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Differential net food utilization by larvae of Sitophilus oryzae and Sitophilus granarius
J. Insect Physid., 1974, Vol. 20, pp. 1937 to 1942. Pergamon Press. Printed in Great &it&
DIFFERENTIAL NET FOOD UTILIZATION BY LARVAE OF SITOPHILUS ORYZAE AND SITOPHILUS
GRANARIUS
*
J. E. BAKER Stored-Product
Insects Research and Development Laboratory, Agric. Res. Serv., USDA, Savannah, Georgia 31403, U.S.A. (Received 8 lMurch 1974)
Abstract-The
net utilization of a meridic diet by Sitophilus oryzae from the newly hatched larva to the pupa was more efficient compared with that of S. granarius. The artificial diet was highly digestible to both species, but S. oryzae converted 24.1 per cent of the ingested food into body substance compared with 12.4 per cent converted by S. granarius. The mean pupal weight of S. granarius (1402 mg) was greater than that of S. oryzue (I.012 mg); however, larvae of S. granarius consumed 2.6 times as much food and excreted 8.1 times as much faecal material to obtain that weight. Even though S. granarius consumed the diet at a greater rate, the relative growth rate of S. oryzae was faster. The intracellular symbiotes present in S. oryzae may have a r81e in the species’ faster development and more efficient utilization of its food. INTRODUCTION
THE GROWTHand development of larvae of the rice weevil, Sitophilus oryzae (L.), are faster than that of larvae of the closely related granary weevil, S. granaries (L.), whether the insects are reared in intact whole grain or on a meridic diet (BAKER and MABIE, 1973a). On the meridic diet S. oryzae developed to the pupal stage approximately 8.5 days sooner than did S. granarius. These differential developmental rates could arise because of differences in size or differences in behavioural mechanisms that result in an increased rate of food intake. However, S. oryxae could be more efficient than 5’. granarius in converting the dietary nutrients into body substance since S. oryxae does utilize certain dietary sterols more efficiently than S. granarius, probably because of an associated bacteria-like symbiote (BAKER, 1974). A detailed study of the net consumption and utilization of the meridic diet by these two species was undertaken to obtain more comparative information. The results are presented in this paper. MATERIALS
AND
METHODS
Newly hatched larvae of S. oryzae and S. granarius were individually reared in numbered glass tubes at 30.0 + 0*5”C and 50 to 60% r.h. on the meridic diet of BAKER and MABIE (1973a). The time (T, days) to pupation was recorded for each * Mention of a commercial or proprietary product in this paper does not constitute an endorsement of this product by the USDA. 1937
1938
J, E. BAKER
larva. The newly formed pupae were carefully removed, dried for 1 hr at lO.YC, and weighed on a Cahn Gram Electrobalance@ (Model G). The dry pupal weight was taken as the net dry weight increase (AI%‘) since the weight of each newly hatched larva was negligible. The amount of faeces in the uneaten diet was calculated by (1) determining the uric acid content of the uneaten diet, and (2) estimating the percentage of uric acid in the larval faeces of both species by using the enzyme-spectrophotometric method and WALDBAUER(1969) as follows : of LIDDLE et al. (1959) and BHATTACHARYA The dried, uneaten food in each tube was tapped out and homogenized in 2.0 ml of 0.6% solution of LisCO,. After centrifugation the supernatant was decanted into a 15 ml centrifuge tube and the residue was similarly extracted three times with 1.0 ml of 0.6% Li,CO,. Because some diet and faeces remained on the sides of the tubes, each tube was crushed in a heavy-duty glass homogenizer and the mixture was extracted twice with 1.0 ml of 0*60/O Li,CO,. These supernatants were combined with the original supernatant and brought to a known volume. Then 1-O ml aliquots were used for the analysis. Type I, purified hog liver uricase (Sigma Chemical Co.) was used as the. enzyme source. The differential absorbance at 292 nm was determined on a Beckman DU-2 spectrophotometer. The uric acid content of each tube was corrected for a trace amount of differential absorbance (equivalent to l-2 ,ug of uric acid) found in the Li,CO, extract of control tubes with diet only. The percentage of uric acid in the larval faeces was determined as follows: Groups of ca. 50 larvae of each species were set up individually on the diet. S. oryxae larvae were removed after 9 days and S. granarius larvae were removed after 15 days. The larvae were cleaned in an air stream and placed in a Petri dish. The faeces that were excreted during the next 16 hr were collected and stored at -20°C. The faecal samples were dried for 1 hr at 105”C, weighed on the electrobalance, and homogenized with O-5 ml L&CO,. After centrifugation, the residues were re-extracted five times with O-2 ml of LisCO,. The combined supernatants were adjusted to 2-O ml and aliquots used for the analysis. The mean percentage of uric acid was calculated from duplicate or triplicate analyses of two faecal The corrected uric acid content of each samples from S. oryxae and S. granaks. tube was divided by the appropriate percentage of uric acid in the pure faeces to obtain the amount of faeces (AS) produced during the total larval feeding period. The amount of dry diet consumed per larva (AF) was determined as follows: The rearing tubes were prepared as described by BAKERand MAHE (1973b) except the tubes were cut shorter-to a length of 2-O to 2.3 cm. Tare weights were determined for each tube by suspending the tubes from a wire loop on the B range of the electrobalance. After the tubes were tared, a newly hatched larva and diet were added and the tube was reweighed. The fresh weight of the diet was determined by difference. The dry weight of the initial diet was determined by using the mean dry matter content of triplicate 1.0 g samples of the diet dried at 105°C for 1 hr. At the end of the feeding period and after the removal of the newly formed pupa, each tube containing the uneaten food plus faeces was dried by
UTILIZATION OF FOODBY
SITOPHILUS
LARVAE
1939
heating for 1 hr at 105°C and weighed. The final dry weight of the uneaten food plus faeces was obtained by subtracting the tare weight of the tube. Six control tubes without larvae were also set up and the mean difference between the initial and final dry weights of these tubes was subtracted from the total weight change determined for each test-tube. Finally, the faecal content of each tube was subtracted from the corrected total weight change of each tube to obtain AF, the dry weight of the food consumed by each larva. Thus, for this study chemical markers and gravimetric procedures were used and the indices obtained were calculated according to GORDON (1968) and WALDBAUER (1968). The consumption index (C.I.) was calculated by using the exponential mean larval fresh weight (GORDON, 1968) and the dry weight of the food consumed. The duration of the larval feeding period was taken as 1 day less than the mean time to pupation to account for the prepupal period. The relative growth rate (G.R.) was calculated by using the exponential mean larval dry weight. RESULTS An average uric acid content of 10.00%
was determined for the larval faeces of S. c~y,z~~e.This figure was the mean of two mean values, 12.10 and 7.90%, found in samples with dry weights of 380 and 1020 pg, respectively. Similarly, uric acid constituted 7.62% of the faeces of S. granaries. This figure was based on means of 6.40 and 8+35~o for faecal samples with weights of 1502 and 1392 ,ug. The AS values calculated with these percentages are given in Table 1. TABLE I-PARAMETERS ANDINDICESCOMPARING THE NET UTILIZATION OF A MRRIDIC DIET BY LARVAEOF S. oryzae ANDS. granarius REARED AT 30°C AND55% r.h.
Species S. oryzae S. granarius
S. oryzae S. granarim
Uric acid excreted
n
Days to pupation
20 14
14.5 * 1.3 21.3 f 2.9
4.35 &-0.23 11.43 f 0.47
A.D.
E.C.I.
E.C.D.
95.8+ o-5 87.7+ 1-o
24.1rt1.4 12.4AZ0.5
24.9+ l-7 14.2iz0.6
20 14
(ccg> 16.7 + 1.9 106.4 + 95
0.172 + 0.019 1.012 c 0.048 1.395 + 0.125 1402 & 0.057
AF, Food consumed; AS, faeces produced; AW, net weight gain; A.D., approximate digestibility; E.W., efficiency of conversion of ingested food (%); and E.C.D., efficiency of conversion of digested food (%). All values are means + S.E. based on dry weights and expressed on a per larva basis.
The exponential mean fresh larval weights were calculated to be 0.369 mg for S. oryxae and O-477 mg for S. granarius. From these values, mean C.I.‘s of O-87 and 1.18 mg dry food consumedjmg fresh body weight per day were calculated for
1940
J. E. BAKER
S. oryxae and S. granarius, respectively. The G.R. for S. oryxae, calculated from the mean time to pupation and the mean AW, was O-513 mg dry weight gain/mg dry body weight per day compared with a G.R. of O-374 for S. granarius. The remaining parameters and indices (A.D. = approximate digestibility, E.C.I. = efficiency of conversion of ingested food into body substance, and E.C.D. = efficiency of conversion of digested food into body substance) are given in Table 1. Larvae of both species secrete a fluid that lines the pupation chambers in the rearing tubes. Corrections for the weight of this material were not attempted, but the E.C.I. and E.C.D. values for both species would be lowered slightly. Also, the AS values were not corrected for urine content. DISCUSSION
Larvae of S. oryxae were more efficient than those of S. granarius in the digestion of the food and the net conversion of both ingested and digested food into body substance as evidenced by the higher A.D., E.C.I., and E.C.D. values. Previous studies indicated that the maximum mean fresh larval weights of both species reared on this diet were similar (ca. 3.00 mg) although adults of S. oryzae weighed ca. 10 per cent less than those of S. granarius (BAKER and MABIE, 1973a, b). In this study A WIT was comparable for both species ; however, the relative growth rate for S. oryxae was greater. The shorter developmental time of S. oryxae was therefore not strictly due to their smaller net weight gain. Similarly, the percentage difference in AW between S. oryxae and S. granarius was not comparable to the much greater amount of food consumed by S. granarius to obtain that weight. Since the rate of consumption of the diet by S. granarius was actually greater than that of S. oryzae, the faster rate of development of S. oryxae is apparently due to the efficiency with which this species utilizes -its food. There was a striking difference in the amount of faeces voided by the two species. S. granarius excreted 6.4 times as much uric acid and 8.1 times as much faecal material as S. oryzae. This difference was also apparent when faecal material was collected. It took many more S. myzae larvae to obtain sufficient quantities of excreta for analysis. If uric acid is one of the major end-products of N metabolism, one can speculate on the fate of uric acid in larvae of S. oryxae as follows : (1) The major portion of the uric acid may be stored during the larval and pupal stages. GUPTA and SINHA (1960) demonstrated storage of uric acid and its salts in the fat body of adult S. granarius. Also, in another curculionid, Anthonomus grandis Boh., tissue levels of uric acid increased with larval age, reached a peak in the pupal stage, and declined in the adult, the major portion apparently being voided in the meconium (MITLIN AND MAULDIN, 1966). (2) A n active uricase present in S. oryzae or released by its associated intracellular symbiotes could also explain the decreased levels of uric acid. MITLIN and WIYGUL (1973) demonstrated that the carbon of 14C-uric acid injected into adult A. grandis was incorporated principally into amino and nucleic acids. In this case, the insects themselves apparently have the requisite enzymes since there are no associated symbiotes described for this species.
UTILIZATIONOF FOODBY SITOPHILUS LARVAF
1941
Because of the relatively large amounts of uric acid excreted by S. granarius and the lack of symbiotes in the Savannah strain of this species (LUM and BAKER, 1973), it probably has no uricase. However, in the present study none of the faecal material of either species was analysed directly for allantoin or allantoic acid. (3) The intracellular symbiotes found in S. oryzae may be able to utilize the haemolymph uric acid as an N source and in the process depress the amount of total uric acid excreted. A similar hypothesis was put forward by JURZITZA (1972) who demonstrated that symbiotic larvae of Lasioderma sewicorne (F.) could utilize dietary uric acid but that larvae whose fungal symbiotes were removed could not. If a similar situation exists in S. oryzae, the bacteria-like micro-organisms may assist the larvae in the metabolism of N or may utilize the uric acid N for the synthesis of amino acids or other nitrogenous compounds required for their own growth. Subsequently, the bacteria could provide the larvae with some factor that allows reactions involved in the breakdown and utilization of food to proceed at a maximum rate. The first two hypotheses do not, by themselves, account for the reduction in AS; however, the third hypothesis could explain both the reduced AS (principally a measure of undigested food) and the reduction in the excretion of uric acid by S. myzae. To obtain the same amount of limiting factor from the diet, the asymbiotic larvae of S. granarius, on the other hand, must consume more food and in the process, waste (or not utilize optimally) many of the nutrients consumed in excess. This situation could also explain the much larger AS of S. granarius. Studies of net food utilization with larvae of Tribolium conjkum reared on cracked or ground wheat gave A.D. values of 58.4 and 52.2 per cent, respectively (BHATTACHARYA and WALDBAUER, 1970). The E.C.D. values on both of these natural diets was cu. 8-O per cent. The physical texture as well as nutrient availability and balance of the artificial diet used in the present study are apparently responsible for its extremely high digestibility by both Sitophilus spp. and the relatively more efficient utilization of nutrients. The utilization of whole grain by these Sitophilus spp. may be somewhat less efficient. Nevertheless, the factors that increase the efficiency of food utilization by S. oryxae on the artificial diet may be of even more importance on certain natural products. Differences in the indices between S. wyxae and S. granarius may be even more exaggerated, especially if the symbiotes of S. oryxae are involved and the diet is suboptimal. Acknowledgement-The author would like to thank D. S. FAUSTfor excellent technical assistance and P. T. M. LUM of this laboratory for helpful discussion during the course of this study.
REFERENCES BAKERJ. E. (1974) Differential sterol utilization by larvae of Sitophilus oryzae and Sitophilus granarius. Ann. ent. Sot. Am. In press. BAKERJ. E. and MABIE J. M. (1973a) Growth responses of larvae of the rice weevil, maize weevil, and granary weevil on a meridic diet. J. econ. Ent. 66, 681-683.
1942
J. E. BAKER
BAKERJ. E. and MABIE J. M. (1973b) Growth and development of larvae of the granary weevil, SitophiZus grunarius (Coleoptera: Curculionidae), on natural and meridic diets. Can. Ent. 105, 249-256. BHATTACHARYA A. K. and WALDBAUER G. P. (1969) Q uantitative determination of uric acid in insect feces by lithium carbonate extraction and the enzymatic-spectrophotometric method. Ann. ent. Sec. Am. 62, 925-927. BHATTACHARYA A. K. and WALDBAUERG. P. (1970) Use of the faecal uric acid method in measuring the utilization of food by Tribolium confusum. J. Insect Physiol. 16, 1983-1990. GORDONH. T. (1968) Quantitative aspects of insect nutrition. A,m. 2002. 8, 131-138. GUPTA P. D. and SINHA R. N. (1960) Excretion and its products in some stored-graininfesting beetles. Ann. ent. Sot. Am. 53, 632-638. JURZITZAV. G. (1972) Die fur&ion der endosymbiontischen Pilze des Tabakkiifers Lasiodwma serricorne F. (Cal., Anobiidae) bei einer aufzucht unter Proteinmangelbedingungen. 2. angew. Ent. 72, 198-205. LIDDLE L., SEEGMILLER J. E., and LASTERL. (1959) The enzymatic spectrophotometric method for determination of uric acid. r. Lab. clin. Med. 54, 903-913. LUM P. T. M: and BAKERJ. E. (1973) Development of mycetomes in larvae of Sitophilus granarius and S. oryzae. Ann. ent. Sot. Am. 66, 1261-1263. MITLIN N. and MAULDINJ. K. (1966) Uric acid in nitrogen metabolism of the boll weevil: a preliminary study. Ann. ent. Sot. Am. 59, 651-653. MITLIN N. and WIYGUL G. (1973) Uric acid in nucleic and amino acid synthesis in the boll weevil, Anthonomus grandis. J_ Insect Physiol. 19, 1569-l 574. WALDBAUERG. P. (1968) The consumption and utilization of food by insects. Adw. Insect Physiol. 5, 229-288.
DIFFERENTIAL NET FOOD UTILIZATION BY LARVAE OF SITOPHILUS ORYZAE AND SITOPHILUS
GRANARIUS
*
J. E. BAKER Stored-Product
Insects Research and Development Laboratory, Agric. Res. Serv., USDA, Savannah, Georgia 31403, U.S.A. (Received 8 lMurch 1974)
Abstract-The
net utilization of a meridic diet by Sitophilus oryzae from the newly hatched larva to the pupa was more efficient compared with that of S. granarius. The artificial diet was highly digestible to both species, but S. oryzae converted 24.1 per cent of the ingested food into body substance compared with 12.4 per cent converted by S. granarius. The mean pupal weight of S. granarius (1402 mg) was greater than that of S. oryzue (I.012 mg); however, larvae of S. granarius consumed 2.6 times as much food and excreted 8.1 times as much faecal material to obtain that weight. Even though S. granarius consumed the diet at a greater rate, the relative growth rate of S. oryzae was faster. The intracellular symbiotes present in S. oryzae may have a r81e in the species’ faster development and more efficient utilization of its food. INTRODUCTION
THE GROWTHand development of larvae of the rice weevil, Sitophilus oryzae (L.), are faster than that of larvae of the closely related granary weevil, S. granaries (L.), whether the insects are reared in intact whole grain or on a meridic diet (BAKER and MABIE, 1973a). On the meridic diet S. oryzae developed to the pupal stage approximately 8.5 days sooner than did S. granarius. These differential developmental rates could arise because of differences in size or differences in behavioural mechanisms that result in an increased rate of food intake. However, S. oryxae could be more efficient than 5’. granarius in converting the dietary nutrients into body substance since S. oryxae does utilize certain dietary sterols more efficiently than S. granarius, probably because of an associated bacteria-like symbiote (BAKER, 1974). A detailed study of the net consumption and utilization of the meridic diet by these two species was undertaken to obtain more comparative information. The results are presented in this paper. MATERIALS
AND
METHODS
Newly hatched larvae of S. oryzae and S. granarius were individually reared in numbered glass tubes at 30.0 + 0*5”C and 50 to 60% r.h. on the meridic diet of BAKER and MABIE (1973a). The time (T, days) to pupation was recorded for each * Mention of a commercial or proprietary product in this paper does not constitute an endorsement of this product by the USDA. 1937
1938
J, E. BAKER
larva. The newly formed pupae were carefully removed, dried for 1 hr at lO.YC, and weighed on a Cahn Gram Electrobalance@ (Model G). The dry pupal weight was taken as the net dry weight increase (AI%‘) since the weight of each newly hatched larva was negligible. The amount of faeces in the uneaten diet was calculated by (1) determining the uric acid content of the uneaten diet, and (2) estimating the percentage of uric acid in the larval faeces of both species by using the enzyme-spectrophotometric method and WALDBAUER(1969) as follows : of LIDDLE et al. (1959) and BHATTACHARYA The dried, uneaten food in each tube was tapped out and homogenized in 2.0 ml of 0.6% solution of LisCO,. After centrifugation the supernatant was decanted into a 15 ml centrifuge tube and the residue was similarly extracted three times with 1.0 ml of 0.6% Li,CO,. Because some diet and faeces remained on the sides of the tubes, each tube was crushed in a heavy-duty glass homogenizer and the mixture was extracted twice with 1.0 ml of 0*60/O Li,CO,. These supernatants were combined with the original supernatant and brought to a known volume. Then 1-O ml aliquots were used for the analysis. Type I, purified hog liver uricase (Sigma Chemical Co.) was used as the. enzyme source. The differential absorbance at 292 nm was determined on a Beckman DU-2 spectrophotometer. The uric acid content of each tube was corrected for a trace amount of differential absorbance (equivalent to l-2 ,ug of uric acid) found in the Li,CO, extract of control tubes with diet only. The percentage of uric acid in the larval faeces was determined as follows: Groups of ca. 50 larvae of each species were set up individually on the diet. S. oryxae larvae were removed after 9 days and S. granarius larvae were removed after 15 days. The larvae were cleaned in an air stream and placed in a Petri dish. The faeces that were excreted during the next 16 hr were collected and stored at -20°C. The faecal samples were dried for 1 hr at 105”C, weighed on the electrobalance, and homogenized with O-5 ml L&CO,. After centrifugation, the residues were re-extracted five times with O-2 ml of LisCO,. The combined supernatants were adjusted to 2-O ml and aliquots used for the analysis. The mean percentage of uric acid was calculated from duplicate or triplicate analyses of two faecal The corrected uric acid content of each samples from S. oryxae and S. granaks. tube was divided by the appropriate percentage of uric acid in the pure faeces to obtain the amount of faeces (AS) produced during the total larval feeding period. The amount of dry diet consumed per larva (AF) was determined as follows: The rearing tubes were prepared as described by BAKERand MAHE (1973b) except the tubes were cut shorter-to a length of 2-O to 2.3 cm. Tare weights were determined for each tube by suspending the tubes from a wire loop on the B range of the electrobalance. After the tubes were tared, a newly hatched larva and diet were added and the tube was reweighed. The fresh weight of the diet was determined by difference. The dry weight of the initial diet was determined by using the mean dry matter content of triplicate 1.0 g samples of the diet dried at 105°C for 1 hr. At the end of the feeding period and after the removal of the newly formed pupa, each tube containing the uneaten food plus faeces was dried by
UTILIZATION OF FOODBY
SITOPHILUS
LARVAE
1939
heating for 1 hr at 105°C and weighed. The final dry weight of the uneaten food plus faeces was obtained by subtracting the tare weight of the tube. Six control tubes without larvae were also set up and the mean difference between the initial and final dry weights of these tubes was subtracted from the total weight change determined for each test-tube. Finally, the faecal content of each tube was subtracted from the corrected total weight change of each tube to obtain AF, the dry weight of the food consumed by each larva. Thus, for this study chemical markers and gravimetric procedures were used and the indices obtained were calculated according to GORDON (1968) and WALDBAUER (1968). The consumption index (C.I.) was calculated by using the exponential mean larval fresh weight (GORDON, 1968) and the dry weight of the food consumed. The duration of the larval feeding period was taken as 1 day less than the mean time to pupation to account for the prepupal period. The relative growth rate (G.R.) was calculated by using the exponential mean larval dry weight. RESULTS An average uric acid content of 10.00%
was determined for the larval faeces of S. c~y,z~~e.This figure was the mean of two mean values, 12.10 and 7.90%, found in samples with dry weights of 380 and 1020 pg, respectively. Similarly, uric acid constituted 7.62% of the faeces of S. granaries. This figure was based on means of 6.40 and 8+35~o for faecal samples with weights of 1502 and 1392 ,ug. The AS values calculated with these percentages are given in Table 1. TABLE I-PARAMETERS ANDINDICESCOMPARING THE NET UTILIZATION OF A MRRIDIC DIET BY LARVAEOF S. oryzae ANDS. granarius REARED AT 30°C AND55% r.h.
Species S. oryzae S. granarius
S. oryzae S. granarim
Uric acid excreted
n
Days to pupation
20 14
14.5 * 1.3 21.3 f 2.9
4.35 &-0.23 11.43 f 0.47
A.D.
E.C.I.
E.C.D.
95.8+ o-5 87.7+ 1-o
24.1rt1.4 12.4AZ0.5
24.9+ l-7 14.2iz0.6
20 14
(ccg> 16.7 + 1.9 106.4 + 95
0.172 + 0.019 1.012 c 0.048 1.395 + 0.125 1402 & 0.057
AF, Food consumed; AS, faeces produced; AW, net weight gain; A.D., approximate digestibility; E.W., efficiency of conversion of ingested food (%); and E.C.D., efficiency of conversion of digested food (%). All values are means + S.E. based on dry weights and expressed on a per larva basis.
The exponential mean fresh larval weights were calculated to be 0.369 mg for S. oryxae and O-477 mg for S. granarius. From these values, mean C.I.‘s of O-87 and 1.18 mg dry food consumedjmg fresh body weight per day were calculated for
1940
J. E. BAKER
S. oryxae and S. granarius, respectively. The G.R. for S. oryxae, calculated from the mean time to pupation and the mean AW, was O-513 mg dry weight gain/mg dry body weight per day compared with a G.R. of O-374 for S. granarius. The remaining parameters and indices (A.D. = approximate digestibility, E.C.I. = efficiency of conversion of ingested food into body substance, and E.C.D. = efficiency of conversion of digested food into body substance) are given in Table 1. Larvae of both species secrete a fluid that lines the pupation chambers in the rearing tubes. Corrections for the weight of this material were not attempted, but the E.C.I. and E.C.D. values for both species would be lowered slightly. Also, the AS values were not corrected for urine content. DISCUSSION
Larvae of S. oryxae were more efficient than those of S. granarius in the digestion of the food and the net conversion of both ingested and digested food into body substance as evidenced by the higher A.D., E.C.I., and E.C.D. values. Previous studies indicated that the maximum mean fresh larval weights of both species reared on this diet were similar (ca. 3.00 mg) although adults of S. oryzae weighed ca. 10 per cent less than those of S. granarius (BAKER and MABIE, 1973a, b). In this study A WIT was comparable for both species ; however, the relative growth rate for S. oryxae was greater. The shorter developmental time of S. oryxae was therefore not strictly due to their smaller net weight gain. Similarly, the percentage difference in AW between S. oryxae and S. granarius was not comparable to the much greater amount of food consumed by S. granarius to obtain that weight. Since the rate of consumption of the diet by S. granarius was actually greater than that of S. oryzae, the faster rate of development of S. oryxae is apparently due to the efficiency with which this species utilizes -its food. There was a striking difference in the amount of faeces voided by the two species. S. granarius excreted 6.4 times as much uric acid and 8.1 times as much faecal material as S. oryzae. This difference was also apparent when faecal material was collected. It took many more S. myzae larvae to obtain sufficient quantities of excreta for analysis. If uric acid is one of the major end-products of N metabolism, one can speculate on the fate of uric acid in larvae of S. oryxae as follows : (1) The major portion of the uric acid may be stored during the larval and pupal stages. GUPTA and SINHA (1960) demonstrated storage of uric acid and its salts in the fat body of adult S. granarius. Also, in another curculionid, Anthonomus grandis Boh., tissue levels of uric acid increased with larval age, reached a peak in the pupal stage, and declined in the adult, the major portion apparently being voided in the meconium (MITLIN AND MAULDIN, 1966). (2) A n active uricase present in S. oryzae or released by its associated intracellular symbiotes could also explain the decreased levels of uric acid. MITLIN and WIYGUL (1973) demonstrated that the carbon of 14C-uric acid injected into adult A. grandis was incorporated principally into amino and nucleic acids. In this case, the insects themselves apparently have the requisite enzymes since there are no associated symbiotes described for this species.
UTILIZATIONOF FOODBY SITOPHILUS LARVAF
1941
Because of the relatively large amounts of uric acid excreted by S. granarius and the lack of symbiotes in the Savannah strain of this species (LUM and BAKER, 1973), it probably has no uricase. However, in the present study none of the faecal material of either species was analysed directly for allantoin or allantoic acid. (3) The intracellular symbiotes found in S. oryzae may be able to utilize the haemolymph uric acid as an N source and in the process depress the amount of total uric acid excreted. A similar hypothesis was put forward by JURZITZA (1972) who demonstrated that symbiotic larvae of Lasioderma sewicorne (F.) could utilize dietary uric acid but that larvae whose fungal symbiotes were removed could not. If a similar situation exists in S. oryzae, the bacteria-like micro-organisms may assist the larvae in the metabolism of N or may utilize the uric acid N for the synthesis of amino acids or other nitrogenous compounds required for their own growth. Subsequently, the bacteria could provide the larvae with some factor that allows reactions involved in the breakdown and utilization of food to proceed at a maximum rate. The first two hypotheses do not, by themselves, account for the reduction in AS; however, the third hypothesis could explain both the reduced AS (principally a measure of undigested food) and the reduction in the excretion of uric acid by S. myzae. To obtain the same amount of limiting factor from the diet, the asymbiotic larvae of S. granarius, on the other hand, must consume more food and in the process, waste (or not utilize optimally) many of the nutrients consumed in excess. This situation could also explain the much larger AS of S. granarius. Studies of net food utilization with larvae of Tribolium conjkum reared on cracked or ground wheat gave A.D. values of 58.4 and 52.2 per cent, respectively (BHATTACHARYA and WALDBAUER, 1970). The E.C.D. values on both of these natural diets was cu. 8-O per cent. The physical texture as well as nutrient availability and balance of the artificial diet used in the present study are apparently responsible for its extremely high digestibility by both Sitophilus spp. and the relatively more efficient utilization of nutrients. The utilization of whole grain by these Sitophilus spp. may be somewhat less efficient. Nevertheless, the factors that increase the efficiency of food utilization by S. oryxae on the artificial diet may be of even more importance on certain natural products. Differences in the indices between S. wyxae and S. granarius may be even more exaggerated, especially if the symbiotes of S. oryxae are involved and the diet is suboptimal. Acknowledgement-The author would like to thank D. S. FAUSTfor excellent technical assistance and P. T. M. LUM of this laboratory for helpful discussion during the course of this study.
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1942
J. E. BAKER
BAKERJ. E. and MABIE J. M. (1973b) Growth and development of larvae of the granary weevil, SitophiZus grunarius (Coleoptera: Curculionidae), on natural and meridic diets. Can. Ent. 105, 249-256. BHATTACHARYA A. K. and WALDBAUER G. P. (1969) Q uantitative determination of uric acid in insect feces by lithium carbonate extraction and the enzymatic-spectrophotometric method. Ann. ent. Sec. Am. 62, 925-927. BHATTACHARYA A. K. and WALDBAUERG. P. (1970) Use of the faecal uric acid method in measuring the utilization of food by Tribolium confusum. J. Insect Physiol. 16, 1983-1990. GORDONH. T. (1968) Quantitative aspects of insect nutrition. A,m. 2002. 8, 131-138. GUPTA P. D. and SINHA R. N. (1960) Excretion and its products in some stored-graininfesting beetles. Ann. ent. Sot. Am. 53, 632-638. JURZITZAV. G. (1972) Die fur&ion der endosymbiontischen Pilze des Tabakkiifers Lasiodwma serricorne F. (Cal., Anobiidae) bei einer aufzucht unter Proteinmangelbedingungen. 2. angew. Ent. 72, 198-205. LIDDLE L., SEEGMILLER J. E., and LASTERL. (1959) The enzymatic spectrophotometric method for determination of uric acid. r. Lab. clin. Med. 54, 903-913. LUM P. T. M: and BAKERJ. E. (1973) Development of mycetomes in larvae of Sitophilus granarius and S. oryzae. Ann. ent. Sot. Am. 66, 1261-1263. MITLIN N. and MAULDINJ. K. (1966) Uric acid in nitrogen metabolism of the boll weevil: a preliminary study. Ann. ent. Sot. Am. 59, 651-653. MITLIN N. and WIYGUL G. (1973) Uric acid in nucleic and amino acid synthesis in the boll weevil, Anthonomus grandis. J_ Insect Physiol. 19, 1569-l 574. WALDBAUERG. P. (1968) The consumption and utilization of food by insects. Adw. Insect Physiol. 5, 229-288.