Dobie Index Research Articles

Breeding for Weevil (Sitophilus Zeamais Motschulsky) Resistance in Maize (Zea mays L)

A study was conducted with an overall objective of establishing the determinants of weevil resistance in maize. Field experiments were done at GART using a North Carolina Design II with three replications and also at Nanga research in Mazabuka. Laboratory experiments were conducted at Zambia Agriculture Research Institute (ZARI) Entomology laboratory and University of Zambia, Food Science Department where insect bioassay and the biochemical tests were done respectively. In the insect bioassay grain hardness, grain weight loss, median development period, F1 progeny emergence and the Dobie index of susceptibility indices were measured. Protein and the Phenolic content were determined under the biochemical tests among genotypes. They were highly significant differences in all the twenty seven genotypes evaluated. Results showed that Parental survival accounted for 78.5 % of the total variation, Phenolic content was strongly and positively correlated (r = 0.423***) with grain hardness providing a good measure of resistance accounting for the 10.9 % of the total variation. The study showed the possibility of breeding maize genotypes with an increased resistance and also susceptible lines had a decreasing Phenolic content but increasing Phenolic content resulted in increased resistance. It was therefore concluded that Phenolic content and parental survival can be used as an indirect selection criteria for weevil resistance.

Resistance of selected Mozambican local and improved maize genotypes to maize weevil, Sitophilus zeamais (Motschulsky)

Maize weevil Sitophilus zeamais (Motschulsky) is one of the most important pests of on-farm stored maize in thetropics, particularly where there is no control of moisture content and application of pesticides. This study aimed todetermine the level of resistance among Mozambican maize genotypes against S. zeamais. A total of 17 genotypes,composed of seven experimental hybrids developed from S5 inbred lines from five populations, one released hybrid,two improved open pollinated varieties (OPV), three landraces from Mozambique, two resistant and two susceptiblechecks from Kenya were screened for their resistance to S. zeamais. Four hybrids were screened at their F1 and F2generations. The genotypes were evaluated in the post-harvest laboratory, Kiboko, Kenya in a completely randomizeddesign, replicated 4 times. Data was collected on the number of emerged insects, seed weight loss, seed damage,median development period; and protein, starch and oil contents. The Dobie index of susceptibility was used to groupthe genotypes. Six of the of the eight hybrids at the F1 generation were resistant while two were moderately resistant.Three of the four hybrids evaluated at the F2 generation were moderately resistant while one was susceptible. Two ofthe five OPVs evaluated (EV8430DMRSR and landrace Kandjerendjere) were resistant. Genotypes with high proteincontent showed resistance while those with high starch contents showed susceptibility. Weevils fed on the resistantgenotype produced low number of F1 progeny, had a high median developmental time, caused low seed damage andlow seed weight loss. The resistant genotypes can be either used as cultivars or as sources of resistance in a breedingprogram for weevil resistance.

Resistance of Rice Varieties to the Stored-Product Insect, Sitophilus zeamais (Coleoptera: Curculionidae).

Four common Portuguese rice varieties--Thaibonnet, Gladio, Albatros, and Eurosis--were tested for their relative susceptibility to Sitophilus zeamais Motschulsky, a common pest of stored rice in Portugal and in tropical countries. Physical (moisture content, hardness, length, and width) and chemical (by attenuated total reflection-Fourier transform infrared spectroscopy) properties of rice kernels were measured. Insect bioassays measured median developmental time, Dobie's index of susceptibility, percentage of damaged grains and weight loss, and progeny developed. This was done for paddy, brown rice, and polished rice for each variety. There were small, but significant, differences in insect resistance among the varieties. However, it was different for paddy and polished rice. In paddy, these differences were correlated with hull damage, and Eurosis was the most susceptible variety. In polished rice, resistance was correlated with hardness, and Thaibonnet was the most susceptible variety. In general, paddy rice was more resistant to insect attack, followed by polished rice and then brown rice. Paddy kernels selected with undamaged hull were completely resistant to attack. Implications for IPM and breeding for resistant varieties are discussed.

English

Sitophilus zeamais Motsch. is  one  of  the  most  important  pests  of  sorghum  in  storage  causing  severe economic damage to the grain. Twenty-one sorghum varieties obtained from Haramaya University Sorghum Improvement Research Program were investigated for their relative resistance to S. zeamais attack. The Dobie index of susceptibility was used to group the varieties into different reaction categories. Among the twenty one sorghum varieties evaluated, only one variety, ‘WB-77’, was regarded as resistant to the weevil. All the remaining varieties were categorized as moderately resistant. Weevils reared on the resistant variety produced a few number of F1 progeny (20.00), had a high median developmental time (42.00 days) and a low percentage of seed damage (2.67) and seed weight loss (0.30). Percentage seed damage and weight loss are significantly and positively correlated with the mean number of F1 progeny emergence and are inversely associated with median developmental time. Consequently, those varieties with high number of F1 progeny scored high percentage of seed damage and weight loss. These results indicated that high lysine content was found to be the predominant factor in sorghum resistance to S. zeamais.   Key words: Sorghum varieties, resistance, insect infestation, Sitophilus zeamais, susceptibility index.  

Susceptibility of different types of sorghums during storage to Sitophilus zeamais Motschulsky

Caryopses belonging to twelve different sorghum cultivars (two red, one brown or high-tannin, two white heterowaxy, two white waxy, two white high-digestible protein and three white with regular endosperm) were selected to study their resistance to Sitophilus zeamais during storage. Five resistance parameters were evaluated: Dobie Index, Total Emerged Insects, Median Development Time, Ratio final/initial insects and Weight Loss. Biophysical characteristics (test weight, 1000 kernel weight, endosperm texture, flotation index, true density, percentage of kernel removed with TADD, anatomical parts, color index and kernel size), chemical composition (starch, amylose, protein, free amino nitrogen and ash), and nutraceutical traits (free and bound phenolics and antioxidant capacity for the free and bound fractions) were obtained. The most resistant cultivars were both red sorghums (RR1 and RR2) and a white cultivar with regular endosperm (WR1) whereas the most susceptible were the brown high-tannin (Sumac), a white waxy (Waxy1) and a white high digestible (HD1). Correlation coefficients among resistance parameters and physicochemical characteristics were calculated, yielding a clear relationship amongst different endosperm texture indicators, endosperm, ash, amylose and free amino nitrogen content, and susceptibility traits. The harder kernels (in terms of vitrousness), higher endosperm percentage, low ash, increased amylose content and reduced free amino nitrogen concentration had more resistance to S. zeamais. Significant relationships among nutraceutical profiles and resistance were not detected, despite the wide range of phenolics in the array of kernels. These results indicated that endosperm structure is predominant in sorghum resistance to S. zeamais.

Evaluation of open pollinated varieties of maize for resistance to the maize weevil in a controlled temperature and humidity laboratory in Zimbabwe

Maize weevil (Sitophilus zeamais Motschulsky) is one of the major maize storage pests among smallholder farmers in eastern and southern Africa (ESA), thereby, reducing total on-farm maize harvests. Experiments were conducted in a controlled temperature and humidity (CTH) laboratory at CIMMYT-Zimbabwe to compare the resistance of new maize open pollinated varieties (OPVs) from regional trials undertaken in ESA to maize weevils. Twenty maize OPVs were shelled and cleaned before being placed in a deep freezer at −20 °C to kill any pests and eggs in the grain from the field. Fifty grams of grain from each OPV was measured after 3 weeks and placed in the CTH lab for conditioning. The samples were placed in 250-mL jars with brass-screened lids and then infested with 32 maize weevils aged between 10 and 14 days. Data were collected for kernel hardness prior to weevil infestation, as kernel hardness to confer resistance to weevils. The components of maize weevil resistance considered were: median development period, Dobie index of susceptibility, weevil emergence, weevil mortality, weevil fecundity, grain weight loss, kernel damage and germination. There were highly significant differences in maize kernel characteristics and median development period (P < 0.001) among the maize OPVs. There were significant differences in the number of damaged kernels, grain weight loss (P < 0.01), weevil mortality and germination (P < 0.05) among the maize OPVs. There were no significant differences found for weevil progeny emergence (F1), Dobie index of susceptibility and weevil fecundity, the last of which was positively and significantly correlated with kernel damage (P < 0.001). The DIS was positively and significantly correlated with weight loss and F1 (P < 0.001). The maize OPVs 07WEEVIL, Chitedze6, Strigoff126, Strigoff128 and ZM625 were found to be resistant; on the other hand, Strigoff140, Strigoff125, Strigoff133, VP05199 and VP074 varieties were highly susceptible.

Yield and agronomic performance of maize hybrids resistant to the maize weevil Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae)

This study evaluated improved maize hybrids (Zea mays L.) with varying level of resistance to the maize weevil, Sitophilus zeamais Motschulsky, for yield and agronomic traits for two seasons. A total of 22 improved maize hybrids and three commercial standards were tested. Out of the 22 tested, two hybrids CKPH08035 (7.4–9.9 t ha−1) and CKPH08039 (7.3–9.8 t ha−1) significantly out yielded the commercial standards WH505 (8.8 t ha−1) and BH140 (5.5 t ha−1). In addition to yield, the improved hybrids also possess desirable traits including good plant height, good plant and ear aspects and good husk cover. The hybrid CKPH08004 had the lowest Dobie index of susceptibility and was regarded as resistant to S. zeamais. Weevils fed with the resistant hybrids produced low numbers of F1 generation weevils, had a high median developmental time and a low percentage of grain damage and grain weight loss. An increasing number of F1 generation resulted in an increasing grain damage and grain weight loss. We found an inverse relationship between the susceptibility index and percent mortality. However, the numbers of F1 generation, percent grain damage and grain weight loss were positively correlated with the susceptibility index. The use of resistant maize hybrids should be promoted in managing S. zeamais in stored maize under subsistence farming conditions in Africa.

English

&nbsp; This study aimed at evaluating commonly used maize varieties, collected from Melkasa and Bako Agricultural Research Centers and Haramaya University, Ethiopia, against the maize weevil&nbsp;Sitophilus zeamais&nbsp;Motsch., one of the most important cosmopolitan stored product pests in maize. A total of 13 improved maize varieties were screened for their relative susceptibility to&nbsp;S. zeamais. The Dobie index of susceptibility was used to group the varieties. The variety, &lsquo;BHQP-542&rsquo;, had the least index of susceptibility and was regarded as resistant. The varieties, &lsquo;Katumani&rsquo;, &lsquo;Melkasa-I&rsquo;, &lsquo;Melkasa-II&rsquo;, &lsquo;Melkasa-III&rsquo;, &lsquo;Coree&rsquo;, &lsquo;BH-541&rsquo;, &lsquo;BH-660&rsquo;, &lsquo;BH-540&rsquo;, &lsquo;Rare-I&rsquo;, &lsquo;Awasa-511&rsquo;, &lsquo;ACV3&rsquo; and &lsquo;ACV6&rsquo;, were moderately resistant. Weevils fed with the resistant variety produced low numbers of F1 progeny, had a high median developmental time and a low percentage of seed damage and seed weight loss. Maize varieties with a high F1 progeny tended to have a short median developmental time. An increasing number of F1 progeny resulted in an increasing seed damage and seed weight loss. We found an inverse relationship between the susceptibility index and percent mortality and median developmental time; however, the numbers of F1 progeny, percent seed damage and seed weight loss were positively related with the susceptibility index. The use of resistant varieties should be promoted in managing&nbsp;S. zeamais&nbsp;in stored maize under subsistence farming conditions in Africa. &nbsp; Key words:&nbsp;Grain damage, maize; resistant variety,&nbsp;Sitophilus zeamais, susceptibility index.

Mapping of QTL Associated with Maize Weevil Resistance in Tropical Maize

ABSTRACTThe maize weevil (MW), Sitophilus zeamais (Motsch.), is an important pest of stored maize (Zea mays L.) in tropical areas. Quantitative trait loci (QTL) associated with MW resistance were analyzed. A total of 163 F2 progeny derived from the cross CML290 (susceptible) × Muneng‐8128 C0 HC1‐18‐2‐1‐1 (resistant), were genotyped with 151 molecular markers. Resistance was evaluated using bioassays on replicated F2:3 progenies. Grain damage (GD), grain weight losses (GWL), MW susceptibility index (Dobie index [DI]), and numbers of adult progeny (AP) were evaluated on grain from F2 lines across two environments. Grain hardness and pericarp/grain ratio were evaluated as putative components of resistance. Composite interval mapping was used for QTL analysis. Five QTL dispersed across the genome explained 28% of the phenotypic variance (σp2) and 50% of the genotypic variance (σg2) for GD. Six QTL were detected with effects on GWL and explained 22% of σp2 and 47% of σg2 Seven QTL (27% σp2 and 78% σg2) were identified for DI and three QTL (10% σp2 and 23% σg2) for AP. Both parents contributed resistance alleles. Genetic effects were mainly dominant (GD, GWL, and DI) and additive (AP). Quantitative trait loci × environment interaction was significant for 50% of the QTL. Since the identified QTL accounted for less than a third of σp2, additional mapping populations are required for mapping MW susceptibility.

Hull characteristics as related to susceptibility of different varieties of rough rice to Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae)

Rhyzopertha dominica (F.), an important pest of stored grains, causes economic damage to rough rice through physical damage to the kernel, resulting in reductions in grain quality. In this test, 28 varieties of commercial rough rice (10 long grain, 11 medium grain, and 7 short grain) were examined for solid, split and cracked hulls, hull thickness, and adult emergence from neonate R. dominica introduced on each individual variety. The percentage of solid hulls ranged from 55.5% on Koshihikari variety to 92.8% on Akita variety, and the percentages of cracked and split hulls were correlated with increased susceptibility. The Dobie index for progeny production showed Wells, Jupiter, and Pirogue varieties as the most tolerant to R. dominica, while Rico and Francis were the most susceptible. The hull thickness of rough rice varied among varieties, but the tolerant varieties appeared to have thicker hulls than the susceptible varieties. There was no difference among rice types (long-, medium-, or short grain) regarding tolerance or susceptibility to R. dominica. Results show that the characteristics of the rough rice hull are important for conferring susceptibility of individual varieties to R. dominica.

Effecs of damage by wheat bug, Aelia germari [Hemiptera : Pentatomidae], on grain quality and on reproductive performance of the rice weevil, Sitophilus oryzae [Coleoptera : curculionidae

The wheat bug, Aelia germari, feeds on developing wheat kernels, causing a loss in baking quality of the harvested wheat (Triticum aestivum). The possible nutritional changes in the bug-damaged kernels after the harvest were tested in a bioassay using the rice weevil Sitophilus oryzae, which feeds on the kernel endosperm. The weevils were reared on baking wheat cultivar 'Manon demias', grown in Algeria, with different levels (0, 4, 8 and 13%) of wheat bug (A. germari) damaged kernels. Rice weevil reared on 4, 8 or 13% damaged wheat had a shorter development time than on wheat with no damage. However, pair-wise correlations showed that Dobie's index for susceptibility of grain for stored-products insect attack, number of progeny per female and kernel weight loss were not correlated with the percentage of damaged kernels. Using multivariate analysis, the general characteristics of high quality grain were associated with long development times for S. oryzae, low number of progeny, high baking strength, high falling number, high nitrogen, low ash content, and low assimilable nitrogen.

Divergent Selection for Resistance to Maize Weevil in Six Maize Populations

Maize weevil (Sitophilus zeamais Motschulsky) is an important pest of maize (Zea mays L.) in the tropics, causing serious losses for many resource‐poor farmers who store grain on‐farm for use as food and seed. This study evaluated whether weevil resistance of six maize populations could be divergently changed by S1 and S2 selection, and assessed the importance of replicating grain samples when screening for resistance. Weevil resistance was evaluated for unreplicated S1 and for replicated and unreplicated S2 lines by infesting 50‐g grain samples with 32 young adult weevils, and then incubating the samples in a controlled temperature and relative humidity (CTH) laboratory. Divergent synthetics were formed by recombining the most resistant 10% and the most susceptible 10% of at least 100 lines screened for weevil resistance for each maize population. Replicated S2 selection was successful for both populations where it was applied, resulting in an average of 16% (P &lt; 0.01), 49% (P &lt; 0.05), and 20% (P &lt; 0.01) difference between divergent synthetics for weevil progeny emerged, grain weight loss, and the Dobie index of susceptibility, respectively. S1 unreplicated selection was successful for two of the six populations, while S2 unreplicated selection was not successful. Reciprocal effects were significant (P &lt; 0.01) for weevil resistance of F1 varietal crosses among the divergently selected synthetics, indicating the influence of maternal effects. Nevertheless, the most resistant crosses were those among the most resistant synthetics, confirming that additive gene action for weevil resistance was important. Our results provide practical insights regarding methodologies and demonstrate that it is possible to improve weevil resistance of maize populations.

Traditional farmers’ knowledge of sorghum (sorghum bicolor [Poaceae]) landrace storability in Ethiopia

Traditional knowledge of grain storage was studied on small farms in Ethiopia. Sixteen of sixty landraces of sorghum collected from small farms in Shewa and Welo regions of Ethiopia were identified by the farmers as being stored sorghum landraces. Farmers were interviewed and asked to rate the storability of these stored-sorghum landraces with respect to the major insect pest of the area, the rice weevil. A farmers’ index of storability was then calculated for each landrace. The landraces were then assessed for rice weevil, Sitophilus oryzae (L.), susceptibility in standardized tests at 70% relative humidity and 27°C. The mean farmers’ index for the 16 landraces was found to be inversely related with the susceptibility parameters of F1 emergence (r2=0.80), oviposition (r2=0.76), weight loss (r2=0.88), and Dobie Index (r2=0.95). It was much less strongly related with the median development period (r2=0.20). The results show that farmer knowledge is an excellent guide to sorghum susceptibility to storage pests.

Soluble phenolic content as an indicator of sorghum grain resistance to Sitophilus oryzae (Coleoptera: Curculionidae)

Eight land races of sorghum collected in Ethiopia were found to show significant variation by genotype in soluble phenolic content. Phenolic content, as catechin equivalents, was measured by the Prussian blue procedure. Variation in storage insect resistance was demonstrated using standardized tests with the rice weevil, Sitophilus oryzae. Significant inverse linear relationships were also found between resistance parameters such as weight loss of grain, the Dobie index of susceptibility, number of eggs laid and progeny emerged and the phenolic content of the grain ( r 2=0.85, 0.55, 0.46 and 0.52 respectively). The results suggest that the soluble phenolic content, which previous studies have shown to consist primarily of proanthocyanidins, can be used as an indicator of resistance.

Is “quality protein” maize more susceptible than normal cultivars to attack by the maize weevil Sitophilus zeamais?

Thirty-one quality protein maize (QPM) populations, pools or varieties were crossed with normal pollen (QPM × NOR) to provide two sets of maize for assessment of the effect of QPM conversion on susceptiblity to Sitophilus zeamais. The QPM maize showed a very high degree of variability from very resistant to very susceptible. The mean Dobie Index, which is a measure of susceptibility to insect attack, of the QPM group was not significantly different from that of the QPM × NOR. The mean number of egg plugs oviposited on maize samples of the two groups was not significantly different. Nine QPM populations or varieties were analysed for protein, tryptophan, oil and phenolic content. Susceptibility was found to be negatively correlated with oil content and phenolics, as has been found in normal maize. Since some QPM genotypes were resistant to weevil attack, the results suggest that it is possible to breed for both quality protein and insect resistance in the same maize cultivar.

Variation in resistance of Mexican landraces of maize to maize weevil Sitophilus zeamais, in relation to taxonomic and biochemical parameters

Accessions representing twenty eight landraces of maize were assessed for susceptibility to the maize weevil, Sitophilus zeamais in standardized resistance tests. Susceptibility parameters such as weight loss of grain, number of insect progeny produced, the Dobie index of susceptibility, and oviposition on grain were found to vary significantly by genotype, with exceptional resistance found in accessions representing the Naltel, Chapalote and Palomero landraces. As in improved genotypes, susceptibility was negatively correlated to phenolic and protein content of the variety tested but positively correlated to moisture content. A detailed analysis of the phenolics revealed the presence of diferulate which may contribute to mechanical resistance of the seed by cross-linking of cell wall hemicelluloses. A canonical discriminant analysis of the resistance data suggests that most of the five landrace groupings are significantly different. The ancient indigenous and prehistoric mestisos groupings are sources of resistant genotypes.

An abbreviated procedure to determine the inherent resistance of maize to Sitophilus zeamais Motsch. (Coleoptera: Curculionidae)

Several experiments on the influence of resistant and susceptible maize on developmental parameters of Sitophilus zeamais Motsch. suggested that the procedure to determine the Dobie index of susceptibility could be shortened by replacing progeny yield with the number of eggs laid per sample and days to mid-emergence (DME) with days to first emergence (DFE). Before experimental validation of the modified index, assumptions regarding homogeneity of variances for DFE, DME and days to last emergence (DLE), degree of association between eggs laid per sample of grain and total progeny yield and between DFE and DME, and β-estimators of host suitability were proven. When compared to the Dobie index, the modified index extended the scale for rating resistance and magnified the differences among maize types. The new index could save time and resources for breeding programs.

Correlation of phenolic acid content of maize to resistance toSitophilus zeamais, the maize weevil, in CIMMYT'S collections

The (E)-ferulic acid content of the grain of nine populations of land races of maize derived from CIMMYT's collections was found to be negatively correlated to susceptibility characteristics towards the maize weevilSitophilus zeamais. Correlation coefficients for six susceptibility parameters and (E)-ferulic acid content were significant and ranged from -0.58 to -0.79. A multiple regression analysis by the SAS forward procedure using the primary seed characteristics associated with susceptibility indicated that the ferulic acid content was the only significant factor in explaining variation in at least two susceptibility parameters: the Dobie index and adult preference. In 15 CIMMYT pools, correlations between four susceptibility parameters and (E)-ferulic acid content were also significant (-0.76 to -0.81). The results suggest that phenolic acid content is a leading indicator of grain resistance or susceptibility to insects and may represent a newly identified mechanism of resistance.