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The susceptibility of t'ef (Eragrostis abyssinica Schard.) to infestation by some insect pests of stored grain
J. stored Prod. Res., 1972, Vol. 8, pp. 177-182. Pergamon Press. Printed in Great Britain,
The Susceptibility of T’ef (Eragrostis abyssinica Schrad.) to Infestation by some Insect Pests of Stored Grain J. A. McFARLANE
and P. DOBIE
Tropical Stored Products Centre (Tropical Products Institute), Slough, Bucks, England (First receiued8 December 197 1, and in&z1 form 8 March 1972) Abstract-Insect-rearing experiments at 27°C and 70% r.h. on t’ef (the grain of Eragrostis abyssinica Schrad.) showed that whole t’ef is unlikely to be infested by Sitophilus oryzae (L.) Rhywpertha dominica (F.) or Sitatroga cerealella (Oliv.). Tribolium castaneum (Herbst) was the only insect, of eight species tested, able to multiply effectively on undamaged whole t’ef where it fed predominantly upon the grain embryo. T. destructor Uyttenb. and Ephestia cautella (Wlk.) showed some ability to produce surviving progeny on whole t’ef but are unlikely to be serious pests. C@tolestes pusillus (Schon.) was able to infest t’ef in the presence of the other insect species and also multiplied effectively on milled t’ef, which suggests that it is the nature of the intact seed that prevents infestation in the absence of T. castaneum. It is likely that other insects with free-living larvae, including Oryra+philus surinamensis (L.) which did not multiply on whole t’ef in these experiments, may be able to infest milled or damaged t’ef. However, none of the other species tested produced surviving larvae in the presence of T. castaneum (on whole t’ef). All eight species produced surviving progeny on whole wheat in separate cultures but in composite cultures 0. surinamensis, E. cautella and S. cerealella were eliminated.
INTRODUCTION
T’EF, THE grain of Eragrostis abyssinica Schrad. is a staple food in Ethiopia where it is greatly esteemed and more widely grown than any other cereal grain. It is used to make a traditional unleavened bread called in&a. The grains are extremely small (about 1 mm long) and have a hard seed-coat. Despite the small grain-size t’ef is a very valuable crop. It has high nutritional value (ROUK and MENGASHA, 1963 ; EBBA, 1969) and appears to be relatively resistant to insect infestation in store. In traditional storage containers insect damage is generally negligible in comparison with other cereal grains and pulses. This has been observed by one of US (J. A. McF) in Ethiopia. The present work investigates this resistance to insect attack with regard to the following species : Sitophilus oryzae (L.), Rhyzopertha dominica (F.), Sitotroga cerealella ((Xiv.), T~bo~i~m castaneum (Herbst), T. destwtor Uyttenb., Uryzaephilus surinamensia (L.), Cvptolestes pwillus (Schon.) and Ephestia cautella (Wlk.). S.P.R. 8/3-n
177
J. A. MCFARLANEand P. DOBIE
178
TABLE 1. INCREASE IN NUMBER OF ADULT INSECTS FROM 10 MALESAND 10 FEMALES IN SEPARATE CULTURES AT 27°C AND 70% r.h.
Species T. castaneum E. cautella T. destructor R. dominica s. oryzae C. pusillus S. cereaklla 0. surinamensis
T’ef 100 100 9 12 8 5 0 0 0 0 0 0 0 0* 0 0
Wheat
(weeks)
68 79 32 33 7 20 1279 1238 917 593 416 478 231 253 126 70
10 10 10 10 15 15 10 10 10 10 11 11 8 8 8 8
*One larva found and accidentally killed 2 weeks after final count on wheat.
METHODS Susceptibility
AND RESULTS
to injktation
Twenty adults of each species, ten male and ten female, all O-7 days old, were placed on separate lots of 50 g t’ef in 750 ml glass jars the tops of which were sealed with filter paper. Each culture was duplicated and, for each species, two control cultures were set up on 50 g wheat. The t’ef was a mixture of several unspecified varieties comparable to the ‘mixed t’ef’ commonly stored and marketed in Ethiopia. The wheat was Manitoba No. 2 containing very few broken grains and very little dust. All the grain had been pre-conditioned to the temperature and humidity of the experiments, i.e. 27°C and 70% r.h. The insects were from insectary stocks maintained at the Pest Infestation Control Laboratory, Slough. Observations were made weekly and insects were counted whenever there was a noticeable increase in insect numbers. The original adults were removed together with the first progeny. All cultures in which no increase was noticeable were kept for 10 weeks and then re-examined and discarded. At the end of the experiment small aggregates of t’ef grains were found in several of the cultures. These were removed, soaked in water until they broke up, and then examined. The results are shown in Table 1. T. castaneum, T. destructor and E. cautella successfully reproduced on t’ef but the latter two species produced few progeny and T. castaneum increased only about five-fold in 10 weeks. Nevertheless, this insect produced more progeny on t’ef than on wheat. Aggregates of t’ef from T. castaneum cultures contained cast larval skins and faeces. Those from 0. surinamensis contained a single larva which was accidentally killed during the separation of the grains, and one R. dominica culture contained a pupa d two late-instar larvae that died during the course of the experiment.
Susceptibility of T’ef to Insect Pests
179
TABLE 2. INCREASE IN NUMBER OF ADULT INSECTS FROM 5 MALESAND 5 FEMALESINMIXEDSPECIES CULTURESAT 27°C AND 70% r.h. FOR 9 WEEKS.
Species
T’ef
Wheat
T. castaneum
41 85 0 0 0 0 0 0 13 4 0 0 0 0
27 21 0 0 72 45 428 328 212 246 0 0 0 0
E. cautella R. dominica s. oryzae C. pu5ilLu.s S. cerealella 0. surinamensis
Microscopic examination of t’ef grains damaged by T. castaneum and T. destructor showed that damage was limited to the region of the embryo which was cleanly excised, leaving a characteristic cup-shaped depression in a high proportion of the grains. Less damage had been done by the small number of E. cautella which survived on t’ef and the damage to specific grains was less localized than that due to T. castaneum.
Multiple infestation of t’ef Five male
and five female
adults of all the species tested singly, excepting T. on to each of two duplicate cultures of 50 g t’ef. Two control cultures were set up on 50 g wheat. All insects were of the same age and origin as those used previously and other conditions were as before. These cultures were examined and all insects counted after 9 weeks. Only T. castaneum and C. pusillus produced surviving progeny (Table 2). C. pusillus thus appeared to be favoured by the presence of one or more of the other species while E. cautella, which produced progeny in separate cultures (Table l), appeared to be adversely affected. On wheat, 0. surinamensis, S. cerealella and E. cautella appeared to be most adversely affected by one or more of the other species present and produced no surviving progeny. R. dominica produced significantly fewer adults, in proportion to the numbers initially introduced, than in the separate cultures (Table 1). T. castaneum and S. oryzae appeared to be unaffected while C. pusillus produced, proportionately, about double the number of adults.
destructor, were put together
Survival on milled t’ef A quantity of t’ef was milled, using a Regent Maskiner Grinder at the finest setting, so that about 60 per cent by weight of the milled grain passed through a sieve of 30 meshes/in. (about 12 meshes/cm). The milled t’ef was then conditioned
180
J. A. MCFARLANEand P. DOBIE
at 70% r.h. Ten male and ten female C. pusi&~ adults were put on to each of two replicate lots of 50 g and two control cultures on whole wheat were set up in the same way. All other conditions were as before. This test was limited to C. pus&s. The resultant population was assessed using a flotation method, after 6 weeks. The productivity of C. pusiZZus on milled t’ef was clearly greater than on whole t’ef alone or in the presence of other insects. From the two replicate trials on milled t’ef 203 and 157 adult C. pusillus were obtained. Larvae were present in the whole wheat control but none of the F, generation had developed to the adult stage during the 6 weeks of the trial. Behuuiour of T. castaneum
in small bulks of t’ef
The ability of T. castaneum to move through small bulks of t’ef was investigated in small cylindrical towers built of glass sections, 5 cm in diameter and 2 -5 cm high, with eleven such sections, taped together, in each tower. The bases of the towers were closed by a screen of 120 meshes/in. (about 47 meshes/cm). One tower was filled with t’ef to the level of the top of the tenth section and another was filled with wheat to the same height. About 500 T. castaneum adults were placed in the top section of each tower which was then sealed with muslin and kept at 27°C and 70% r.h. for 4 days. At the end of this period the towers were divided up into their component sections and the number of insects in each was counted. The experiment was repeated with adults placed initially at the base of each tower and then again with T. castaneum larvae, first at the top and then at the base of each tower. The numbers of larvae introduced varied among trials, but no significant mortalities were detected during the trials. Movement of the insects through the t’ef was observed in all tests (Table 3). The numbers moving and the distance moved varied, but the results indicate that 7’. castaneum adults and larvae can move upwards and downwards through t’ef in a depth of at least 25 cm. The more marked upward redistribution of the adult insects in t’ef does, however, suggest that these insects are less disposed to remain at this depth in t’ef as compared with wheat.
DISCUSSION
The purpose of these experiments was to show whether or not the insects tested were likely to infest t’ef and not to obtain precise bionomic data. The duration of the tests was therefore determined rather arbitrarily but it may be argued reasonably that any of these particular species that failed to multiply in the period shown in Table 1 at 27°C and 70% r.h. would be unlikely to do infest this grain. The larvae of S. oryzae, R. dominica and S. cerealella normally develop within a cereal grain and their inability to develop effectively on t’ef is likely to be due to the extremely small grain size. The immature stages of R. dominica and S. cerealella found in aggregates of t’ef grains suggest that occasional progeny of these species might develop completely on t’ef but multiplication remains unlikely. It is interesting that the larvae of these two species should be able to create a more congenial environment for development in this way but the larvae of both species are known to
Susceptibility of T’ef to Insect Pests
181
TABLE 3. DISTRIBUTION OF T. castaneum IN SMALLGRAINBULKSAFTER4 DAYSAT 27”C, 70% r.h. (All figures are means of two replicates)
Depth at which found after 4 days (cm) O-2.5 2.5-5-O 5-o-7.5 7.5-10.0 lO*O-12.5 12.5-15.0 15.0-17.5 17.5-20.0 20.0-22.5 22.5-25.0 Total
Depth at which found after 4 days (cm) O-2.5 2.5-5.0 5-o-7.5 7.5-10.0 10.0-12.5 12.5-15.0 15*0-17.5 17.5-20.0 20.0-22.5 22.5-25-O Total
(i) DISTRIBUTION OF ADULTS Adults introduced onto tops of towers T’ef Wheat
Adults introduced onto bottoms of towers T’ef Wheat
457 26 4 3 2 2 1 1 1 2
411 18 12 15 18 15 10 1 0 0
250 29 9 9 5 3 2 2 6 178
7 1 1 1 1 1 4 17 56 410
499
500
491
499
(ii) DISTRIBUTION OF LARVAE Larvae introduced onto tops of towers T’ef Wheat
Larvae introduced into bottoms of towers T’ef Wheat
200 31 15 6 2 1 0 0 0 3
59 24 10 9 10 21 19 32 42 24
26 17 7 7 4 5 8 12 18 151
60 33 20 19 15 21 33 65 79 92
258
250
255
437
be more actively migratory than those of Sitophilus spp and migration from one grain to another, and the clumping together of grains or grain fragments, has been previously observed with S. cerealella on milled rice and paddy by one of us (J. A. McF). 0. surinammsis and C. pusillus do not have a cryptic larval stage and it is probable that these species are unable to infest whole t’ef grains because of the hard seed-coat. This is supported by the observation that C. pusz& could infest milled t’ef and t’ef grains that had been damaged by other insects. E. cautella and T. destructor produced surviving progeny in small numbers but the populations of these insects had not doubled in 10 and 15 weeks respectively. They
182
J. A. MCFARLANEand P. DOBIE
are unlikely to be serious pests of stored t’ef but could spoil the grain to some extent. The failure of these two species, and of 0. surinamensis, to produce progeny in the presence of other insects does not rule out the possibility that they, like C. pusillus, might multiply effectively in milled t’ef, or mechanically damaged t’ef. Further tests to elucidate this would be worthwhile. T. castaneum infested whole t’ef quite heavily, although the multiplication rate was less than ten-fold in 10 weeks which is a low rate for this insect at 27°C. It could be a significant pest of stored t’ef, since the small size of the t’ef grain does not prevent movement through it. The characteristic damage to the t’ef grain embryo by T. castaneum and the absence of such damage by E. cautella is of interest because, on whole wheat, similar damage is very characteristic of infestation by E. cautella while T. castaneum is a more general feeder. The elimination of E. cautella, 0. surinamensis and S. cerealella, and the relative success of C. pusillus in the mixed cultures on whole wheat is of interest. Competition or predation by the other species present is the likely explanation. T. castaneum adults were frequently observed feeding on dead E. cautella adults, and on one occasion they were observed attacking a pair of copulating adult E. cautella. They may also have eaten the eggs of these and other insects. Acknowledgements-The t’ef used in this work was provided by the Ethiopian Institute of Agricultural Research in Addis Ababa.
REFERENCES EBBA, T. (1969) T’ef (Eragrotis tef). The cultivation, usage, and some of the known diseases and insect pests. Part 1. Bull. Exp. Stn imp. Coil. agric. mch. Arts, Ethiopia 60, l-56. ROUK, H. F. and MENGESHA,H. (1963) An introduction to T’ef (Erugrotis abyssinica Schrad.) a nutritious cereal grain of Ethiopia. Bull. Exp. Stn imp. Coil. agric. mech. Arts, Ethiopia 26, 1-21.
The Susceptibility of T’ef (Eragrostis abyssinica Schrad.) to Infestation by some Insect Pests of Stored Grain J. A. McFARLANE
and P. DOBIE
Tropical Stored Products Centre (Tropical Products Institute), Slough, Bucks, England (First receiued8 December 197 1, and in&z1 form 8 March 1972) Abstract-Insect-rearing experiments at 27°C and 70% r.h. on t’ef (the grain of Eragrostis abyssinica Schrad.) showed that whole t’ef is unlikely to be infested by Sitophilus oryzae (L.) Rhywpertha dominica (F.) or Sitatroga cerealella (Oliv.). Tribolium castaneum (Herbst) was the only insect, of eight species tested, able to multiply effectively on undamaged whole t’ef where it fed predominantly upon the grain embryo. T. destructor Uyttenb. and Ephestia cautella (Wlk.) showed some ability to produce surviving progeny on whole t’ef but are unlikely to be serious pests. C@tolestes pusillus (Schon.) was able to infest t’ef in the presence of the other insect species and also multiplied effectively on milled t’ef, which suggests that it is the nature of the intact seed that prevents infestation in the absence of T. castaneum. It is likely that other insects with free-living larvae, including Oryra+philus surinamensis (L.) which did not multiply on whole t’ef in these experiments, may be able to infest milled or damaged t’ef. However, none of the other species tested produced surviving larvae in the presence of T. castaneum (on whole t’ef). All eight species produced surviving progeny on whole wheat in separate cultures but in composite cultures 0. surinamensis, E. cautella and S. cerealella were eliminated.
INTRODUCTION
T’EF, THE grain of Eragrostis abyssinica Schrad. is a staple food in Ethiopia where it is greatly esteemed and more widely grown than any other cereal grain. It is used to make a traditional unleavened bread called in&a. The grains are extremely small (about 1 mm long) and have a hard seed-coat. Despite the small grain-size t’ef is a very valuable crop. It has high nutritional value (ROUK and MENGASHA, 1963 ; EBBA, 1969) and appears to be relatively resistant to insect infestation in store. In traditional storage containers insect damage is generally negligible in comparison with other cereal grains and pulses. This has been observed by one of US (J. A. McF) in Ethiopia. The present work investigates this resistance to insect attack with regard to the following species : Sitophilus oryzae (L.), Rhyzopertha dominica (F.), Sitotroga cerealella ((Xiv.), T~bo~i~m castaneum (Herbst), T. destwtor Uyttenb., Uryzaephilus surinamensia (L.), Cvptolestes pwillus (Schon.) and Ephestia cautella (Wlk.). S.P.R. 8/3-n
177
J. A. MCFARLANEand P. DOBIE
178
TABLE 1. INCREASE IN NUMBER OF ADULT INSECTS FROM 10 MALESAND 10 FEMALES IN SEPARATE CULTURES AT 27°C AND 70% r.h.
Species T. castaneum E. cautella T. destructor R. dominica s. oryzae C. pusillus S. cereaklla 0. surinamensis
T’ef 100 100 9 12 8 5 0 0 0 0 0 0 0 0* 0 0
Wheat
(weeks)
68 79 32 33 7 20 1279 1238 917 593 416 478 231 253 126 70
10 10 10 10 15 15 10 10 10 10 11 11 8 8 8 8
*One larva found and accidentally killed 2 weeks after final count on wheat.
METHODS Susceptibility
AND RESULTS
to injktation
Twenty adults of each species, ten male and ten female, all O-7 days old, were placed on separate lots of 50 g t’ef in 750 ml glass jars the tops of which were sealed with filter paper. Each culture was duplicated and, for each species, two control cultures were set up on 50 g wheat. The t’ef was a mixture of several unspecified varieties comparable to the ‘mixed t’ef’ commonly stored and marketed in Ethiopia. The wheat was Manitoba No. 2 containing very few broken grains and very little dust. All the grain had been pre-conditioned to the temperature and humidity of the experiments, i.e. 27°C and 70% r.h. The insects were from insectary stocks maintained at the Pest Infestation Control Laboratory, Slough. Observations were made weekly and insects were counted whenever there was a noticeable increase in insect numbers. The original adults were removed together with the first progeny. All cultures in which no increase was noticeable were kept for 10 weeks and then re-examined and discarded. At the end of the experiment small aggregates of t’ef grains were found in several of the cultures. These were removed, soaked in water until they broke up, and then examined. The results are shown in Table 1. T. castaneum, T. destructor and E. cautella successfully reproduced on t’ef but the latter two species produced few progeny and T. castaneum increased only about five-fold in 10 weeks. Nevertheless, this insect produced more progeny on t’ef than on wheat. Aggregates of t’ef from T. castaneum cultures contained cast larval skins and faeces. Those from 0. surinamensis contained a single larva which was accidentally killed during the separation of the grains, and one R. dominica culture contained a pupa d two late-instar larvae that died during the course of the experiment.
Susceptibility of T’ef to Insect Pests
179
TABLE 2. INCREASE IN NUMBER OF ADULT INSECTS FROM 5 MALESAND 5 FEMALESINMIXEDSPECIES CULTURESAT 27°C AND 70% r.h. FOR 9 WEEKS.
Species
T’ef
Wheat
T. castaneum
41 85 0 0 0 0 0 0 13 4 0 0 0 0
27 21 0 0 72 45 428 328 212 246 0 0 0 0
E. cautella R. dominica s. oryzae C. pu5ilLu.s S. cerealella 0. surinamensis
Microscopic examination of t’ef grains damaged by T. castaneum and T. destructor showed that damage was limited to the region of the embryo which was cleanly excised, leaving a characteristic cup-shaped depression in a high proportion of the grains. Less damage had been done by the small number of E. cautella which survived on t’ef and the damage to specific grains was less localized than that due to T. castaneum.
Multiple infestation of t’ef Five male
and five female
adults of all the species tested singly, excepting T. on to each of two duplicate cultures of 50 g t’ef. Two control cultures were set up on 50 g wheat. All insects were of the same age and origin as those used previously and other conditions were as before. These cultures were examined and all insects counted after 9 weeks. Only T. castaneum and C. pusillus produced surviving progeny (Table 2). C. pusillus thus appeared to be favoured by the presence of one or more of the other species while E. cautella, which produced progeny in separate cultures (Table l), appeared to be adversely affected. On wheat, 0. surinamensis, S. cerealella and E. cautella appeared to be most adversely affected by one or more of the other species present and produced no surviving progeny. R. dominica produced significantly fewer adults, in proportion to the numbers initially introduced, than in the separate cultures (Table 1). T. castaneum and S. oryzae appeared to be unaffected while C. pusillus produced, proportionately, about double the number of adults.
destructor, were put together
Survival on milled t’ef A quantity of t’ef was milled, using a Regent Maskiner Grinder at the finest setting, so that about 60 per cent by weight of the milled grain passed through a sieve of 30 meshes/in. (about 12 meshes/cm). The milled t’ef was then conditioned
180
J. A. MCFARLANEand P. DOBIE
at 70% r.h. Ten male and ten female C. pusi&~ adults were put on to each of two replicate lots of 50 g and two control cultures on whole wheat were set up in the same way. All other conditions were as before. This test was limited to C. pus&s. The resultant population was assessed using a flotation method, after 6 weeks. The productivity of C. pusiZZus on milled t’ef was clearly greater than on whole t’ef alone or in the presence of other insects. From the two replicate trials on milled t’ef 203 and 157 adult C. pusillus were obtained. Larvae were present in the whole wheat control but none of the F, generation had developed to the adult stage during the 6 weeks of the trial. Behuuiour of T. castaneum
in small bulks of t’ef
The ability of T. castaneum to move through small bulks of t’ef was investigated in small cylindrical towers built of glass sections, 5 cm in diameter and 2 -5 cm high, with eleven such sections, taped together, in each tower. The bases of the towers were closed by a screen of 120 meshes/in. (about 47 meshes/cm). One tower was filled with t’ef to the level of the top of the tenth section and another was filled with wheat to the same height. About 500 T. castaneum adults were placed in the top section of each tower which was then sealed with muslin and kept at 27°C and 70% r.h. for 4 days. At the end of this period the towers were divided up into their component sections and the number of insects in each was counted. The experiment was repeated with adults placed initially at the base of each tower and then again with T. castaneum larvae, first at the top and then at the base of each tower. The numbers of larvae introduced varied among trials, but no significant mortalities were detected during the trials. Movement of the insects through the t’ef was observed in all tests (Table 3). The numbers moving and the distance moved varied, but the results indicate that 7’. castaneum adults and larvae can move upwards and downwards through t’ef in a depth of at least 25 cm. The more marked upward redistribution of the adult insects in t’ef does, however, suggest that these insects are less disposed to remain at this depth in t’ef as compared with wheat.
DISCUSSION
The purpose of these experiments was to show whether or not the insects tested were likely to infest t’ef and not to obtain precise bionomic data. The duration of the tests was therefore determined rather arbitrarily but it may be argued reasonably that any of these particular species that failed to multiply in the period shown in Table 1 at 27°C and 70% r.h. would be unlikely to do infest this grain. The larvae of S. oryzae, R. dominica and S. cerealella normally develop within a cereal grain and their inability to develop effectively on t’ef is likely to be due to the extremely small grain size. The immature stages of R. dominica and S. cerealella found in aggregates of t’ef grains suggest that occasional progeny of these species might develop completely on t’ef but multiplication remains unlikely. It is interesting that the larvae of these two species should be able to create a more congenial environment for development in this way but the larvae of both species are known to
Susceptibility of T’ef to Insect Pests
181
TABLE 3. DISTRIBUTION OF T. castaneum IN SMALLGRAINBULKSAFTER4 DAYSAT 27”C, 70% r.h. (All figures are means of two replicates)
Depth at which found after 4 days (cm) O-2.5 2.5-5-O 5-o-7.5 7.5-10.0 lO*O-12.5 12.5-15.0 15.0-17.5 17.5-20.0 20.0-22.5 22.5-25.0 Total
Depth at which found after 4 days (cm) O-2.5 2.5-5.0 5-o-7.5 7.5-10.0 10.0-12.5 12.5-15.0 15*0-17.5 17.5-20.0 20.0-22.5 22.5-25-O Total
(i) DISTRIBUTION OF ADULTS Adults introduced onto tops of towers T’ef Wheat
Adults introduced onto bottoms of towers T’ef Wheat
457 26 4 3 2 2 1 1 1 2
411 18 12 15 18 15 10 1 0 0
250 29 9 9 5 3 2 2 6 178
7 1 1 1 1 1 4 17 56 410
499
500
491
499
(ii) DISTRIBUTION OF LARVAE Larvae introduced onto tops of towers T’ef Wheat
Larvae introduced into bottoms of towers T’ef Wheat
200 31 15 6 2 1 0 0 0 3
59 24 10 9 10 21 19 32 42 24
26 17 7 7 4 5 8 12 18 151
60 33 20 19 15 21 33 65 79 92
258
250
255
437
be more actively migratory than those of Sitophilus spp and migration from one grain to another, and the clumping together of grains or grain fragments, has been previously observed with S. cerealella on milled rice and paddy by one of us (J. A. McF). 0. surinammsis and C. pusillus do not have a cryptic larval stage and it is probable that these species are unable to infest whole t’ef grains because of the hard seed-coat. This is supported by the observation that C. pusz& could infest milled t’ef and t’ef grains that had been damaged by other insects. E. cautella and T. destructor produced surviving progeny in small numbers but the populations of these insects had not doubled in 10 and 15 weeks respectively. They
182
J. A. MCFARLANEand P. DOBIE
are unlikely to be serious pests of stored t’ef but could spoil the grain to some extent. The failure of these two species, and of 0. surinamensis, to produce progeny in the presence of other insects does not rule out the possibility that they, like C. pusillus, might multiply effectively in milled t’ef, or mechanically damaged t’ef. Further tests to elucidate this would be worthwhile. T. castaneum infested whole t’ef quite heavily, although the multiplication rate was less than ten-fold in 10 weeks which is a low rate for this insect at 27°C. It could be a significant pest of stored t’ef, since the small size of the t’ef grain does not prevent movement through it. The characteristic damage to the t’ef grain embryo by T. castaneum and the absence of such damage by E. cautella is of interest because, on whole wheat, similar damage is very characteristic of infestation by E. cautella while T. castaneum is a more general feeder. The elimination of E. cautella, 0. surinamensis and S. cerealella, and the relative success of C. pusillus in the mixed cultures on whole wheat is of interest. Competition or predation by the other species present is the likely explanation. T. castaneum adults were frequently observed feeding on dead E. cautella adults, and on one occasion they were observed attacking a pair of copulating adult E. cautella. They may also have eaten the eggs of these and other insects. Acknowledgements-The t’ef used in this work was provided by the Ethiopian Institute of Agricultural Research in Addis Ababa.
REFERENCES EBBA, T. (1969) T’ef (Eragrotis tef). The cultivation, usage, and some of the known diseases and insect pests. Part 1. Bull. Exp. Stn imp. Coil. agric. mch. Arts, Ethiopia 60, l-56. ROUK, H. F. and MENGESHA,H. (1963) An introduction to T’ef (Erugrotis abyssinica Schrad.) a nutritious cereal grain of Ethiopia. Bull. Exp. Stn imp. Coil. agric. mech. Arts, Ethiopia 26, 1-21.