close to the wild flies from Halkidiki and Crete. Their torpor temperatures were measured in a similar manner to that for the wild strains. The results are shown in Fig. 4. There were no significant differences between the torpor temperatures of normal and irradiated flies at any time during the winter. Furthermore. their torpor temperatures on a given date were not significantlq different from those of the wild flies from Halkidiki and Crete. Thus neither rearing in the laboratory at a constant temperature of 25 C for 15 generations. nor sturilising doses of ;-irradiation affected the ability of II. olrr~ adults to acclimate normally to winter trmperaturcs. DISCUSSION The results indicate that during long term exposure to low temperatures adults of D. &au have notable powers of acclimation. The maximum changes of torpor threshold are close to 0 to 5 C for a 1 C change of acclimation temperature. Apart from D. rrymi. a closely related species which has similar powers of acclimation (ME.ATS.1976). it is much higher than that reported for other insects and not much less than the highest values for vertebrate poikilotherms (FRY. 19671. In an earlier study, MEATS (1973) found that adults of D. crxoni that had been reared at 95 C could acclimate rapidly to acute changes of temperature but the changes in thresholds were much smaller (i.e. 0.13 to 0.18 per 1 C). Thus, flies that had been kept at various temperatures between 7 C and 40 C for relatively short periods (4 to 30 hr) all had the same torpor temperature when cooled slowly. This was different from the result obtained with D. olrrrr after slow cooling. for the Ries had different torpor thresholds depending upon their prior acclimation temperature. However. the difference is probably due to the fact that in the cast of D. oltwc the flies had been acclimatised for much longer periods (i.e. IO days) before testing and despite the claim of MEATS (1973) that D. rr~~)r~icould acclimate rapidly after an abrupt transfer from one constant temperature to another it is now known that like D. o/ecw.it takes days or even weeks to fully acclimate after transfer to a lower temperature (MEAIS. 1976). Apparently thero is a mechanism in both species which can produce rapid but minor changes to the basic atate of acclimation that is different from the process that controls long term acclimation. This phenomenon of rapid acclimation was observed in D. dew during the experiments in which the torpor thresholds of flies that had been chilled either rapidly or &~wly were compared. Flies cooled slowly (i.e. less
AND GFOKGE
ZERVAS
than 1 C per min) had torpor thresholds almost I C less than flies from the same batch that were chilled rapidly. This rapid or ‘acute’ acclimation, differs from long term acclimation in that it is readily reversible on returning the Ries to warmer temperatures in both D. olcu (unpublished observations) and D. tryoni (MEATS. 1973). As such it seems to have been evolved to enhance the performance of the flies during the daily fluctuations in temperature which may be quite different from the seasonal trends. Long term acclimation. however. is more important in enabling the flies to survive for long periods during the relatively harsh climatic conditions they experience during the winter. It stems likely that rapid acclimation involves only minimal changes in the metabolic state of the fly. whereas long term acclimation is accompanied by relatively major changes in metabolism and physiology. Long term acclimation is obviously going to be most beneficial if it can anticipate seasonal trends in temperature and enable torpor thresholds to keep below the daily minimum temperatures for as long as possible. a situation which was seen in the flies kept under natural conditions. Observations on reacclimation after abrupt transfer to lower temperatures gives some indication of how this is brought about. When transferred to new temperatures of 15 C and abobe. torpor temperature reaches a new equilibrium within a reasonably short time. However. when Hies arc transferred to temperatures below 15 C the process of re-acclimation continues over a number of weeks and finally results in the very low torpor thresholds that are typical of those observed in overwintering flies kept under natural conditions. Even with constant conditions therefore. temperatures below a certain level. which is not particularly extreme. cause a progressive decline in torpor temperature which can only be reversed by a return to warmer conditions. The effects of fluctuating temperatures were not studied in this project but the similarities between long term acclimation to constant low temperatures in D. olrrrr and D. rrprti (MEATS. 1976) leads one to expect that the elect of fluctuating temperatures would also bc broadly similar. Thus the acclimation model recently proposed by MEATS (1976) to explain long term acclimation in D. rr!mi might well apply. in a general sense. to the situation in D. oltw. l[,kr~o~~/r~/yrrt~r,lr\ - This worh was carried out while the senior author was a consultant to the UNDP Project GRE.‘6’%525 “Research on the control of olive pests and diseases in continental Greece. Crete and Corfu”. The facilities were provided by the Greek Atomic Energy Commission.
REFERENCES ANIXKSO?~ R. L. and MUTCHMOR J. A. (lY711 Temperature acclimation in Triholiltrtl and Mu\c-~ at locomotory. metabolic and enzyme levels. ,I. /u\<~cI PIt!..\icll. 17, ‘X%321’).
Acclimation of D. olrae to low temperatures DEHNELP. A. and SEGALE. (1956) Acclimation of oxygen consumption to temperature in the American cockroach (Periplaneta
an~ericana).
Biol.
Bul/..
Woods
Hole
111,
53-61. FKY F. E. J. (1967) Responses of vertebrate poikilotherqs to temperaiure. In T&rmobio&y (Ed. by ROSEA. H.) pp. 375-409. Academic Press. London. MEATSA. (1973) Rapid acclimation to low temperatures in the Queensland fruit fly, Dacus trymi. J. Insect Physiol. 19. 1903~1911. MEATSA. (1974) Apparatus for the exposure and observation of insects at known constant or changing temperatures, humidities and wind speeds. Lab. Practice 33, 119. MEATSA. (1977) Developmental and long term acclimation to cold by the Queensland fruit fly (Dacus trymi) at
653
constant and fluctuating temperature. .I. Insect Ph!xiol. In press NUTTALLR. M. (1970) The effect of acclimation upon the survival of Ptinus tectus and Tenrhrio molitor when exposed to low temperatures. E~~tomologia cup. appl. 13. 217-222. TSITSIPISJ. A. (1975) Mass rearing of the olive fly, Ducus o/rue (Gmel.) at “Demokritos”. In Fruit FIy Control h!,
thr Sterile Insect Technique.
Proc.
Sgmp.
Kevnu Nov.
1973. I.A.E.A. Vienna.
TZANAKAKISM. E.. ECONOMOPOULOS A. P.. and TSITSIPIS J. A. (1970) Rearing and nutrition of the olive fly. I. Improved larval diet and simple containers. J. WJI~.Ent. 63. 317-318.
Physir,/.. 1977. Vol. 23. pp. 649 to 653. Pergumon
Press. Printed irk Greut Brituin.
ACCLIMATION OF DIFFERENT STRAINS OF THE OLIVE FLY, DACUS OLEAE, TO LOW TEMPERATURES BRIAN S. FLETCHER* and GEORGE ZERVAS Demokritos Nuclear
Research
Centre,
(Received
Aghia
Paraskevi,
Atticus.
Athens,
Greece
28 Mrcs 1976)
Abstract-Male and female D. olrar have similar powers of acclimation when exposed to low temperatures. Their torpor thresholds depend upon the temperature to which they have been acclimatised. During slow cooling (i.e. less than 1°C per min) they are capable of some rapid acclimation which enables them to lower their torpor threshold by almost 1°C degree. as compared with when they are chilled quickly. After abrupt transfer from WC to a different temperature, acclimation takes some time to be accomplished. At 15°C and above it occurs within IO days but at temperatures below this, progressive acclimation lowers the torpor thresholds to the very low levels typical of flies overwintering under natural conditions. During this long term acclimation torpor thresholds may change by almost 0.5’C per 1’ C change of acclimation temperature. No differences were observed in the ability of either flies from northern and southern Greece. or normal and y-irradiated laboratory reared flies to acclimate to winter conditions in the field. In all cases. torpor thresholds were progressively lowered in advance of the decline in weekly minimum temperatures.
INTRODUCTION Ducrrs oleae (Gmel.) is an important pest of olive fruits in many countries of the Mediterranean region. It is able to overwinter successfully in the adult stage even though it has to cope with both large seasonal changes and wide daily fluctuations in temperature (e.g. 0 to 26°C in 24 hr). Acclimation to constant low temperatures in the laboratory has been demonstrated in a variety of insects including Pvriplaneta amrricana (DEHNEL and SE<;AL,1956). Tetwhrio ttzolitor (NUTTAL, 1970), Triholiuttt cot2fiwttt and Musca dotnestica (ANDERSONand M~IXHMOOR, 1971), and Dacus trpni (MEATS, 1973. 1976). With the exception of D. tryoni, however, very little work has been carried out on the long term acclimation that might be expected to accompany seasonal temperature changes in the field. We studied both short term and long term acclimation of adult D. o/rue to low temperatures in a variety of situations in the laboratory and in the field. Comparisons were made between flies collected from northern and southern Greece to see if there were any geographical differences in their ability to acclimate to winter temperatures. Also, because control by the sterile male release technique is being devel-
mine if rearing at constant temperatures for a number of generations. or sterilising doses of y-irradiation affected their ability to acclimate to winter temperatures under field conditions. MATERIALS
AND METHODS
The laboratory flies used during this study came from a culture at the ‘Demokritos’ centre which had been reared at 25’C for 15 generations on an artificial diet (TZANAKAKIS et cd. 1970; TSITSIPIS. 1975). The sterilised flies were irradiated with 11 K-rads of C06’ y-rays in nitrogen during the late pupal stage. The two wild strains were bred from olives collected in Halkidiki. Northern Greece and Chania, Crete in October 1974. On emergence, the flies were separated into batches of 200 (sex ratio 1 :I) and placed in cages (30 x 30 x 3Ocm) provisioned with sugar. protein and water. Control flies were kept at 25-C in a large constant temperature room. Flies maintained at other constant temperatures were transferred to environmental cabinets. which had been preset to the required temperature, and maintained under a 12: 12 light/dark regime. The flies to be kept under natural conditions were oped for this species. similar studies were made with transferred in cages, shortly after emergence. on to normal and irradiated laboratory reared flies to detera series of shelves in an open space near the laboratory. They were protected from rain. but otherwise *Present address-C.S.I.R.O., Division of Entomology. completely exposed to the elements. A daily record Zoology Building. University of Sydney, N.S.W. 2006. Auswas kept of the air temperatures at the site. tralia. 649
BRIAN S. FLETC.HE.H
650 I:\1 irmrtiorl qf’ torpor
tefnpewtuw
Acclimation to different temperatures was studied by observing the changes in the torpor temperature of the flies. The torpor temperature of individual flies was determined by observing the temperature at which they ceased to readopt a normal standing posture when disturbed during cooling and the temperature at which they took up a standing posture again on rewarming. On average. Aies became torpid during cooling at a temperature a few tenths of a degree less than that at which they became active again on re-warming. Normally there was a good correlation between the two values for individual flies and therefore the mean was taken as the torpor temperature. The measurements were carried out in an apparatus adapted from that of MEATS (1974). Ten flies could be accomodated at the same time. They were attached with adhesive to thin wooden rods (Zmm diam.) by the dorsal surface of the thorax and allowed to take up a normal standing position on a metal platform in the observation chamber. A stream of air (I I.:min) that had been equilibrated to the temperature of the water bath. and then humidified. was passed through the chamber to regulate the temperature. Heating of the water bath was regulated by a thermostatically controlled coil and lowering of the temperature was achieved by adding crushed ice. if desired. rates of cooling greater than I C down to 5 & 6 C and 0.3 to 0.5 C per minute below this could be obtained. The flies were introduced into the observation chamber at the appropriate temperature and chilled at the required rate. Their torpor temperatures were then detcrmincd by frequently raising and lowering them from the substratum by means of the rods to which thcv were attached, and observing if they moved th& legs. RESULTS Preliminary measurements carried out on normal and irradiated laboratory reared flies that were acclimatised at 25 C indicated that the torpor temperatures of males and females were not significantly different. Even so. in all subsequent experiments five males and five females were tested in each replicate in case sexual differences might occur in more extreme conditions. However, no sexual differences in the extent or rate at which flies acclimated were observed in any of the conditions tested. In other experiments. the torpor temperatures of batches of normal and irradiated laboratory reared flies acclimatised to 25 C were measured at intervals throughout life (i.e. from newly emerged until nine weeks). Except that some batches of young flies (0 to IO hr post-eclosion) had slightly higher torpor temperatures. there were no significant changes in torpor temperature as the flies aged. remaining around 1.5 ) 0.4 C.
P 4
/ ,,$f i”I
5
5
10
15
ACCLIMATIZATION
20
Fig. I. Torpor temperatures of flies after and
rapid
cooling
(01 in relation temperatures.
25
30
TEMPERATURE
to
slow
their
cooling
(0)
acclimation
To determine the torpor temperature of flies acclimatised to a series of different constant temperatures. batches of flies that had been reared at 25 C were placed when 2 weeks old. into a series of constant temperature regimes ranging from 5 to 30 C. The torpor temperatures of the flies were then measured lOdays later, as it had been established earlier that acclimation to new temperatures took several days at least. The torpor temperatures were measured under two sets of conditions. Firstly, when the flies were placed into the apparatus at their acclimatisation temperature and then chilled slowly (i.e. less than 1 C per min). Secondly when flies were transferred in to the apparatus at a temperature 2 to 3 C above their torpor temperature (as indicated by the first series of experiments) and then chilled further. Different flies from the same batches were used for the two series of experiments. The results are shown in Fig. I. There was a direct relationship between the acclimatisation temperature and the torpor temperature. The lower the acclimatisation temperature the lower the torpor temperature. except at the lower end of the range (i.e. with acclimatisation temperatures of lo‘ and 5’~‘C)when after slow chilling the torpor temperatures were not significantly different from each other. There was a marked difference in the torpor temperature of flies from the same acclimatisation temperature. depending upon whether they were chilled slowly or rapidly. Flies cooled slowly became torpid at a lower temperature than those chilled rapidly. which indicates that a certain amount of re-acclimation occurs while the flies are actually being chilled. Ilnltke D. rrymi (MEATS. 197?), however, where after slow chilling flies acclimatised to temperatures
651
Acclimation of D. olrur to low temperatures between 30 and 8’ C had the same torpor temperature. in D. olrcle the torpor temperature increased with increasing acclimatisation temperatures when these were above 1o’C. Rcltes qf’ ucclitmrtion
to new constant
temperatures
rates of re-acclimation which occurred when batches of flies were transferred straight from 2.5 C into a series of constant temperatures ranging between 30 and 5’C are shown in Fig. 2. The torpor temperatures were measured using the slow rate of chilling. The results clearly indicate that even after a relatively small abrupt change in the temperature at which they are maintained the flies take several days to fully re-acclimate. Another interesting feature is that when flies were transferred into temperatures at the lower end of the range (i.e. 10 and 5 C), the torpor temperatures of the different batches converged so that after the first 2 to 3 days there were no significant differences. Furthermore the torpor temperatures of these batches of flies (i.e. those maintained at 10’ and 5 C) continued to decline throughout the entire period measurements were made. Thus 15 days after the transfer from 25°C. the torpor temperatures of flies from both batches were around 4.8’C and 39 days after transfer they had dropped to 3 C. The
7‘1~’ trcclimution of’ Hakidiki o/elk, to winter temperatures
and Crete
strains
of’ D.
Batches of flies that emerged from fruit collected in Halkidiki. Northern Greece and Crete were exposed in cages to natural winter conditions 2 to 3 days after emergence (i.e. mid-November). Other cages of flies from the same sources were kept at 25 C. The torpor temperatures of the different batches of flies were then measured at intervals throughout the winter, using the slow method of cooling. The flies were placed into the apparatus at a temperature which corresponded to the outdoor temperature at
I
I
I
I
16
30
NOV
FROM
25°C
F.ig. 2. Changes in torpor thresholds of flies transferred from ‘5 C to a series of different temperatures. The torpor thresholds of flies transferred to 1O’C and 5 C continued to decline over a period of many weeks.
25
JAN
temperatures
minimum
8
22
8
MAR
FEB.
of flies from Crete and Halki-
temperatures.
the time of the experiment. The results, along with the average weekly minimum temperatures are shown in Fig. 3. There were no significant differences in the torpor temperatures of flies from Halkidiki and Crete even when the temperature was very low. In both cases. the torpor temperatures of the flies decreased as the temperature dropped at the beginning of winter so that they remained below the average minimum temperatures. Thus. although on certain occasions the temperature may drop below the torpor temperature (e.g. 5 and h Jan. when the minimum tempcraturc was close to 0 C) for most of the time even during the night the flies avoid becoming torpid because of their ability to re-acclimate in advance of the decreasing temperatures. The wild flies kept at 25 C during the winter had torpor temperatures around 7.4 C which were similar to those of the laboratory reared flies kept under the same conditions.
-.
Normal
O-Q
lrrodmted
'*w-?
r' : :
I 30
NOV TRANSFER
11
diki kept under natural conditions during the winter months. The dotted line indicates the trend in weekly mean
16
AFTER
28 DEC
Fig. 3. Torpor
\
DAYS
14
I 28
14
DEC
11
25
JAN.
L 22
8 FEB
L 8 MAR
Fig. 4. Torpor temperatures of normal and y-irradiated flies that had been reared in the laboratory at 25 C for 15 generations when kept under natural conditions during the winter months. The mean weekly minimum temperatures are also shown.
652
BRIAN
S.
FLt’KHER
Batches of recently emerged laboratory reared flies (I to 1 days old). half of which had been irradiated during the pupal stage were placed outside in small cages close to the wild flies from Halkidiki and Crete. Their torpor temperatures were measured in a similar manner to that for the wild strains. The results are shown in Fig. 4. There were no significant differences between the torpor temperatures of normal and irradiated flies at any time during the winter. Furthermore. their torpor temperatures on a given date were not significantlq different from those of the wild flies from Halkidiki and Crete. Thus neither rearing in the laboratory at a constant temperature of 25 C for 15 generations. nor sturilising doses of ;-irradiation affected the ability of II. olrr~ adults to acclimate normally to winter trmperaturcs. DISCUSSION The results indicate that during long term exposure to low temperatures adults of D. &au have notable powers of acclimation. The maximum changes of torpor threshold are close to 0 to 5 C for a 1 C change of acclimation temperature. Apart from D. rrymi. a closely related species which has similar powers of acclimation (ME.ATS.1976). it is much higher than that reported for other insects and not much less than the highest values for vertebrate poikilotherms (FRY. 19671. In an earlier study, MEATS (1973) found that adults of D. crxoni that had been reared at 95 C could acclimate rapidly to acute changes of temperature but the changes in thresholds were much smaller (i.e. 0.13 to 0.18 per 1 C). Thus, flies that had been kept at various temperatures between 7 C and 40 C for relatively short periods (4 to 30 hr) all had the same torpor temperature when cooled slowly. This was different from the result obtained with D. olrrrr after slow cooling. for the Ries had different torpor thresholds depending upon their prior acclimation temperature. However. the difference is probably due to the fact that in the cast of D. oltwc the flies had been acclimatised for much longer periods (i.e. IO days) before testing and despite the claim of MEATS (1973) that D. rr~~)r~icould acclimate rapidly after an abrupt transfer from one constant temperature to another it is now known that like D. o/ecw.it takes days or even weeks to fully acclimate after transfer to a lower temperature (MEAIS. 1976). Apparently thero is a mechanism in both species which can produce rapid but minor changes to the basic atate of acclimation that is different from the process that controls long term acclimation. This phenomenon of rapid acclimation was observed in D. dew during the experiments in which the torpor thresholds of flies that had been chilled either rapidly or &~wly were compared. Flies cooled slowly (i.e. less
AND GFOKGE
ZERVAS
than 1 C per min) had torpor thresholds almost I C less than flies from the same batch that were chilled rapidly. This rapid or ‘acute’ acclimation, differs from long term acclimation in that it is readily reversible on returning the Ries to warmer temperatures in both D. olcu (unpublished observations) and D. tryoni (MEATS. 1973). As such it seems to have been evolved to enhance the performance of the flies during the daily fluctuations in temperature which may be quite different from the seasonal trends. Long term acclimation. however. is more important in enabling the flies to survive for long periods during the relatively harsh climatic conditions they experience during the winter. It stems likely that rapid acclimation involves only minimal changes in the metabolic state of the fly. whereas long term acclimation is accompanied by relatively major changes in metabolism and physiology. Long term acclimation is obviously going to be most beneficial if it can anticipate seasonal trends in temperature and enable torpor thresholds to keep below the daily minimum temperatures for as long as possible. a situation which was seen in the flies kept under natural conditions. Observations on reacclimation after abrupt transfer to lower temperatures gives some indication of how this is brought about. When transferred to new temperatures of 15 C and abobe. torpor temperature reaches a new equilibrium within a reasonably short time. However. when Hies arc transferred to temperatures below 15 C the process of re-acclimation continues over a number of weeks and finally results in the very low torpor thresholds that are typical of those observed in overwintering flies kept under natural conditions. Even with constant conditions therefore. temperatures below a certain level. which is not particularly extreme. cause a progressive decline in torpor temperature which can only be reversed by a return to warmer conditions. The effects of fluctuating temperatures were not studied in this project but the similarities between long term acclimation to constant low temperatures in D. olrrrr and D. rrprti (MEATS. 1976) leads one to expect that the elect of fluctuating temperatures would also bc broadly similar. Thus the acclimation model recently proposed by MEATS (1976) to explain long term acclimation in D. rr!mi might well apply. in a general sense. to the situation in D. oltw. l[,kr~o~~/r~/yrrt~r,lr\ - This worh was carried out while the senior author was a consultant to the UNDP Project GRE.‘6’%525 “Research on the control of olive pests and diseases in continental Greece. Crete and Corfu”. The facilities were provided by the Greek Atomic Energy Commission.
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constant and fluctuating temperature. .I. Insect Ph!xiol. In press NUTTALLR. M. (1970) The effect of acclimation upon the survival of Ptinus tectus and Tenrhrio molitor when exposed to low temperatures. E~~tomologia cup. appl. 13. 217-222. TSITSIPISJ. A. (1975) Mass rearing of the olive fly, Ducus o/rue (Gmel.) at “Demokritos”. In Fruit FIy Control h!,
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TZANAKAKISM. E.. ECONOMOPOULOS A. P.. and TSITSIPIS J. A. (1970) Rearing and nutrition of the olive fly. I. Improved larval diet and simple containers. J. WJI~.Ent. 63. 317-318.