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Some observations on the loss in weight caused by Sitophilus granarius (L.) (Coleoptera, Curculionidae) to wheat under constant experimental conditions
3. Stored Prod. Res., 1965, Vol. 1, pp. 193-l 95.
Some Observations
Pergamon
Press Ltd.
Printed
in Great
Britain.
on the Loss in Weight
Caused by Sitophilus granarius (L.) (Coleoptera,
Curculionidae)
under Constant
Experimental
to Wheat Conditions*
E. T. HURLOCK Ministry
of Agriculture, Fisheries and Food, Infestation Control Tolworth, Surbiton, Surrey, England (Received 22 January,
Laboratory,
1965)
Abstract-( 1) An investigation into the weight of grain eaten by a small population of Sitophilus larvae at three different temperatures is described. (2) Assessment of the damage caused was made both by weighing and by radiography. There were no significant differences between the amounts of damage caused at the three temperatures. (3) The average amount of grain consumed by each larva during development from egg to adult was 28.7 mg. The mean weight of each adult produced was 2.7 mg. 1. INTRODUCTION
MANY attempts have been made in the past to estimate the amount of damage caused by insect infestation of stored foods and most of these have been reviewed by PAKKIN (1956). Those based on weight loss vary mainly from 5 to 10 per cent of weight per annum of all foodstuffs in store consumed by all types of insects. However the bases on which these estimates have been made are often obscure and very few reliable data giving the actual amount of damage caused by insects, to stored food appear to have been published. CLEARE (1962) has published some interesting data concerning the weight losses caused by the attacks of Sitotroga cerealella Oliv., Rhyzopertha dominica F., to stored paddy, and of Sitophilus oryzae (L.) to stored rice, in British Guiana. He has attempted to translate these losses into terms of monetary loss. It is the purpose of this paper to provide data showing the amount of wheat consumed by a known small population of Sitophilus granarius (L.) living under constant conditions of temperature and humidity. 2. MATERIAL
AND
METHODS
ITifteen lots of dust-free Capelle wheat, previously conditioned for three weeks at 25°C and 70 per cent relative humidity and shown by X-ray to be free from existing
*Crown
copyright
reserved. 193
E. T. HURLOCK
I !M
infestation, but not necessarily from past damage, were accurately weighed and made to be as near 100 g as possible. Each lot was placed in a glass jar of 5 cm i.d. and 12 cm i.h., fitted with a ventilated screw cap closed with copper-gauze. The grain in each jar was about 5 cm deep. Weevils were sexed according to the characters given by HALSTEAD (1963), and into each of twelve jars of grain were placed ten female weevils none of which was more than three weeks old. These had come from a stock culture and were presumed fertile. Since 100 g of this wheat contains about 2000 grains there were 200 grains per female. Five jars, i.e. four infested and one uninfested control jar, were then placed in a constant temperature and humidity room at 25°C and 70 per cent r.h.; five more at 27.8°C and 70 per cent r.h. ; and the remaining five at 3lal”C and 70 per cent r.h. The level of infestation present in the jars was unlikely to cause the temperature of the grain to deviate from these figures. The female weevils were allowed to lay eggs on the wheat fbr seven days and they were then removed by sieving. The wheat in the jars was observed daily and when fresh adults emerged they were removed by sieving, to prevent further egg-laying, and were weighed. A daily count of these emergences was kept. At the end of the experiment, i.e. when there had been no fresh emergences for five consecutive days in any jar, the wheat in all the jars was mechanically sieved to remove dust and frass, and both the wheat and the dust and frass were weighed. Finally the damaged grain was examined radiographically in order to compare the extent of the damage in the infested wheat with that in the controls. 3. RESULTS At the end of the experimental period it was found that the weight of wheat in two of the control jars had increased and in the other had decreased slightly, probably due to changes in moisture content. The initial weights of wheat in the four other jars associated with each control jar were therefore adjusted by a small percentage. The means and standard errors for the percentage weight loss, the numbers and average weights of emergent beetles, the percentage of damaged grains and the average weight of grain eaten by each weevil are shown in Table 1. TABLE
1.
Loss IN WEIGHT EMERGED
Temperature f”C,!
Percentage loss in weight of wheat (g) Mean
25 27.8 31.1
6.7 5.5 6.3
* S.E. = standard
S.E.” 0.8 1.3 0.09
OF WHEAT, WEEVILS,
PERCENTAGE
ANDTHE
Percentage damaged grains
OF~DAMAGED
AMOUNTEATEN
Number of emerged weevils
Mean
S.E.
Mean
11.4 9.6 11.6
1.3 1.3 0.8
234 183 227
S.E. 23.6 21.9 11.3
GRAINS, NUMBER
AND
WEIGHT
OF
BYEACHLARVA
Weight of emerged weevils (mg)
Loss in weight of wheat per weevil emerged (mg)
Mean
SE.
Mean
SE.
2.8 2.6 2.6
0.04 0.04 0.00
28.6 29.6 27.9
0.2 0.7 0.7
error.
In the assessment of the percentage of damaged grains the average number of damaged grains in the control samples was determined (=25) and this figure subtracted from the number of damaged grains found in the experimental samples.
Observations
on the Loss in Weight
4. DISCUSSION
AND
Caused by Sitophilus granariur (L.)
195
CONCLUSIONS
There is no significant difference between the results obtained at the different temperatures. The average loss in weight of wheat per weevil emerged was 28.7 mg and the mean weight of the adults was 2.7 mg. These figures are somewhat higher than those given by RICHARDS (1947). The amount eaten by each larva is approximately one half of the weight of a grain (mean weight 49 mg) of the wheat used in this experiment, and a weight loss of only 1 kg of wheat could result in the ultimate appearance of about 35,000 weevils weighing, in total, approximately 91 g. 1Vith regard to the actual numbers of grain found by X-ray examination to be damaged, those in the control jars were hardly damaged at all (highest count 1.5 per cent) but of those in the remaining jars an average of 10.9 per cent were found to be weevil-damaged, and of the damaged grains nearly all were either one half or more than one half eaten away. Comparison of the numbers of damaged grains and the number of emerging weevils suggests that in these cultures there must have been a number of grains supporting two larvae, and indeed, these are visible on the X-ray plates. This effect is particularly marked at the highest temperature but also occurs at the intermediate terrperature. It does not occur so often at 25°C. l’hus under the conditions of this experiment, 1,000 weevils would consume 28.7 g of grain from the time the eggs were laid to the time that they emerged as adults. This population represents slightly less than the entire egg-production of only 10 “a\ erage” female weevils. Since the numbers of egg laying weevils in even a light infestation may reach many thousands per ton of grain the direct damage done by the:r progeny is considerable, quite apart from the indirect damage caused by resultant heating, loss of germination and the results of moisture migration. REFERENCES CLE ARE, L. D. (1962) Damage and Loss caused by Insects to Stored Rice and Paddy in British Guiana. Rice Storage Investigations, Publication No. 4., Dept. of Agriculture, British Guiana. HALSTEAD, D. G. H. (1963) External sex differences in stored products coleoptera. Bull. ent. Res. 54, 119-134. PARKIN, E. A. (1956) Stored product entomology (the assessment and reduction of losses caused by insects to stored foodstuffs). A. Rev. Ent. 1, 223-240. RICIIARDS, 0. W. (1947) Observations on grain weevils, Calandra (Col. Curculionidae)-1 : General biology and oviposition. Proc. zool. Sot. Lond. 117, l-43.
Some Observations
Pergamon
Press Ltd.
Printed
in Great
Britain.
on the Loss in Weight
Caused by Sitophilus granarius (L.) (Coleoptera,
Curculionidae)
under Constant
Experimental
to Wheat Conditions*
E. T. HURLOCK Ministry
of Agriculture, Fisheries and Food, Infestation Control Tolworth, Surbiton, Surrey, England (Received 22 January,
Laboratory,
1965)
Abstract-( 1) An investigation into the weight of grain eaten by a small population of Sitophilus larvae at three different temperatures is described. (2) Assessment of the damage caused was made both by weighing and by radiography. There were no significant differences between the amounts of damage caused at the three temperatures. (3) The average amount of grain consumed by each larva during development from egg to adult was 28.7 mg. The mean weight of each adult produced was 2.7 mg. 1. INTRODUCTION
MANY attempts have been made in the past to estimate the amount of damage caused by insect infestation of stored foods and most of these have been reviewed by PAKKIN (1956). Those based on weight loss vary mainly from 5 to 10 per cent of weight per annum of all foodstuffs in store consumed by all types of insects. However the bases on which these estimates have been made are often obscure and very few reliable data giving the actual amount of damage caused by insects, to stored food appear to have been published. CLEARE (1962) has published some interesting data concerning the weight losses caused by the attacks of Sitotroga cerealella Oliv., Rhyzopertha dominica F., to stored paddy, and of Sitophilus oryzae (L.) to stored rice, in British Guiana. He has attempted to translate these losses into terms of monetary loss. It is the purpose of this paper to provide data showing the amount of wheat consumed by a known small population of Sitophilus granarius (L.) living under constant conditions of temperature and humidity. 2. MATERIAL
AND
METHODS
ITifteen lots of dust-free Capelle wheat, previously conditioned for three weeks at 25°C and 70 per cent relative humidity and shown by X-ray to be free from existing
*Crown
copyright
reserved. 193
E. T. HURLOCK
I !M
infestation, but not necessarily from past damage, were accurately weighed and made to be as near 100 g as possible. Each lot was placed in a glass jar of 5 cm i.d. and 12 cm i.h., fitted with a ventilated screw cap closed with copper-gauze. The grain in each jar was about 5 cm deep. Weevils were sexed according to the characters given by HALSTEAD (1963), and into each of twelve jars of grain were placed ten female weevils none of which was more than three weeks old. These had come from a stock culture and were presumed fertile. Since 100 g of this wheat contains about 2000 grains there were 200 grains per female. Five jars, i.e. four infested and one uninfested control jar, were then placed in a constant temperature and humidity room at 25°C and 70 per cent r.h.; five more at 27.8°C and 70 per cent r.h. ; and the remaining five at 3lal”C and 70 per cent r.h. The level of infestation present in the jars was unlikely to cause the temperature of the grain to deviate from these figures. The female weevils were allowed to lay eggs on the wheat fbr seven days and they were then removed by sieving. The wheat in the jars was observed daily and when fresh adults emerged they were removed by sieving, to prevent further egg-laying, and were weighed. A daily count of these emergences was kept. At the end of the experiment, i.e. when there had been no fresh emergences for five consecutive days in any jar, the wheat in all the jars was mechanically sieved to remove dust and frass, and both the wheat and the dust and frass were weighed. Finally the damaged grain was examined radiographically in order to compare the extent of the damage in the infested wheat with that in the controls. 3. RESULTS At the end of the experimental period it was found that the weight of wheat in two of the control jars had increased and in the other had decreased slightly, probably due to changes in moisture content. The initial weights of wheat in the four other jars associated with each control jar were therefore adjusted by a small percentage. The means and standard errors for the percentage weight loss, the numbers and average weights of emergent beetles, the percentage of damaged grains and the average weight of grain eaten by each weevil are shown in Table 1. TABLE
1.
Loss IN WEIGHT EMERGED
Temperature f”C,!
Percentage loss in weight of wheat (g) Mean
25 27.8 31.1
6.7 5.5 6.3
* S.E. = standard
S.E.” 0.8 1.3 0.09
OF WHEAT, WEEVILS,
PERCENTAGE
ANDTHE
Percentage damaged grains
OF~DAMAGED
AMOUNTEATEN
Number of emerged weevils
Mean
S.E.
Mean
11.4 9.6 11.6
1.3 1.3 0.8
234 183 227
S.E. 23.6 21.9 11.3
GRAINS, NUMBER
AND
WEIGHT
OF
BYEACHLARVA
Weight of emerged weevils (mg)
Loss in weight of wheat per weevil emerged (mg)
Mean
SE.
Mean
SE.
2.8 2.6 2.6
0.04 0.04 0.00
28.6 29.6 27.9
0.2 0.7 0.7
error.
In the assessment of the percentage of damaged grains the average number of damaged grains in the control samples was determined (=25) and this figure subtracted from the number of damaged grains found in the experimental samples.
Observations
on the Loss in Weight
4. DISCUSSION
AND
Caused by Sitophilus granariur (L.)
195
CONCLUSIONS
There is no significant difference between the results obtained at the different temperatures. The average loss in weight of wheat per weevil emerged was 28.7 mg and the mean weight of the adults was 2.7 mg. These figures are somewhat higher than those given by RICHARDS (1947). The amount eaten by each larva is approximately one half of the weight of a grain (mean weight 49 mg) of the wheat used in this experiment, and a weight loss of only 1 kg of wheat could result in the ultimate appearance of about 35,000 weevils weighing, in total, approximately 91 g. 1Vith regard to the actual numbers of grain found by X-ray examination to be damaged, those in the control jars were hardly damaged at all (highest count 1.5 per cent) but of those in the remaining jars an average of 10.9 per cent were found to be weevil-damaged, and of the damaged grains nearly all were either one half or more than one half eaten away. Comparison of the numbers of damaged grains and the number of emerging weevils suggests that in these cultures there must have been a number of grains supporting two larvae, and indeed, these are visible on the X-ray plates. This effect is particularly marked at the highest temperature but also occurs at the intermediate terrperature. It does not occur so often at 25°C. l’hus under the conditions of this experiment, 1,000 weevils would consume 28.7 g of grain from the time the eggs were laid to the time that they emerged as adults. This population represents slightly less than the entire egg-production of only 10 “a\ erage” female weevils. Since the numbers of egg laying weevils in even a light infestation may reach many thousands per ton of grain the direct damage done by the:r progeny is considerable, quite apart from the indirect damage caused by resultant heating, loss of germination and the results of moisture migration. REFERENCES CLE ARE, L. D. (1962) Damage and Loss caused by Insects to Stored Rice and Paddy in British Guiana. Rice Storage Investigations, Publication No. 4., Dept. of Agriculture, British Guiana. HALSTEAD, D. G. H. (1963) External sex differences in stored products coleoptera. Bull. ent. Res. 54, 119-134. PARKIN, E. A. (1956) Stored product entomology (the assessment and reduction of losses caused by insects to stored foodstuffs). A. Rev. Ent. 1, 223-240. RICIIARDS, 0. W. (1947) Observations on grain weevils, Calandra (Col. Curculionidae)-1 : General biology and oviposition. Proc. zool. Sot. Lond. 117, l-43.