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The effect of thermal acclimation and relative humidity on the oxygen consumption of three Sitophilus species
THE EFFECT OF THERMAL ACCLIMATION AND RELATIVE HUMIDITY ON THE OXYGEN CONSUMPTION OF THREE SITOPHILUS SPECIES D. E. EVANS CSIRO
Divisionof Entomology. P.O. Box 1700. Canberra C‘ity.A.C.T. 2601, Audrali;i
The oxygen consumption of adult Sirophilus oryrtw (L.). S. qr
INTRODUCTION THE RELATIONSHIPbetween
oxygen consumption and temperature in adult rice weevils. Sitophilus oryzae (L.), has been studied by BIRCH (1947) and by EVANS ( 1977. 1979). BIRCH measured the oxygen consumption of warm-acclimated S. o~~~tre at several temperatures whereas EVANS measured the consumption of both warm and cold-acclimated weevils and found that warm-acclimated weevils used more oxygen than cold-acclimated weevils. EVANS (1977) obtained similar results with Sitophil~~ cl~trnc~ius (L.) and found that acclimation from 27 to 15”C, in both species. took 8- 10 days and re-acclimation to 27 C only 6-8 days. Later, EVANS (1979) showed, by closely restraining the weevils during respirometry, that cold-acclimation lowered the resting metabolism of S. o~~:tre at measurement temperatures below 23 C and that of S. gramrrius below 25 C. He was able to quantify the changes in active and resting metabolism that occurred when weevils were transferred from 27 to 15°C. The objective of the present study was to examine the effect of thermal acclimation and relative humidity on the respiration of the adults of three species of grain weevil. namel) S. or~~c~e. S. yrvrnarius and Sitophilus ~ecrrnuis Motsch. Respiration rate-temperature relationships were determined at 70 and 50”,, r.h. These humidities provided respective grain equilibrium moisture contents (m.c.) of 14.2 and 11.3”,,. To allow comparison with earlier studies (EVANS, 1977, 1979) the oxygen uptake of S. or~xe and S. ~JI.LIIIOI.~II.S acclimated at 70”,, r.h. was also measured at 94:” r.h. Several authors (CLOUDSLEY-THOMPSON. 1970; HAZEL and PROSSER. 1974: NEWEI.L. 1973: PROSSER, 1973; WIESER, 1973) have recently discussed thermal acclimation.
METHODS The S. oryzae and S. granarius used were taken from long-term CSIRO laboratory stocks: the S. zeamais were obtained from Savannah, U.S.A. in 1975. Two cultures of each species were established on 14.2% m.c. Olympic wheat at 27°C and 70?,, r.h. with an 87
88
D. E. EVANS
interval of 2 weeks between cultures. Several hundred < 24-hr-old weevils were obtained daily by sieving. The weevils from the two cultures were mixed so that the test insects would be representative of more than one culture and then redivided into two groups. One group was held at 27°C and 70% r.h. on 14.2”,; m.c. wheat and the other at 27°C and 500,; r.h. on 11.3?; m.c. wheat. After 7 days, some of each group were used for respirometry and the others were transferred to 15’C. Whilst at that temperature, the weevils were held at the same relative humidity as they had been at 27°C. The respirometry was repeated after S. oryzae had been acclimated for 2 weeks at 15’C and after the other species had been held at 15’C for 3 weeks. Rearing and acclimation temperatures were held within + 0.2”C and the desired relative humidities were maintained by saturated salt solutions. Oxygen consumption was measured with electrolytic respirometers (WINTERINGHAM, 1959; EVANS,1977). Sulphuric acid of appropriate concentrations was used as the electrolyte and provided a relative humidity of 94, 70 or 509;; within the respirometer flasks. The electrolyte was changed weekly. Carbon dioxide produced by the weevils was absorbed with Mallcosorb (Mallinkrodt). In experiments with unrestrained weevils, 10 unsexed weevils of known weight were placed with 25 grains of wheat of the appropriate moisture content in each respirometer flask. In other experiments, 10 S. oryzae were confined within the respirometers, without wheat, in 10 mm lengths of 2.8 mm bore polyethylene tube, the ends of which were closed with wire gauze. Preliminary experiments showed that the weevils aggregated at the ends of the small tubes and moved little. The experiments were set up and ended at approximately the same time of day. The weevils were allowed at least 1 hr to settle before a given measurement started: each measurement lasted 20 i 1 hr. Respirometer temperatures were maintained within i_ O.l’C. The statistical significance of the difference between the oxygen consumption of warmand cold-acclimated weevils at a given measurement temperature/humidity regimen was assessed by r-test. Following one-way analysis of variance, the significance of differences between treatment (i.e. humidity) means at a given measurement temperature was assessed by comparing the mean differences with their standard errors and referring to the t-distribution. Where variances could not be assumed to be equal, homogeneity of variances was examined, as appropriate, by F-test or by Bartlett’s test. RESULTS S. oryxe
The oxygen consumption of adult S. oryzae that had been reared and acclimated at 27°C and 70 or 50T;r.h. was measured at several constant temperatures immediately TABLE I. THEOXYGEN CONSUMPTION (&J OF CINRESTRAINED. WARM- AND COLD-ACCLIMATED S. or!'~urADULTS HELD FOR 20HR AT FOUR CONSTANT TEMPERATURES AND THREI:RELATIVE HUMIDITIES
Qo, and standard Measurement temperature
(~1 rng-
’ hr-‘)*
at 37 C
Acclimated
at 15 C
( C)
94”,, r.h.
70”,, r.h.
50”,, r.h.
94”,, r.h.
70” 0 r.h.
50” ,I r.h.
15
1.97 * 0.10 (11) 3.93 + 0.18 (12) 6.27 + 0.34 (11) 7.51 * 0.35 (12)
2.09 * 0.10 (II) 4.38 * 0.19 (11) 6.75 k 0.34 (11) 8.09 + 0.37 (11)
1.87 f 0.10 (11) 3.72 f 0.19 (11) 5.86 + 0.34 Ill) 8.34 * 0.35 (12)
1.51 + 0.08 (12) 3.10 * 0.15 (12) 5.52 + 0.27 (12) 6.69 ) 0.26 (12)
1.59 * 0.08 (111 3.37 * 0.15 (12) 5.92 + 0.27 (12) 6.59 + 0.26 (12)
1.61 + 0.08 (I?) 3.15 k 0.16 (11) 4.93 f 0.27 (17) 7.38 + 0.26 (12)
20 25 30
*
Acclimated
error
Values are based on the number of replicates indicated in parentheses: each replicate comprised 10 weevils of known fresh weight where W = 1.7 mg. Standard errors were derived from one-way analysis of variance. The Qo,s of weevils acclimated at 70”~r.h. were determined at 70 and 949,;r.h. whereas those of weevils acclimated at SO”/, r.h. were determined only at that humidity.
Thermal
Acclimation
R4TkkTEMPFRATL’RE
TARI E 7.
and Relative
REGRESSION
COMPONENTS
LINRESTRAINED s. 0ry;ur
Rclattvse humidity f”,,)
70
I5 50
37 15
* Where Q,,
‘,
( c-1 37 I5 37
is expressed
of Three Sit~~p/tr/~~.~ Spectes
AND
Q,,,s
FOR
THE
OXYGEN
X9
(‘ONSUMPTION
_ I.2833 - 1.5115 - 1.3561 - I.6911 - 1.0133 ~ 0.9660 in ;tl rng-
Ok
AT THREE RELATIVE HIIMIDITIES
Regression components tlogmQoz = 0 + hX + c?i2)*
Acclimation temperature
44
Humidity
h
Qlu Correlation coefficient
C’
0.132X
0.1468 0.1482 0.167X
-0.00’2 - 0.00~3 - 0.0024 - 0.00’8
0.1084
- 0.0015
0.0959
-0.0012
’ hr- I. and .!’ is measurement
0.9999 O.YYX9 O.YY9Y O.YYX6 O.YYY7 O.YYY4
temperature
15-25 C
20 30 C
3 .2 3.7 1.2 37 ;:I ‘.I
I .I) 2.2
I .x 2.0 2.2 ‘..3
in C
before and 2 weeks after the weevils were transferred to acclimate at 1.5 C. The consumption of weevils held at 700,, r.h. was determined at 94 as well as 70”,, r.h.; the consumption of weevils held at .50”,, r.h. was measured at that humidity. Un~est~~i~rtl ~~erils. At a given temperature and humidity, warm-acclimated (WA) weevils consumed significantly more oxygen (p < 0.05) than cold-acclimated (CA) weevils except in regimens of 50”,, r.h./lS”C and 50”,, r.h.j30’C (Table 1). WA weevils consumed consistently, but not always significantly, more oxygen at 70”,, r.h. than they did at 94”,, r.h. At 50”,, r.h.. over measurement temperatures of 15 to 35 C, WA weevils used consistently less oxygen than they did at 94”,,r.h. but only the values for 20 C differed significantly (p = 0.05). At 50”,, r.h./30 C, however, consumption was greater than at either 70 or 949,) r.h./30 ‘C. The oxygen uptake of CA weevils at 70”,, r.h. was greater between measurement temperatures of 15 and 25 C than it was at 94”,,r.h. but the differences between values for the two humidities were not statistically significant. At temperatures of 20 and 25 C uptake was lower at 50”,, r.h. than at 70”,, r.h., the values at 25 C differed significantly (p = 0.02). CA weevils held at 15 and 30 C consumed more oxygen at 50”,, r.h. than they did at either 70 or 94”,,r.h. When measured at 30 C. consumption at 50”,,r.h. was significantly greater than at 70”,, r.h. (p = 0.05). The relationships between oxygen consumption (Qo,) and measurement temperature were found to be described satisfactorily for each relative humidity (Table 2) by the quadratic function Y= N + hX + cX’ where Yis log,, Qo, in ,~l mgg’ hr-‘. X is the measurement temperature in C and the coefficients (I and c have negative values. QIos were computed for the temperature ranges 15 to 25 C and 20 to 30 C (Table 2): values for CA weevils were a little greater at 94 and 70”,, r.h. than those for WA weevils. i.e. CA weevils were somewhat more sensitive to temperature change than WA weevils at those humidities. From 25 to 3O”C, the mean Qto at 50”,,r.h. for both WA and CA weevils was 2.1 compared with 1.5 and 1.3 respectively at 94 and 70”,, r.h. Rruftuirwd ueeri/.s. Analysis of variance indicated that relative humidity had no significant effect on oxygen consumption at a given temperature. Because the humidity variTAI+I.I. 3. THF oxY(;r-2
~CJSIIMPTION
(Qo,)
OF RESTRAINFI),
WARM-
fiOR ?o HR AT FOUR COYSTANT
QO,and
Acclimation temperature
standard
AND COLD-ACCLIMATED
S. or~‘xt’
41)~ LTS HELD
TEMPFRATl‘RES
error ($ mg
I hr I)*
( C)
15 c
20 c
75 C
30 c
37 IS
0.70 * 0.02 0.58 * 0.01
1.01 f 0.01 0.92 * 0.01
1.4? * 0.03 1.46 * 0.02
1.90 + 0.02 1.84 i_ 0.03
* Values are based on the pooled data for 94. 70 and 50% r.h. and are for 36 replicates, each of 10 weevils. except for 77 C-acclimated weevils at 15 C which is for 34 replicates, The Qo.s of weevils acclimated at 70”,, r.h. were determined at 70 and 94”,, r.h. whereas those of weevils acclimated at 50”,, r.h. were determined only at that humidity.
90
D.E.
EVANS
ANDCOLD-AC‘C‘LIMATEDS. yrunarilts THREE RELATIVE HUMIDITIES
TARLE~.THEOXYGENCONSUMPTION(Q~~)OFUNRESTRAINED,WARMHELD FOR 20HR Al-FIVE CONSTANTTEMPERATURESAND
Qo, and standard Measurement temperature (‘C) 10 15 20 25 30
Acclimated 94”,, r.h. 0.70 2.19 3.51 5.98 7.55
+ 0.03 + 0.10 _+ 0.16 ) 0.22 k 0.18
(~1 rng-
’ hr I)*
at 27 C
Acclimated
70”;, r.h. 0.69 2.11 3.95 6.81 8.49
error
+ + k 5 +
50”,, r.h.
0.03 0.10 0.17 0.22 0.18
0.52 1.79 3.45 5.43 7.58
f * & * +
0.40 1.20 2.12 4.12 6.52
at 15 C
70” I, r.h.
94” 0 r.h.
0.03 0.10 0.16 0.22 0.18
ADULTS
+ 0.01 * 0.03 + 0.05 + 0.06 _+ 0.13
0.44 1.24 2.33 4.71 7.06
50” 0 r.h.
_t 0.01 f 0.03 2 0.05 * 0.06 _+ 0.13
0.38 1.13 2.06 4.04 6.62
* 0.01 +_ 0.03 + 0.05 + 0.06 & 0.13
* Values are based on 12 replicates except for the value for 27 ‘C-acclimated weevils at 2OC and 70”” r.h. which is for 11 replicates: each replicate comprised 10 weevils of known fresh weight where FJ = 2.6 m.g. Standard errors were derived from one-way analysis of variance. The Qois of weev*ils acclimated at 70”,, r.h. were determined at 70 and 94”,, r.h. whereas those of weevils acclimated at 50”,, r.h. were determined only at that humidity.
antes were not significantly different, the data for the three humidities were pooled. Mean values for oxygen uptake are given in Table 3. WA weevils used significantly more oxygen (p =
TABLE
Relative humidity f~<,) 94 70 50
* Where
Acclimation temperature
Regression components (log,, Qo, = u + hX + c/Y’)* (I
h
(’
27 15 27 15 27 15
- 1.2927 - 1.4170 - 1.3828 - 1.3750 - 1s-I93 -1.4156
0.1377 0.1 184 0.1466 0.1177 0.1565 0.1154
-0.0022 -0.0015 - 0.0023 -0.0015 - 0.0025 -0.0014
in ~1 mg-r
hr-‘.
and X
1smeasurement
OF UN-
Q,u Correlation coefficient
( Cl
Qo, is expressed
CONSUMPTION
0.99 I I 0.9976 0.9990 0.9989 0.9975 0.998 I
temperature
in “C.
ICr20 C 5.0 5.3 5.7 5.3 6.7 5.4
XL30 2.2 3.1 2.1 3.0 2.2 3.2
c
Thermal
T.4~1E 6. THF OXYGFN
Acclimation
CONSUMPTION
and Relative
Humidity
(Qol)OF UNRESTRAINED. WARM-
HE1.D FOR 20 HR AT FIVE CONSTANT
Sirophilus
of Three
Species
AND COLD-ACTLIMATED
TEMPERATURES AND TWO
YI s. zeumuis
ADLILTS
RELATIVE HUMlDITiES
-._-
Qol and standard Measurement trmperaturc
Acclimated
1 C)
0.42 0.99 2.24 3.56 5.00
I5 20 25 .;(I
+ * + + *
@I mg -’
0.38 0.79 1.76 2.86 4.03
at 15’ C
70”,, r.h.
50”,, r.h.
0.01 0.02 0.04 0.05 0.08
hr-‘I* Acclimated
at 27 C
70”,, r.h.
II)
error
f 0.01 _+ 0.0’ + 0.04 2 0.05 & 0.08
* Laluch xc based on 18 replicates: each replicate comprised in = 2.4 mg. Standard errors were derived from one-way analysis io”, r.h. prior to measurement at those humidities.
0.30 0.75 1.69 2.68 3.58
* + f f *
10 weevils variance.
50” 0 r.h.
0.01 0.01 0.03 0.03 0.05 of
0.28 0.65 1.48 2.38 3.16 known
fresh
& 0.01 * 0.01 * 0.02 * 0.03 +_ 0.05
weight
Weevilswereacclimatedat
where 70 or
was noted that observed values at 2Cr’C were consistently somewhat lower than values expected from the quadratic functions. At 94”,, r.h., for example, the observed values of QO, at 20 C for WA and CA weevils respectively were 3.51 and 2.12 ~1 mg-’ hr- ’ compared with corresponding expected values of 3.86 and 2.29 ~1 mg- ’ hr- ‘. At 70”,, and at 50”:cr.h.. particularly, the QlOs of WA weevils were greater than those of CA weevils between 10 and 20°C. By contrast, the Q t ,g of WA weevils over 20 to 30’-‘C were consistently lower than those of CA weevils. The QlOs of WA weevils over 10 to 20°C increased from 5.0 to 6.7 as humidity decreased from 94 to 50”;r.h. but differed little with humidity over 20 to 30°C. By contrast. the Qlos of CA weevils changed little with humidity over either temperature range.
The oxygen consumption of unrestrained adult S. ZXKLZ~Sthat were acclimated at 27 C and 70 or 50”,, r.h. was measured immediately before and 3 wk after transfer to 15 C. Qo: was determined at five constant temperatures in relative humidities of 70 or 50”,, (Table 6). WA weevils consumed significantly more oxygen (p < 0.001) than CA weevils at all measurement temperatures regardless of humidity. Consumption was significantly greater at 70”,,r.h. (p = 0.001) than at 50”,,r.h. irrespective of both measurement and acclimation temperature. The relationships between the logarithm of oxygen consumption and measurement temperature were again adequately described by quadratic functions (Table 7). The Qlos of WA and CA weevils were similar at both humidities. DISCUSSJON Llnrestrained adults of all three Sitophil~rs species consumed more oxygen in almost all experimental regimens when warm-acclimated than they did when cold-acclimated: this finding accords with previous observations at 94’,, r.h. (EVANS. 19771. TAB!,
7. RArf-TCMI'I~RAKRI
REGRESSION cOMPDNENTS RLSTRAINED S. mzmuis
AND Qlos FOR THE OXYGEN AT 70 AND so",,R.H.
CONSUMPTION
OF I:N'-
-.--~ Regression Rcla~i\e Acclimation tcmpcrature 111lllll~iI~~ I”,,)
( CI
70
27
50
IS 2; 15
* Where
Qo, is expressed
(log,,
u - I .4428 - 1.6689 - 1.3755 - 1.6442 in ~1 mg-’
components
Q,o
Q<)> = tl + hX + <,X2)*
hr-‘,
h
c
0.1232 0.1339 0.1076 0.1264
-0.0017 - 0.0020 -0.0010 -0.0018
and X is measurement
Correlation coefficient 0.9994 0.9996 0.9989 0.9992 temperature
I@20 5.3 5.6 5.3 5.3 in ‘C.
c
15-25 3.6 3.6 3.6 3.6
c
20-30 2.2 2.1 2.3 2.1
C
92
D. E. EVANS
S. oryzae generally used more oxygen, when unrestrained, at 70:/Or.h. than at 949;, r.h., the humidity at which previous observations were made (EVANS, 1977). The relatively high oxygen consumption of unrestrained S. oryzao in regimens of 50”,,r.h./30”C when warm-acclimated and SOY,;r.h./lYC and 3O’C when cold-acclimated suggests that there was an interaction between aridity and relatively low and high experiment temperatures. The fact that the mean Q 10 for WA and CA weevils over 25-30°C was greater at SOY,r.h. than at 94 and 70”/:,r.h. is a further indication of an interaction. Because relative humidity had no significant effect on the oxygen uptake of restrained S. ortlzae, it is clear that the relatively high consumption of unrestrained weevils under some regimens must be attributed to differences in locomotor activity. The similarity of the oxygen consumption of WA and CA restrained S. oryzae at 25 and at 3O’C confirms other studies with this species (EVANS,1979). The values for oxygen consumption at 94:; r.h. were greater in absolute terms than those observed or expected by interpolation from a previous study (EVANS, 1977). For reasons that are not understood, the metabolic rate of grain weevils may differ even when apparently identical rearing and experimental procedures are used. The differences may extend not only to the absolute Qo, values observed but also to the proportional changes that follow cold-acclimation. For example, the QoJs of 27 ‘C-acclimated weevils at 94:/;, r.h. before, immediately after and 28 days after transfer to 15°C were earlier found to be 5.91, 1.17 and 0.81 pl mg-’ hr-’ (EVANS, 1977). Equivalent values from the present study were 6.99 (estimated value), 1.97 and 1.51 ~_dmg- ’ hr- ‘. Acclimation at 15 ‘C lowered active metabolism by 31”, in the earlier study and by 237, in the present. The measurement of oxygen consumption in both unrestrained and restrained S. or).zae in this study, using weevils from the same series of cultures, allows an earlier conclusion on the relative importance of resting and active metabolism in cold-acclimation to be examined. EVANS(1977) concluded that 0.15 ~1 mg-’ hr- ‘, or 42”o, of the change of 0.36 1.11 mg- ‘hr-’ (i.e. the diffe rence between the QOJs of WA and CA weevils measured at 15’C) in the consumption of unrestrained weevils acclimated from 27 to 15 C was due to a reduction in resting metabolism. In the present study, the equivalent value (from Table 3) was found to be 0.12 ~1 mg-’ hr-‘, or 26?,,,, of the overall change of 0.46 111 mg-’ hr- ’ (from Table 1). On the bases of the two estimates, it appears reasonable to conclude that about one-third of the change in overall metabolism that occurs during acclimation at 15’C is due to reduced basal metabolism and about two-thirds is due to decreased locomotor activity. The oxygen consumption of unrestrained WA S. grunarius at 949,,r.h. was similar to that expected from the previous study (EVANS, 1977) but the CA weevils in the present study used considerably more oxygen at 20°C and above than in the earlier experiment. There was evidence of an interaction between humidity and experiment temperature in that the oxygen consumption of both WA and CA weevils was relatively high when measured at 509, r.h. and 3O’C. Humidity and acclimation temperature also appeared to interact in that the Qlos of WA weevils over 10 to 20°C changed with humidity whereas those of CA weevils did not. Furthermore, the Qlo of WA weevils between 10 and 2OC at 50’?; r.h. was higher than that of CA weevils. The finding that observed Qo, values for S. granarius at 20°C were consistently lower than expected values supports an earlier suggestion (EVANS,1977) that there is a discontinuity in the respiration rate-temperature relationship of S. granarius at about 20°C. Similarly, the lower Qlos of CA weevils between 10 and 20°C at 70 and 50q/,r.h. support the conclusion (EVANS, 1977) that cold-acclimation in S. granaritts involves a lowered sensitivity to temperature changes below about 20°C. The rate-temperature relationships of S. zeamais are similar to those of S. oryzar in that Qlos were higher at low than at high temperatures and single quadratic functions fitted the data well. The Qlos for WA and CA S. zeamais were similar and it seems. therefore, that cold-acclimation in this species involved lowered rates of consumption but not changed temperature sensitivity. Unlike S. oryzae and S. granarius, the oxygen consumption of S. zeamais at 50% was consistently lower than at 7O?dr.h. and there was no
Thermal Acclimation and Relative Humidity of Three
Sitophihrs
Species
Y?
evidence of an interaction between humidity and either experiment or acclimation temperature. It could be, however, that an interaction might be revealed if consumption were measured at temperatures above 30°C. The applicability, in terms of absolute QOJ values, of the results of this study to grain storage conditions are difficult to assess. Even when differences between populations of a given species (EVANS, 1977) are allowed for, experimental estimates of respiration may vary greatly with the type of respirometer used and the methods employed, particularly as regards the amount of activity allowed to the test organisms within the respirometer (PETRUSEWICZ and MACFADYEN, 1970; WIGHTMAN, 1977). For example, BIRCH (1947) and SIVGH L’Itrl. (1976) confined adult S. or~zur in cloth bags within Warburg respirometers and obtained Qoz values for 30”C-acclimated weevils (assuming a fresh weight of 1.7 mg at 3O’C;7Op, r.h. per weevil) equivalent to about 3.2 and 2.4 ~1 mg- ’ hr-’ respectively By contrast, unrestrained and restrained 27.C-acclimated S. ory~r in the present study consumed respectively 8.1 and 1.9 111mg- I hr-’ at 30 C,/‘70”,,r.h. (Tables 1 and 3). Clearly. further work is required to determine the effect of locomotor activity on the respiration of weevils in ‘typical’ grain storage conditions where weevils are free to move within a grain mass and the modifying influences upon such activity of biological factors (e.g. the presence of other weevils) and physical factors (e.g. the presence of boundaries that contribute to aggregation (SURTEES. 1964)), and differences in grain packing that may influence rates of dispersal (HOWE, 1951). Information from such studies may then permit comparison of metabolism measured directly by respirometry and indirectly by feeding studies in which. typically. a single weevil is confined within a small capsule (SINGH rt al., 19761. -l~~~r~~~~c~/crlyrr,~c,lrs~ I am most grateful to ROSLYN COOPER and GLORIA M~ILLIGAN for carrying out the expertment< described and to the Australian Wheat Board for financial support.
REFERENCES BIRCII. L. C. (1947) The oxygen consumption of the small strain of C&ntlrtr crrj’xr L. and Rhiroprrfhr domi,lic[r Fab. as affected by temperature and humidity. Ecnlog~~ 28, l7- 25. CI.O[ ~SLEY-THOMPSON. J. L. ( 1970) Terrestrial invertebrates. In Cumpurtrrirr Ph~~.sio/oyy of’ Thrmlorr~t~tltrtioil (Edited by G. C. WHITTOW). pp. 15-77. Academic Press. New York. E\ANS. D. E. (1977) Some aspects of acclimation to low temperatures in the grain weevils Sitophi/w\ oryxc (L.) and S. qrunwiu.s (L.). Aust. J. Ecol. 2, 309-3 18. E\ AKS. D. E. (1979) Further studies on acclimation to low temperatures m the grain weevils Sitophilus orvxc (L.1 and Sirqhilus qranurius (L.). Aust. J. Ecol. in press. HA2t.L. J. R. and PROSStR. C. L. (1974) Molecular mechanisms of temperature compensation in poikilotherms. F%~.sio/. Rec. 54. 62&677. Howr:. R. W. (1951) The movement of grain weevils through grain. Bull. cwt.Rus. 42, 125-134. NI wI.Lt.. R. C. (1973) EnvIronmental factors affecting the acclimatory responses of ectotherms. In Efl>c,r\ of Trmpewturr on Ectothermic Oryunisms (Edited by W. WIESER). pp. I51 164. Springer Verlag. Berlin. PI rRI.sfwI(‘z, K. and MACFADYEN. A. (1970) Prorluctirir!~ of Trrrrstricrl .-lrlimtr/\. Principles lrrd Mcthor/.~ Blackwell. Oxford. PROSSER. C. L. (1973) Temperature. In Compurutiw Animal PI1ysioloq~ (Edited by C. L. PROSSER). pp. 363 42X. Saunders, Philadelphia. SI\G~I. N. B.. CAMPBELL. A. and SINHA, R. N. (1976) An energy budget of Sitophih or~‘xc (Coleoptera: Curculionidae). .4nn. r,lt. Sot. .Amer. 69, 503-5 I?. SKIRTERS.G. (1964) Laboratory studies on dispersion behaviour of adult beetles in grain---VI. Three-dimen\ional analysis of five species in a uniform bulk. Bull. ent. Res. 55, l6l& 171. Wlt:ShR. W. (1973) Temperature relations of ectotherms: a speculative review. In @/&t,s q/’ Trmprraturr OH Ectothrrmic~ Orgunisms (Edited by W. WIESER), pp. l&23. Springer Verlag. Berlin. WIGHTMAN. J. A. (1977) Respirometery techniques for terrestrial invertebrates and their application to energetic studies. Nrn~ Zecllund J. Zoo/. 4, 453469. WINTliRINGHAw, F. P. W. (1959)An electrolytic respirometer for insects. Ltrh. Pruct. 8, 372- 375.
Divisionof Entomology. P.O. Box 1700. Canberra C‘ity.A.C.T. 2601, Audrali;i
The oxygen consumption of adult Sirophilus oryrtw (L.). S. qr
INTRODUCTION THE RELATIONSHIPbetween
oxygen consumption and temperature in adult rice weevils. Sitophilus oryzae (L.), has been studied by BIRCH (1947) and by EVANS ( 1977. 1979). BIRCH measured the oxygen consumption of warm-acclimated S. o~~~tre at several temperatures whereas EVANS measured the consumption of both warm and cold-acclimated weevils and found that warm-acclimated weevils used more oxygen than cold-acclimated weevils. EVANS (1977) obtained similar results with Sitophil~~ cl~trnc~ius (L.) and found that acclimation from 27 to 15”C, in both species. took 8- 10 days and re-acclimation to 27 C only 6-8 days. Later, EVANS (1979) showed, by closely restraining the weevils during respirometry, that cold-acclimation lowered the resting metabolism of S. o~~:tre at measurement temperatures below 23 C and that of S. gramrrius below 25 C. He was able to quantify the changes in active and resting metabolism that occurred when weevils were transferred from 27 to 15°C. The objective of the present study was to examine the effect of thermal acclimation and relative humidity on the respiration of the adults of three species of grain weevil. namel) S. or~~c~e. S. yrvrnarius and Sitophilus ~ecrrnuis Motsch. Respiration rate-temperature relationships were determined at 70 and 50”,, r.h. These humidities provided respective grain equilibrium moisture contents (m.c.) of 14.2 and 11.3”,,. To allow comparison with earlier studies (EVANS, 1977, 1979) the oxygen uptake of S. or~xe and S. ~JI.LIIIOI.~II.S acclimated at 70”,, r.h. was also measured at 94:” r.h. Several authors (CLOUDSLEY-THOMPSON. 1970; HAZEL and PROSSER. 1974: NEWEI.L. 1973: PROSSER, 1973; WIESER, 1973) have recently discussed thermal acclimation.
METHODS The S. oryzae and S. granarius used were taken from long-term CSIRO laboratory stocks: the S. zeamais were obtained from Savannah, U.S.A. in 1975. Two cultures of each species were established on 14.2% m.c. Olympic wheat at 27°C and 70?,, r.h. with an 87
88
D. E. EVANS
interval of 2 weeks between cultures. Several hundred < 24-hr-old weevils were obtained daily by sieving. The weevils from the two cultures were mixed so that the test insects would be representative of more than one culture and then redivided into two groups. One group was held at 27°C and 70% r.h. on 14.2”,; m.c. wheat and the other at 27°C and 500,; r.h. on 11.3?; m.c. wheat. After 7 days, some of each group were used for respirometry and the others were transferred to 15’C. Whilst at that temperature, the weevils were held at the same relative humidity as they had been at 27°C. The respirometry was repeated after S. oryzae had been acclimated for 2 weeks at 15’C and after the other species had been held at 15’C for 3 weeks. Rearing and acclimation temperatures were held within + 0.2”C and the desired relative humidities were maintained by saturated salt solutions. Oxygen consumption was measured with electrolytic respirometers (WINTERINGHAM, 1959; EVANS,1977). Sulphuric acid of appropriate concentrations was used as the electrolyte and provided a relative humidity of 94, 70 or 509;; within the respirometer flasks. The electrolyte was changed weekly. Carbon dioxide produced by the weevils was absorbed with Mallcosorb (Mallinkrodt). In experiments with unrestrained weevils, 10 unsexed weevils of known weight were placed with 25 grains of wheat of the appropriate moisture content in each respirometer flask. In other experiments, 10 S. oryzae were confined within the respirometers, without wheat, in 10 mm lengths of 2.8 mm bore polyethylene tube, the ends of which were closed with wire gauze. Preliminary experiments showed that the weevils aggregated at the ends of the small tubes and moved little. The experiments were set up and ended at approximately the same time of day. The weevils were allowed at least 1 hr to settle before a given measurement started: each measurement lasted 20 i 1 hr. Respirometer temperatures were maintained within i_ O.l’C. The statistical significance of the difference between the oxygen consumption of warmand cold-acclimated weevils at a given measurement temperature/humidity regimen was assessed by r-test. Following one-way analysis of variance, the significance of differences between treatment (i.e. humidity) means at a given measurement temperature was assessed by comparing the mean differences with their standard errors and referring to the t-distribution. Where variances could not be assumed to be equal, homogeneity of variances was examined, as appropriate, by F-test or by Bartlett’s test. RESULTS S. oryxe
The oxygen consumption of adult S. oryzae that had been reared and acclimated at 27°C and 70 or 50T;r.h. was measured at several constant temperatures immediately TABLE I. THEOXYGEN CONSUMPTION (&J OF CINRESTRAINED. WARM- AND COLD-ACCLIMATED S. or!'~urADULTS HELD FOR 20HR AT FOUR CONSTANT TEMPERATURES AND THREI:RELATIVE HUMIDITIES
Qo, and standard Measurement temperature
(~1 rng-
’ hr-‘)*
at 37 C
Acclimated
at 15 C
( C)
94”,, r.h.
70”,, r.h.
50”,, r.h.
94”,, r.h.
70” 0 r.h.
50” ,I r.h.
15
1.97 * 0.10 (11) 3.93 + 0.18 (12) 6.27 + 0.34 (11) 7.51 * 0.35 (12)
2.09 * 0.10 (II) 4.38 * 0.19 (11) 6.75 k 0.34 (11) 8.09 + 0.37 (11)
1.87 f 0.10 (11) 3.72 f 0.19 (11) 5.86 + 0.34 Ill) 8.34 * 0.35 (12)
1.51 + 0.08 (12) 3.10 * 0.15 (12) 5.52 + 0.27 (12) 6.69 ) 0.26 (12)
1.59 * 0.08 (111 3.37 * 0.15 (12) 5.92 + 0.27 (12) 6.59 + 0.26 (12)
1.61 + 0.08 (I?) 3.15 k 0.16 (11) 4.93 f 0.27 (17) 7.38 + 0.26 (12)
20 25 30
*
Acclimated
error
Values are based on the number of replicates indicated in parentheses: each replicate comprised 10 weevils of known fresh weight where W = 1.7 mg. Standard errors were derived from one-way analysis of variance. The Qo,s of weevils acclimated at 70”~r.h. were determined at 70 and 949,;r.h. whereas those of weevils acclimated at SO”/, r.h. were determined only at that humidity.
Thermal
Acclimation
R4TkkTEMPFRATL’RE
TARI E 7.
and Relative
REGRESSION
COMPONENTS
LINRESTRAINED s. 0ry;ur
Rclattvse humidity f”,,)
70
I5 50
37 15
* Where Q,,
‘,
( c-1 37 I5 37
is expressed
of Three Sit~~p/tr/~~.~ Spectes
AND
Q,,,s
FOR
THE
OXYGEN
X9
(‘ONSUMPTION
_ I.2833 - 1.5115 - 1.3561 - I.6911 - 1.0133 ~ 0.9660 in ;tl rng-
Ok
AT THREE RELATIVE HIIMIDITIES
Regression components tlogmQoz = 0 + hX + c?i2)*
Acclimation temperature
44
Humidity
h
Qlu Correlation coefficient
C’
0.132X
0.1468 0.1482 0.167X
-0.00’2 - 0.00~3 - 0.0024 - 0.00’8
0.1084
- 0.0015
0.0959
-0.0012
’ hr- I. and .!’ is measurement
0.9999 O.YYX9 O.YY9Y O.YYX6 O.YYY7 O.YYY4
temperature
15-25 C
20 30 C
3 .2 3.7 1.2 37 ;:I ‘.I
I .I) 2.2
I .x 2.0 2.2 ‘..3
in C
before and 2 weeks after the weevils were transferred to acclimate at 1.5 C. The consumption of weevils held at 700,, r.h. was determined at 94 as well as 70”,, r.h.; the consumption of weevils held at .50”,, r.h. was measured at that humidity. Un~est~~i~rtl ~~erils. At a given temperature and humidity, warm-acclimated (WA) weevils consumed significantly more oxygen (p < 0.05) than cold-acclimated (CA) weevils except in regimens of 50”,, r.h./lS”C and 50”,, r.h.j30’C (Table 1). WA weevils consumed consistently, but not always significantly, more oxygen at 70”,, r.h. than they did at 94”,, r.h. At 50”,, r.h.. over measurement temperatures of 15 to 35 C, WA weevils used consistently less oxygen than they did at 94”,,r.h. but only the values for 20 C differed significantly (p = 0.05). At 50”,, r.h./30 C, however, consumption was greater than at either 70 or 949,) r.h./30 ‘C. The oxygen uptake of CA weevils at 70”,, r.h. was greater between measurement temperatures of 15 and 25 C than it was at 94”,,r.h. but the differences between values for the two humidities were not statistically significant. At temperatures of 20 and 25 C uptake was lower at 50”,, r.h. than at 70”,, r.h., the values at 25 C differed significantly (p = 0.02). CA weevils held at 15 and 30 C consumed more oxygen at 50”,, r.h. than they did at either 70 or 94”,,r.h. When measured at 30 C. consumption at 50”,,r.h. was significantly greater than at 70”,, r.h. (p = 0.05). The relationships between oxygen consumption (Qo,) and measurement temperature were found to be described satisfactorily for each relative humidity (Table 2) by the quadratic function Y= N + hX + cX’ where Yis log,, Qo, in ,~l mgg’ hr-‘. X is the measurement temperature in C and the coefficients (I and c have negative values. QIos were computed for the temperature ranges 15 to 25 C and 20 to 30 C (Table 2): values for CA weevils were a little greater at 94 and 70”,, r.h. than those for WA weevils. i.e. CA weevils were somewhat more sensitive to temperature change than WA weevils at those humidities. From 25 to 3O”C, the mean Qto at 50”,,r.h. for both WA and CA weevils was 2.1 compared with 1.5 and 1.3 respectively at 94 and 70”,, r.h. Rruftuirwd ueeri/.s. Analysis of variance indicated that relative humidity had no significant effect on oxygen consumption at a given temperature. Because the humidity variTAI+I.I. 3. THF oxY(;r-2
~CJSIIMPTION
(Qo,)
OF RESTRAINFI),
WARM-
fiOR ?o HR AT FOUR COYSTANT
QO,and
Acclimation temperature
standard
AND COLD-ACCLIMATED
S. or~‘xt’
41)~ LTS HELD
TEMPFRATl‘RES
error ($ mg
I hr I)*
( C)
15 c
20 c
75 C
30 c
37 IS
0.70 * 0.02 0.58 * 0.01
1.01 f 0.01 0.92 * 0.01
1.4? * 0.03 1.46 * 0.02
1.90 + 0.02 1.84 i_ 0.03
* Values are based on the pooled data for 94. 70 and 50% r.h. and are for 36 replicates, each of 10 weevils. except for 77 C-acclimated weevils at 15 C which is for 34 replicates, The Qo.s of weevils acclimated at 70”,, r.h. were determined at 70 and 94”,, r.h. whereas those of weevils acclimated at 50”,, r.h. were determined only at that humidity.
90
D.E.
EVANS
ANDCOLD-AC‘C‘LIMATEDS. yrunarilts THREE RELATIVE HUMIDITIES
TARLE~.THEOXYGENCONSUMPTION(Q~~)OFUNRESTRAINED,WARMHELD FOR 20HR Al-FIVE CONSTANTTEMPERATURESAND
Qo, and standard Measurement temperature (‘C) 10 15 20 25 30
Acclimated 94”,, r.h. 0.70 2.19 3.51 5.98 7.55
+ 0.03 + 0.10 _+ 0.16 ) 0.22 k 0.18
(~1 rng-
’ hr I)*
at 27 C
Acclimated
70”;, r.h. 0.69 2.11 3.95 6.81 8.49
error
+ + k 5 +
50”,, r.h.
0.03 0.10 0.17 0.22 0.18
0.52 1.79 3.45 5.43 7.58
f * & * +
0.40 1.20 2.12 4.12 6.52
at 15 C
70” I, r.h.
94” 0 r.h.
0.03 0.10 0.16 0.22 0.18
ADULTS
+ 0.01 * 0.03 + 0.05 + 0.06 _+ 0.13
0.44 1.24 2.33 4.71 7.06
50” 0 r.h.
_t 0.01 f 0.03 2 0.05 * 0.06 _+ 0.13
0.38 1.13 2.06 4.04 6.62
* 0.01 +_ 0.03 + 0.05 + 0.06 & 0.13
* Values are based on 12 replicates except for the value for 27 ‘C-acclimated weevils at 2OC and 70”” r.h. which is for 11 replicates: each replicate comprised 10 weevils of known fresh weight where FJ = 2.6 m.g. Standard errors were derived from one-way analysis of variance. The Qois of weev*ils acclimated at 70”,, r.h. were determined at 70 and 94”,, r.h. whereas those of weevils acclimated at 50”,, r.h. were determined only at that humidity.
antes were not significantly different, the data for the three humidities were pooled. Mean values for oxygen uptake are given in Table 3. WA weevils used significantly more oxygen (p =
TABLE
Relative humidity f~<,) 94 70 50
* Where
Acclimation temperature
Regression components (log,, Qo, = u + hX + c/Y’)* (I
h
(’
27 15 27 15 27 15
- 1.2927 - 1.4170 - 1.3828 - 1.3750 - 1s-I93 -1.4156
0.1377 0.1 184 0.1466 0.1177 0.1565 0.1154
-0.0022 -0.0015 - 0.0023 -0.0015 - 0.0025 -0.0014
in ~1 mg-r
hr-‘.
and X
1smeasurement
OF UN-
Q,u Correlation coefficient
( Cl
Qo, is expressed
CONSUMPTION
0.99 I I 0.9976 0.9990 0.9989 0.9975 0.998 I
temperature
in “C.
ICr20 C 5.0 5.3 5.7 5.3 6.7 5.4
XL30 2.2 3.1 2.1 3.0 2.2 3.2
c
Thermal
T.4~1E 6. THF OXYGFN
Acclimation
CONSUMPTION
and Relative
Humidity
(Qol)OF UNRESTRAINED. WARM-
HE1.D FOR 20 HR AT FIVE CONSTANT
Sirophilus
of Three
Species
AND COLD-ACTLIMATED
TEMPERATURES AND TWO
YI s. zeumuis
ADLILTS
RELATIVE HUMlDITiES
-._-
Qol and standard Measurement trmperaturc
Acclimated
1 C)
0.42 0.99 2.24 3.56 5.00
I5 20 25 .;(I
+ * + + *
@I mg -’
0.38 0.79 1.76 2.86 4.03
at 15’ C
70”,, r.h.
50”,, r.h.
0.01 0.02 0.04 0.05 0.08
hr-‘I* Acclimated
at 27 C
70”,, r.h.
II)
error
f 0.01 _+ 0.0’ + 0.04 2 0.05 & 0.08
* Laluch xc based on 18 replicates: each replicate comprised in = 2.4 mg. Standard errors were derived from one-way analysis io”, r.h. prior to measurement at those humidities.
0.30 0.75 1.69 2.68 3.58
* + f f *
10 weevils variance.
50” 0 r.h.
0.01 0.01 0.03 0.03 0.05 of
0.28 0.65 1.48 2.38 3.16 known
fresh
& 0.01 * 0.01 * 0.02 * 0.03 +_ 0.05
weight
Weevilswereacclimatedat
where 70 or
was noted that observed values at 2Cr’C were consistently somewhat lower than values expected from the quadratic functions. At 94”,, r.h., for example, the observed values of QO, at 20 C for WA and CA weevils respectively were 3.51 and 2.12 ~1 mg-’ hr- ’ compared with corresponding expected values of 3.86 and 2.29 ~1 mg- ’ hr- ‘. At 70”,, and at 50”:cr.h.. particularly, the QlOs of WA weevils were greater than those of CA weevils between 10 and 20°C. By contrast, the Q t ,g of WA weevils over 20 to 30’-‘C were consistently lower than those of CA weevils. The QlOs of WA weevils over 10 to 20°C increased from 5.0 to 6.7 as humidity decreased from 94 to 50”;r.h. but differed little with humidity over 20 to 30°C. By contrast. the Qlos of CA weevils changed little with humidity over either temperature range.
The oxygen consumption of unrestrained adult S. ZXKLZ~Sthat were acclimated at 27 C and 70 or 50”,, r.h. was measured immediately before and 3 wk after transfer to 15 C. Qo: was determined at five constant temperatures in relative humidities of 70 or 50”,, (Table 6). WA weevils consumed significantly more oxygen (p < 0.001) than CA weevils at all measurement temperatures regardless of humidity. Consumption was significantly greater at 70”,,r.h. (p = 0.001) than at 50”,,r.h. irrespective of both measurement and acclimation temperature. The relationships between the logarithm of oxygen consumption and measurement temperature were again adequately described by quadratic functions (Table 7). The Qlos of WA and CA weevils were similar at both humidities. DISCUSSJON Llnrestrained adults of all three Sitophil~rs species consumed more oxygen in almost all experimental regimens when warm-acclimated than they did when cold-acclimated: this finding accords with previous observations at 94’,, r.h. (EVANS. 19771. TAB!,
7. RArf-TCMI'I~RAKRI
REGRESSION cOMPDNENTS RLSTRAINED S. mzmuis
AND Qlos FOR THE OXYGEN AT 70 AND so",,R.H.
CONSUMPTION
OF I:N'-
-.--~ Regression Rcla~i\e Acclimation tcmpcrature 111lllll~iI~~ I”,,)
( CI
70
27
50
IS 2; 15
* Where
Qo, is expressed
(log,,
u - I .4428 - 1.6689 - 1.3755 - 1.6442 in ~1 mg-’
components
Q,o
Q<)> = tl + hX + <,X2)*
hr-‘,
h
c
0.1232 0.1339 0.1076 0.1264
-0.0017 - 0.0020 -0.0010 -0.0018
and X is measurement
Correlation coefficient 0.9994 0.9996 0.9989 0.9992 temperature
I@20 5.3 5.6 5.3 5.3 in ‘C.
c
15-25 3.6 3.6 3.6 3.6
c
20-30 2.2 2.1 2.3 2.1
C
92
D. E. EVANS
S. oryzae generally used more oxygen, when unrestrained, at 70:/Or.h. than at 949;, r.h., the humidity at which previous observations were made (EVANS, 1977). The relatively high oxygen consumption of unrestrained S. oryzao in regimens of 50”,,r.h./30”C when warm-acclimated and SOY,;r.h./lYC and 3O’C when cold-acclimated suggests that there was an interaction between aridity and relatively low and high experiment temperatures. The fact that the mean Q 10 for WA and CA weevils over 25-30°C was greater at SOY,r.h. than at 94 and 70”/:,r.h. is a further indication of an interaction. Because relative humidity had no significant effect on the oxygen uptake of restrained S. ortlzae, it is clear that the relatively high consumption of unrestrained weevils under some regimens must be attributed to differences in locomotor activity. The similarity of the oxygen consumption of WA and CA restrained S. oryzae at 25 and at 3O’C confirms other studies with this species (EVANS,1979). The values for oxygen consumption at 94:; r.h. were greater in absolute terms than those observed or expected by interpolation from a previous study (EVANS, 1977). For reasons that are not understood, the metabolic rate of grain weevils may differ even when apparently identical rearing and experimental procedures are used. The differences may extend not only to the absolute Qo, values observed but also to the proportional changes that follow cold-acclimation. For example, the QoJs of 27 ‘C-acclimated weevils at 94:/;, r.h. before, immediately after and 28 days after transfer to 15°C were earlier found to be 5.91, 1.17 and 0.81 pl mg-’ hr-’ (EVANS, 1977). Equivalent values from the present study were 6.99 (estimated value), 1.97 and 1.51 ~_dmg- ’ hr- ‘. Acclimation at 15 ‘C lowered active metabolism by 31”, in the earlier study and by 237, in the present. The measurement of oxygen consumption in both unrestrained and restrained S. or).zae in this study, using weevils from the same series of cultures, allows an earlier conclusion on the relative importance of resting and active metabolism in cold-acclimation to be examined. EVANS(1977) concluded that 0.15 ~1 mg-’ hr- ‘, or 42”o, of the change of 0.36 1.11 mg- ‘hr-’ (i.e. the diffe rence between the QOJs of WA and CA weevils measured at 15’C) in the consumption of unrestrained weevils acclimated from 27 to 15 C was due to a reduction in resting metabolism. In the present study, the equivalent value (from Table 3) was found to be 0.12 ~1 mg-’ hr-‘, or 26?,,,, of the overall change of 0.46 111 mg-’ hr- ’ (from Table 1). On the bases of the two estimates, it appears reasonable to conclude that about one-third of the change in overall metabolism that occurs during acclimation at 15’C is due to reduced basal metabolism and about two-thirds is due to decreased locomotor activity. The oxygen consumption of unrestrained WA S. grunarius at 949,,r.h. was similar to that expected from the previous study (EVANS, 1977) but the CA weevils in the present study used considerably more oxygen at 20°C and above than in the earlier experiment. There was evidence of an interaction between humidity and experiment temperature in that the oxygen consumption of both WA and CA weevils was relatively high when measured at 509, r.h. and 3O’C. Humidity and acclimation temperature also appeared to interact in that the Qlos of WA weevils over 10 to 20°C changed with humidity whereas those of CA weevils did not. Furthermore, the Qlo of WA weevils between 10 and 2OC at 50’?; r.h. was higher than that of CA weevils. The finding that observed Qo, values for S. granarius at 20°C were consistently lower than expected values supports an earlier suggestion (EVANS,1977) that there is a discontinuity in the respiration rate-temperature relationship of S. granarius at about 20°C. Similarly, the lower Qlos of CA weevils between 10 and 20°C at 70 and 50q/,r.h. support the conclusion (EVANS, 1977) that cold-acclimation in S. granaritts involves a lowered sensitivity to temperature changes below about 20°C. The rate-temperature relationships of S. zeamais are similar to those of S. oryzar in that Qlos were higher at low than at high temperatures and single quadratic functions fitted the data well. The Qlos for WA and CA S. zeamais were similar and it seems. therefore, that cold-acclimation in this species involved lowered rates of consumption but not changed temperature sensitivity. Unlike S. oryzae and S. granarius, the oxygen consumption of S. zeamais at 50% was consistently lower than at 7O?dr.h. and there was no
Thermal Acclimation and Relative Humidity of Three
Sitophihrs
Species
Y?
evidence of an interaction between humidity and either experiment or acclimation temperature. It could be, however, that an interaction might be revealed if consumption were measured at temperatures above 30°C. The applicability, in terms of absolute QOJ values, of the results of this study to grain storage conditions are difficult to assess. Even when differences between populations of a given species (EVANS, 1977) are allowed for, experimental estimates of respiration may vary greatly with the type of respirometer used and the methods employed, particularly as regards the amount of activity allowed to the test organisms within the respirometer (PETRUSEWICZ and MACFADYEN, 1970; WIGHTMAN, 1977). For example, BIRCH (1947) and SIVGH L’Itrl. (1976) confined adult S. or~zur in cloth bags within Warburg respirometers and obtained Qoz values for 30”C-acclimated weevils (assuming a fresh weight of 1.7 mg at 3O’C;7Op, r.h. per weevil) equivalent to about 3.2 and 2.4 ~1 mg- ’ hr-’ respectively By contrast, unrestrained and restrained 27.C-acclimated S. ory~r in the present study consumed respectively 8.1 and 1.9 111mg- I hr-’ at 30 C,/‘70”,,r.h. (Tables 1 and 3). Clearly. further work is required to determine the effect of locomotor activity on the respiration of weevils in ‘typical’ grain storage conditions where weevils are free to move within a grain mass and the modifying influences upon such activity of biological factors (e.g. the presence of other weevils) and physical factors (e.g. the presence of boundaries that contribute to aggregation (SURTEES. 1964)), and differences in grain packing that may influence rates of dispersal (HOWE, 1951). Information from such studies may then permit comparison of metabolism measured directly by respirometry and indirectly by feeding studies in which. typically. a single weevil is confined within a small capsule (SINGH rt al., 19761. -l~~~r~~~~c~/crlyrr,~c,lrs~ I am most grateful to ROSLYN COOPER and GLORIA M~ILLIGAN for carrying out the expertment< described and to the Australian Wheat Board for financial support.
REFERENCES BIRCII. L. C. (1947) The oxygen consumption of the small strain of C&ntlrtr crrj’xr L. and Rhiroprrfhr domi,lic[r Fab. as affected by temperature and humidity. Ecnlog~~ 28, l7- 25. CI.O[ ~SLEY-THOMPSON. J. L. ( 1970) Terrestrial invertebrates. In Cumpurtrrirr Ph~~.sio/oyy of’ Thrmlorr~t~tltrtioil (Edited by G. C. WHITTOW). pp. 15-77. Academic Press. New York. E\ANS. D. E. (1977) Some aspects of acclimation to low temperatures in the grain weevils Sitophi/w\ oryxc (L.) and S. qrunwiu.s (L.). Aust. J. Ecol. 2, 309-3 18. E\ AKS. D. E. (1979) Further studies on acclimation to low temperatures m the grain weevils Sitophilus orvxc (L.1 and Sirqhilus qranurius (L.). Aust. J. Ecol. in press. HA2t.L. J. R. and PROSStR. C. L. (1974) Molecular mechanisms of temperature compensation in poikilotherms. F%~.sio/. Rec. 54. 62&677. Howr:. R. W. (1951) The movement of grain weevils through grain. Bull. cwt.Rus. 42, 125-134. NI wI.Lt.. R. C. (1973) EnvIronmental factors affecting the acclimatory responses of ectotherms. In Efl>c,r\ of Trmpewturr on Ectothermic Oryunisms (Edited by W. WIESER). pp. I51 164. Springer Verlag. Berlin. PI rRI.sfwI(‘z, K. and MACFADYEN. A. (1970) Prorluctirir!~ of Trrrrstricrl .-lrlimtr/\. Principles lrrd Mcthor/.~ Blackwell. Oxford. PROSSER. C. L. (1973) Temperature. In Compurutiw Animal PI1ysioloq~ (Edited by C. L. PROSSER). pp. 363 42X. Saunders, Philadelphia. SI\G~I. N. B.. CAMPBELL. A. and SINHA, R. N. (1976) An energy budget of Sitophih or~‘xc (Coleoptera: Curculionidae). .4nn. r,lt. Sot. .Amer. 69, 503-5 I?. SKIRTERS.G. (1964) Laboratory studies on dispersion behaviour of adult beetles in grain---VI. Three-dimen\ional analysis of five species in a uniform bulk. Bull. ent. Res. 55, l6l& 171. Wlt:ShR. W. (1973) Temperature relations of ectotherms: a speculative review. In @/&t,s q/’ Trmprraturr OH Ectothrrmic~ Orgunisms (Edited by W. WIESER), pp. l&23. Springer Verlag. Berlin. WIGHTMAN. J. A. (1977) Respirometery techniques for terrestrial invertebrates and their application to energetic studies. Nrn~ Zecllund J. Zoo/. 4, 453469. WINTliRINGHAw, F. P. W. (1959)An electrolytic respirometer for insects. Ltrh. Pruct. 8, 372- 375.