FACTORS OF RESISTANCE IN SORGHUM AGAINST SITOTROGA CEREALELLA (OLIV.) AND SITOPHILUS ORYZAE
(L.)
LAWRENCE E. WONGO
Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506, USA
(Received 21 June 1989; revised 19 December 1989)
Abstract—The structural components and physical characteristics of sorghum kernels were studied as factors of resistance against Sitotroga cerealella (Oliv.) and Sitophilus oryzae (L.).
Specifically, the effects of kernel glumes, weight, size and hardness on larval entry site, oviposition site, emergence site, progeny production, and kernel damage and weight loss, were investigated.
The larvae of 5. cerealella entered kernels primarily in the germ end and its periphery. Females of S. oryzae oviposited eggs mainly in the endosperm portion of kernels. Larval entry into kernels, and subsequent development of 5. cerealella was enhanced in kernels that were enclosed by glumes. In addition, S. cerealella caused greater damage and weight loss in kernels with glumes. In contrast, kernels with glumes yielded less adult progeny of 5. oryzae, and sustained less kernel damage and weight loss. The emergence holes of S. cerealella were located mainly in the crown end, whereas those of 5. oryzae were mostly in the endosperm portion of kernels. The heavier and larger kernels produced heavier and larger adult progeny of both species of insect.
Key Words: Sorghum, resistance, stored-grain insects, glumes, germ, endosperm, kernel hardness Resume—Les composants de structure et les caracteristiques physiques des grains de sorgho etaient etud ies comme facteurs de resistance contre Sitotroga cerealella (Oli v.) et Sitophilus oryzae (L.). Specifiquement, on a examine les effets des glumesdu grain, du poids du grain, de la grandeur du grain et de sa durete sur l'endroit d'entree de la larve, de 1'endroit de la ponte, de Popercule de sortie, de la production de progeniture, des degats au grain et du perte de poids du grain. Les larves de S. cerealella sont entrees dans les grains surtout pres du germe et sa peripherie. Les femelles de 5. oryzae ont pondu les oeufs surtout pres de 1'endosperme des grains. L'entree des larves dans les grains et le developpement ulterieur de 5. cerealella etaient ameliores dans les grains renfermes dansdes glumes. En outre, 5. cerealella a provoque des degats et des pertes de poids plus importants dans les grains avec glumes. Par contraste, les grains avec glumes donnaient moins de progenitures adultes de S. oryzae, et ont subi moins de degats et de perte de poids. Les opercules de S. cerealella se trouvaient surtout au sommet du grain, tandis que celles de S. oryzae se trouvaient dans 1'endosperme des grains. Les grains plus lourds et plus grands ont produit des adultes plus lourds et plus grands dans les deux especes d'insectes.
Mots Clefs: Sorgho, resistance, insectes de grains stockes, glumes, germe, endosperme, durete des grains
179
INTRODUCTION
For many years, studies on cereal grains storage have dealt with environmental factors that influence stored-grain insects and changes in populations of insects, following the use of chemical control methods. The structure and physical attributes of cereal grains have been regarded as important only in relation to kernel function, and to their processing and utilization as foods (Rooney, 1973; Rooney and Sullins, 1976). Recent emphasis on finding alternative nonchemical control methods have led to the realization that some cereal grain varieties may be inherently resistant to insect damage during storage. As a result, it is now widely accepted that kernel structure and its physical characteristics may be important, as well, in resistance against stored- grain insect attack and quality control in grain storage.
Several investigators have identified or suggested various cereal kernel structural components, such as tight-fitting husks, hulls or glumes, as imparting resistance against attack by stored-grain insects in rice (Breese, 1964), corn (Eden, 1952), barley (Boles and Pomeranz, 1979), pearl millet (Kossou, 1981) and sorghum (Rogers and Mills, 1974; Wongo and Pedersen, 1988). In addition, other kernel structural and physical characteristics reported to be associated with resistance against stored-grain insects include kernel pericarp (White, 1975; Gomez etal., 1983), kernel hardness (Doggett, 1957; Russell, 1962; Davey, 1965; Fadelmula, and Hober, 1983), kernel size (Reddy, 1950; Doggett, 1957; Richards, 1948; Ewer, 1945; Surtees, 1965), and kernel surface texture (TyagiandGirish, 1972; Gomez etal., 1983). These factors actas mechanical barriers that prevent access of the insects to kernels or make them unsuitable for oviposition, thus reducing the number of eggs laid and the insects' productivity (Dobie, 1984).
The study reported here was undertaken to examine the effects of various sorghum kernel structural components and physical characteristics on Sitoiroga cerealella (Oli v.) and Sitophilus oryzae (L.), two of the most important species of insects commonly encountered in stored sorghum grain.
MATERIALS AND METHODS Sorghum grain
Grain used in the study was from the 1984 sorghum crop, grown on the Kansas State University Agronomy Research Farm in Manhattan. The
cultivars represented variations in head or panicle type, kernel structural components and kernel physical characteristics. Prior to application of treatments, sorghum grain was disinfested in a deep freezer at -15° C for 4 days, and spread out in thin layers on several trays to dry. The grain was then equilibrated at 25 ± 1° C and 67± 3% r.h. for 2 weeks in a controlled temperature and humidity (CTH) chamber, where final moisture content varied from 12.7 and 13.5%, wet-weight basis (w.b.). All tests were conducted in the CTH chamber using a completely randomized experimental design, with three replications per treatment.
Species of insect
The species of insects used, the Angoumois grain moth, Sitotroga cerealella (Oliv.); and the rice weevil, Sitophilus oryzae (L.), were from established stock cultures which were maintained on hard red winter wheat for several generations in the Stored-Product Insects Research Laboratory, Department of Entomology, Kansas State University. However, before being added to grain samples, insects were reared on sorghum grain for three more generations. Such conditioning was deemed necessary in order to avoid any short-term changes in insect behaviour or biology associated with the change of host grain (Dobie, 1974).
Site of S. cerealella larval entry and S. oryzae oviposition
Fifty sound kernels were selected randomly from each of the cultivars BKs5, Shallu, 80Econ 31, and BTx623, and placed in 48 x 48 x 20 mm plastic boxes, fitted with screened lids to allow free air movement. Kernel samples were infested with 75 eggs (1-24 hr old) of S. cerealella, collected on oviposition strips (Mills, 1965a). Grain samples were left undisturbed in the CTH chamber for 10 days, for eggs to hatch and larvae to penetrate kernels. A larval entry site was identified as a tiny whitish cocoon, spun by the larva as it chewed its way into a kernel. A Bausch and Lomb binocular microscope was used to locate larval entry site on each kernel. Another batch of 50kernelsper sample of each cultivar was infested with five female and two male active S. oryzae adults (1-2 weeks old;
Dobie, 1974), and the mated females were allowed to oviposit for 5 days. Females were assumed to be mated, but males were included for possible unmated females. The sexes were separated using rostrum (snout) characteristics (Halstead, 1963). To
detect an oviposition site on a kernel, a solution that differentially stains an eggplug was used. Kernels were immersed in aqueous solutions containing 20 ppm of the alkaloid, berberine sulfate, for a period of 2-3 min for the selective staining of the eggplug (Milner et al., 1950). Eggplugs absorb the stain more readily than the pericarp, fluoresce bright yellow under ultra-violet light, and are easily identified under a binocular microscope.
Site of adult insect emergence
For recording the location of adult insect emergence site or hole, each kernel was arbitrarily divided into three sections designated as germ, endosperm and crown, all of which are known to be different in their structure, physical characteristics and chemical composition. A binocular microscope was used to determine the location of adult insect emergence site on each kernel.
Effect of kernel glumes
Five cultivars, namely, BKs5, Shallu, Feterita, SC109 15, and R3338 were used in this test. For each cultivar 100 kernels, with or without glumes, were weighed and placed in 0.13-1 babyfood jars, fitted with screened lids to allow free air movement. Each sample was then infested with 100 eggs (1-24 hr old) of 5. cerealella. A second batch of 100 kernels per sample was infested with 10 female and four male active S. oryzae adults (1-2 weeks old), and the mated females were allowed to oviposit for 5 days.
The number of emerged adult progeny, number of damaged kernels, and percentage loss in weight of kernels were recorded for samples infested with each species of insect.
Effect of kernel size and weight
To evaluate the effect of kernel size and weight on the adult progeny of each species of insect, cultivar Feterita was chosen because of the obvious variation in size of its kernels. U.S. Standard Nos. 7 (2.83 mm) and 10 (2.00 mm) sieves were used to separate grain samples into three size groups: large (42.7 mg), medium (32.1 mg), and small (21.6 mg).
The average kernel weight of each size group was determined by the Thousand-Kernel-Weight Method. Fifty sound kernels were selected from each size group under a dissecting microscope, placed in 48 x 48 x 20 mm plastic boxes and infested with 75 eggs (1-24 hr old) of S. cerealella. Another batch of 50 kernels per sample was infested with five female
and two male acti ve5. oryzae adults (1-2 weeks old), and the mated females were allowed to oviposit for 5 days. The first 10 adults to emerge in each sample were immediately anaesthetized for 3-5 min by refrigeration, and then weighed individually on a precision balance to the nearest 0.1 mg. This technique provided weight records of live emerged adults from the three size groups of kernels at the same relative period of development.
The size of each emerged 5. cerealella adult was determined by measuring the left wing dorso- ventrally, between the tips of the anterior and posterior margins, excluding the fringe. For each S.
oryzae adult, the length of the elytron was measured dorso-longitudinally between the tips of the anterior and posterior margins, excluding the carina. S.
cerealella adults were sexed on the basis of the characteristic fringehairs surrounding theclaspersat the tip of the abdomen, in the male (Mills, 1965b). S.
oryzae adults were sexed as described by Halstead (1963). Size measurement and sex determination were made with the aid of a binocular microscope, equipped with an ocular micrometer and 45x magnification.
Effect of kernel hardness
The relative hardness of sorghum kernels was determined on those selected at random from each of cultivars Shallu, R3338, BKs5, B Redlan, and 80Econ 31. Each kernel was placed in the pocket of a kernel hardness tester (Kiya Seisakusho Ltd., Japan), and pressure was applied gradually until the kernel broke with a cracking sound. Kernel hardness was measured as breaking strength in kg and was the mean of 20 observations for each cultivar. Cultivars were also rated according to endosperm texture on a scale of 1 (corneous) to 5 (starchy or floury), from Polaroid photographs (Film type 677) of transverse sections of kernels. Samples of grain ofeachcultivar were dispensed in 75-g batches into wide-mouthed, 0.5-1 jars and infested with 200 eggs (1-24 hr old) of 5. cerealella, which were removed 10 days later after egg-hatch. Another set of samples was infested, each with 75 unsexed S. oryzae adults (1-2 weeks old), which were allowed to feed and oviposit for 5 days and then removed. Each treatmentcombination was replicated three times in a completely randomized design. Beginning from the4th week after exposure to insects, grain samples were checked for emerged adults, which were removed and counted. Sample inspection was continued for another 20 days, at which time the experiment was terminated to exclude second generation progeny.
Statistical analysis
Data from all tests were subjected to analysis of variance by PROC ANOVA procedure of the Statistical Analysis System (SAS) package for computer data analysis (SAS Institute, 1985).
Separation of treatment means was done using Fisher's least significant difference (LSD) procedure when the F-test showed that significant differences had occurred (Snedecor and Cochran,
1980).
RESULTS
Site of S. cerealella larval entry and S. oryzae oviposit ion
Significantly more (P<0.05) larval penetration sites were concentrated in the germ than in the endosperm, in all cultivars tested (Table 1). The highest number of entry sites was 39 in cultivar BTx623, followed by 80Econ 31 with 34 entry sites.
Cultivar BKs5 had the lowest number of entry sites (21.0). The adult S. oryzae female oviposited significantly more (P<0.05) eggs in the endosperm than in the germ area, in all sorghum cultivars (Table
1). In addition, cultivar BTx623 had the most eggs (38.7) oviposited in the endosperm, whereas BKs5 had only half as many eggs deposited in the same portion of the kernel. There was evidence of significant (P<0.05) interaction effects between cultivar and site of larval penetration and oviposition for the two species of insect.
Table 1. Location of S. cerealella larval penetration andS.
oryzae oviposition site on sorghum kernels
Location on kernel
Sorghum cultivars
BKs5 Shallu BTx623 80Econ31Mean S. cerealella
Germ Endosperm S. oryzae
Germ Endosperm
21.0 a 12.5 b
9.7 a 18.0 b
23.7 14.0
9.7 20.7 a b
a b
39.0 a 15.0b
8.0 a 38.7 b
34.0 a 16.7 b
9.7 a 30.0 b
29.4 a 14.6 b
8.9 a 26.9 b Means in column, within species, followed by different letters are significantly different (P<0.05) based on Fisher's LSD procedure.
Site of adult insect emergence
Significantly more (P<0.05) S. cerealella emergence holes were located in the crown or stylar region of the kernel than in the germ and endosperm areas combined, irrespective of the presence or absence of glumes covering kernels (Table 2). Mean number of emergence holes recorded in the germ or endosperm were equally low, and not statistically different (P>0.05). The mean number of kernels with S. oryzae emergence holes in the endosperm was significantly greater (f<0.05) than those with emergence holes in the germ or crown areas (Table 2). No statistical difference (P>0.05) was found between the germ and crown ends of kernel, with respect to mean number of emergence holes located in either portion of the kernel. As with larval penetration, there was also a significant (P<0.05) cultivar by site interaction in the location of S.
cerealella emergence holes; however, no interaction effects were found between cultivar and site of emergence of 5. oryzae.
Table 2. Location of emergence site of adults of S.
cerealella and S. oryzae in sorghum kernels with or without glumes
Location of emergence site
With glumes (% of total)
Without glumes (% of total) S. cerealella
Crown Endosperm Germ S. oryzae
Crown Endosperm Germ
92.5 a 6.6 b 0.2 b
17.1b 75.7 a 6.3 b
96.6 a 4.4 b 3.9 b
25.3 b 51.5 a 23.3 b Means in column, within species, followed by different letters are significantly different {P<Q.05) based on Fisher's LSD procedure.
Effect of kernel glumes
Data for all cultivars were combined in the analysis of variance to reflect only the effect of presence or absence of glumes on kernel infestation and insect development of either species of insect.
There were significantly more (Z'<0.05) adult progeny of S. cerealella, more damaged kernels, and
higher percentage loss in weight recorded for kernels with glumes than those without glumes (Table 3).
For S. oryzae, significantly more (P<0.05) adult progeny, damaged kernels and higher percentage loss in weight were observed for kernels without glumes than those with glumes (Table 3).
Table 3. Means of adult progeny, damaged kernels and percentage weight losses of sorghum grain, with or without glumes, infested with S. cerealella or S. oryzae
Kernels No. adult presented progeny
S. cerealella With glumes Without glumes S. oryzae
With glumes Without glumes
52.5 a 20.6 b
9.0 a 17.7 b
No. damaged kernels
58.7 a 20.3 b
11.1a 20.2 b
Percentage weight loss
39.9 a 12.5 b
6.3 a 11.2 a Means in column, within species, followed by different letters are significantly different (/><0.05) based on Fisher's LSD procedure.
Effect of kernel size and weight
Average adult wing or elytron length and body weight were used in assessing the effects of kernel size and weight on adult progeny of each species of insect. Both wing length and body weight of S.
cerealella differed significantly (P<0.05) among the three kernel size groups (Table 4). The elytron length and body weight of the adult progeny of S. oryzae produced in the largest and heaviest kernels were significantly greater (P<0.05) than those yielded by the smallest and lightest kernels (Table 4). In general, the elytra of adult S. oryzae females were longer than those of the male weevils.
Effect of kernel hardness
The hardness values in kg obtained by using the hardness tester revealed significant differences (P<0.05) among kernels of different cultivars (Table 5). Similarly, the mean number of S. cerealella and S. oryzae progeny produced in grain samples indicated that there were significant differences (P<0.05) among the cultivars tested (Table 5).
Cultivar Shallu, which had the hardest kernels yielded the least progeny, whereas 80Econ 31, which had the softest kernels produced the most progeny of both species of insect. Progeny yielded by the remaining sorghum cultivars also reflected the various levels of kernel hardness as determined by the hardness tester.
Table 4. Means of wing or elytron length and body weight of adult progeny of S. cerealella and S. oryzae produced in sorghum kernels of different size and weight
Kernel size
S. cerealella Large Medium Small S. oryzae
Large Medium Small
Kernel weight (mg)
42.7 32.1 21.6
42.7 32.1 21.6
Adult wing/elytron length
(mm)
Male Female 4.6 a 5.1a 4.3 ab 4.7 b 3.8 b 4.1c
1.7 a 1.6 a 1.5 a 1.6 a 1.2 c 1.3 c
Adult
Male 2.4 a
1.9 b 1.4 b
2.3 a 1.6 b 1.1c
body weight (mg)
Female 4.1 a 3.3 b 2.5 b
2.5 a 2.2 ab 1.6 b Means incolumn, within species, followed by different letters are significantly different (P<0.05) based on Fisher's LSD procedure.
Table 5. Effect of sorghum kernel hardness and comeousness on adult progeny of 5. cerealella and S. oryzae
Sorghum cultivar
Shallu R3338 BKs5 SC109 15 80Econ31
Kernel hardness
(kg)
9.3 a 7.9 b 7.0 be 6.8 c 6.2 cd
Endosperm texture*
AC MC MC MS AS
Number of emerged adult progeny
S. cerealella 115.7 a 133.5 be 124.5 b 138.7 c 139.2 c
S. oryzae 79.2 a 177.8 b 210.0 be 279.2 c 396.3 d
Means in column, within species, followed by different letters are significantly different (P<0.05) based on Fisher's LSD procedure.
*AC = almost corneous.
MC = moderately corneous.
MS= moderately starchy.
AS = almost starchy.
DISCUSSION
The larvae of S. cerealella, unlike the adults, have powerful mandibles with which they chew into grain. First-instar larvae move over the grain and then chew their way in to kernels, where subsequent development takes place. The concentration of larval entry sites in the germ end of kernels may be indicative of its relative softness (S urtees, 1964) and the presence of a thin pericarp layer over the germ or embryo. Mills (1965c) and Khare and Mills (1968) observed that S. cerealella larvae that had early access to wheat, sorghum or corn germ, generally had more rapid development, fewer larval instars and larger adults than those that entered and fed on the endosperm during early development. The choice of the germ as the main site of larval entry into the kernel may be explained by this developmental requirement where, forS. cerealella, early access to the nutrient-rich germ is critical. When kernels were partially or completely enclosed by glumes, larval entry was facilitated just as was reported in pearl millet and sorghum (Kossou, 1981; Wongo and Pedersen, 1988). For a larva to successfully penetrate a kernel, it must have protection and some form of anchorage while doing so. The glumes provide such protection, especially at the germ end of the kernel. Visual examination of sorghum kernels using a binocular microscope and in scanning electron micrographs revealed the
presence of a rough pericarp over the germ end, possibly, as a result of shrivelling due to drying. This rough surface may provide the larva with the foothold needed in boring through the pericarp at the germ end of the kernel.
The oviposition site selected by Sitophilus spp.
has been the subject of a number of studies, particularly in wheat (Sharifi, 1972; Tyagi and Girish, 1972). The distribution of eggplugs in our study showed that the adultS. oryzae female did not deposit its eggs indiscriminately, but rather selectively in a specific area of the kernel. In general, the endosperm was the site most used for oviposition. Of all eggplugs recorded, 69% were in the endosperm, and only 30.6% were in the germ and its periphery. Sharifi (1972) reported that in wheat infested with the maize weevil, 72% of eggs appeared to be deposited in the endosperm, 21% in the germ perimeter, and only 7 % in the germ centre.
He also noted a relatively high mortality of first- instar larvae hatched in the centre of the germ. When rice weevil eggs were deposited in holes made in the germ, or close to the germ, initial larval mortality was observed to be high, but when kernels were inoculated in the endosperm, mortality decreased and development was normal even though larvae fed on the germ during the last instars. Therefore, although the nutrient content of the germ may have an adverse effect on young larvae feeding on kernels, it is not only suitable for late instars but, may be
essential for normal and successful development.
This may explain why the adult 5. oryzae female preferred to oviposit its eggs in the endosperm.
Tipping et al., (1986) found a positive correlation between deep feeding by female maize weevils and oviposition. Urello (1989) has suggested that such deep feeding may indicate that prior to oviposition, the female assesses the quality of the substrate in order to ensure the nutritional welfare of the larva. A wrong assessment could result in detrimental effects. Factors in the germ responsible for the negative effect on oviposition and the mechanism by which the adult S. oryzae female chooses the preferred site of oviposition needs to be studied more thoroughly. Both chemical and mechanical factors may be involved; however, given the fact that the germ is often softer and covered with a thinner pericarp layer than other areas of the kernel, it would appear that this would make it more suitable for oviposition.
With respect to the location of S. cerealella emergence site, the results showed that emergence occurred overwhelmingly in the crown area of the kernel. Kossou (1981) has suggested that the orientation of the larva of this species of insect along the long axis of the kernel might be responsible for the location of most of its emergence holes in the crown end. However, the reason for this larval behaviour is not yet very well understood.
Obviously, for kernels whose germ ends are covered with glumes, it would be difficult for the larva to prepare its emergence window by cutting through the tough, leathery glumes. In a few instances in this study, some adults emerged in the germ end of some kernels without the glumes. For both species of insect, the results show highly significant^-cO.OOl) correlations of both the site of larval penetration and oviposition with the site of adult emergence. With respect to S. cerealella, the correlation of larval penetration with site of adult emergence was negative (-0.78). This negative correlation reflects the tendency of the species to emerge mainly at the crown end of the kernel irrespective of the site of larval penetration. In contrast, 5. oryzae site of oviposition was positively (0.64) correlated with site of adult emergence, indicating that this species consistently emerged in the same area of kernel where it also laid most of its eggs.
The effect of glumes on insect infestation and development may be determined, in part, by the species of insect. An explanation has already been given by Wongo and Pedersen (1988) as to how glumes can provide protection and anchorage for the larva, thus ensuring its survival, penetration, and
subsequent development. Support for this observation is found in the study carried out by Kossou (1981), in which he found that, larval survival and subsequent development was enhanced and, greater numbers of adult progeny were produced in pearl millet lines with glumes compared to those without glumes. On the contrary, glumes seemed to interfere with oviposition by 5. oryzae, as evidenced by the reduced numbers of adult progeny yielded by glumed kernels. This finding is consistent also with that of Rogers and Mills (1974) who suggested that sorghum kernels completely enclosed by glumes were mainly immune to attack by S.
oryzae. Depending on the extent of coverage, glumes can reduce the surface area of kernels exposed to the adultS. oryzae female foroviposition. Furthermore, the size and cylindrical body of this species of insect cannot allow it to physically get inside the narrow space between a kernel and its glumes to lay eggs.
The average wing or elytron length was used in assessing the effects of kernel size and weight on the two species of insect. As would be expected, the largest and heaviest kernels produced the largest and heaviest 5. cerealella adults. Conversely, the smallest and lightest kernels produced adults that had the shortest wing span and least body weight. S.
cerealella male adults were smaller than the females, even when reared in kernels of similar size and weight. Without exception, the heaviest males weighed less than the lightest females. Warren (1954) found that the average weight of S. cerealella adults in sorghum ranged from 1.5 to 2.1 mg for males, and 2.4 to 3.3 for females, depending on the variety of sorghum. Shazali (1987) reported adult progeny mean body weights of 3.8 and 2.7 mg, in largeand small sorghum grain, respectively. Surtees (1965) found that at 25°C, heavier adults of the granary wee\il,Sitophilus granarius (L.), (up to 3.1 mg) emerged from heavier grains, whereas adults bred on broken grain averaged only 1.1 mg. When sorghum varieties were mixed, oviposition preference was greatest for the largest kernels and least for the smallest ones. At the same time, it was found that, the smaller the seeds, the shorter and lighter the weevils that emerged (Russell, 1962).
Richards (1944) stated that rice weevils, Calandra oryzae (L.), vary greatly in size and it is a natural assumption that the kind of grain in which they breed is the main cause of variation. The disparity in adult wing length and body weight may be attributed to factors such as the differences in the size and weight of kernels, related nutritional content of kernels, prevailing environmental conditions and species of insect. A review by Maceljski and Korunic (1971)
indicated that body length depended upon the type of food consumed by the weevils, hence, under given conditions, S. oryzae may be larger or smaller. Data in the present study showed that parent insects from the same stock culture can produce adult progeny with different sizes and weights when reared on sorghum kernels with different size and weight.
Shazali (1982) reported S. oryzae adult weights of 1.4 and 1.1 in large and small grain sorghum, respectively. He explained that, since only one adult of either S. cerealellaorS. oryzae can develop within a sorghum kernel, the larger and heavier kernels would supply more food and space needed for larval growth and development.
Results of the kernel hardness test revealed significant differences among some cultivars, as reflected in numbers of progeny of both species of insect. Clearly, the harder the kernel, the more difficult it is for cither the larva of S. cerealella to penetrate, or the adult S. oryzae female to oviposit and successfully complete development. Rout (1973) found a positive relationship between per cent of weight loss by pearling sorghum, and percent newly-hatched larvae of S. cerealella surviving to the adult stage. Doggett (1957), Russell (1962) and Davey (1965) reported an inverse relationship between hardness and susceptibility to attack by S.
oryzae, with varietal differences of insect yield reaching twenty-fold. Russell (1966) observed a shortening of longevity on the harder grains, which could be a consequence of starvation.
The present study indicates that sorghum kernel structural components and physical attributes can provide good physical barrier to attack by stored- grain insects, although this is not true for all species.
Indeed, stored-grain entomologists have been trying for many years to persuade plant breeders to incorporate postharvest resistance factors inherent in cereal grain varieties to improve their storage- ability and the availability of food grains, particularly in the Third World. However, ithasonly been recently that plant breeders and stored-grain entomologists have begun to cooperate in this effort.
The emphasis is on the exploitation of some identified resistance factors in plant breeding programmes to produce varieties which are high- yielding, nutritionally adequate, acceptable to consumers and, at the same time, resistant to attack by stored-grain insects. There are already examples of a few such effective international or national breedingprogrammes dedicated to the achievement of this objective.
Acknowledgements—The author gratefully acknowledges Mr. Joe Wilson for technical
assistance rendered in various ways throughout the conduct of this study, and the Department of Grain Science and Industry, Kansas State University, for generously providing resources and research facilities.
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