A comparison of two methods of assessment of maize varietal resistance to the maize weevil, Sitophilus zeamais Motschulsky, and the influence of kernel hardness and size on susceptibility

Journal of Stored Products Research 37 (2001) 287–302

A comparison of two methods of assessment of maize varietalresistance to the maize weevil, Sitophilus zeamais Motschulsky,and the influence of kernel hardness and size on susceptibility

Irene Gudrupsa, Sian Floydb, Jennifer G. Klingc, Nilsa A. Bosque-Perezc,1,J.E. Orcharda,*

aFood Security Department, Natural Resources Institute, Central Avenue, Chatham Maritime, Chatham,

Kent ME4 4TB, UKbOffice of the Directorate, Central Avenue, Chatham Maritime, Chatham, Kent, ME4 4TB, UK

c International Institute of Tropical Agriculture, Oyo Road, PMB 5320, Ibadan, Nigeria

Accepted 27 July 2000

Abstract

Fifty-two maize varieties were screened for resistance to infestation by the maize weevil, Sitophiluszeamais, using assessment methods proposed by Dobie (J. Stored Products Res. 10 (1974) 183–197) andUrrelo et al. (J. Stored Products Res. 26 (1990) 100). The two methods gave similar assessments of maizesusceptibility to S. zeamais. The Dobie method is preferred due to the lower total time required forassessment of relative susceptibility of maize varieties. The greatest disadvantage of the Urrelo method isthe intensive labour requirements in the early stages of a trial, when numbers of eggs have to be counted,although it has the advantage that the assessment may be terminated upon emergence of the first F1 adult.Two explanatory variables, kernel size and hardness, were investigated to determine whether they may beused as indicators of resistance. Results suggested that kernel size is the more important in determiningresistance to attack by S. zeamais, with large kernels appearing to show greater resistance than small ones.Contrary to expectations, of the varieties tested, including local, hybrid and improved open pollinated(IOP) varieties, the local varieties were generally more susceptible. This may be related to kernel size, as allIOPs and hybrids tested had large kernels, whereas the majority of the local varieties had small ones. Noclear relationship between weevil susceptibility and kernel hardness could be detected, although there wasan indication that differences associated with kernel size varied depending on kernel hardness. None of the

1Present address N.B.P.: Department of Plant, Soil, and Entomological Sciences, College of Agriculture, University

of Idaho, Moscow, ID 83844-2339, USA.

*Corresponding author. Fax: +44-1634-880066/77.E-mail address: [email protected] (J.E. Orchard).

0022-474X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 2 2 - 4 7 4 X ( 0 0 ) 0 0 0 3 1 - X

varieties tested showed high levels of resistance to attack by S. zeamais. # 2001 Elsevier Science Ltd. Allrights reserved.

Keywords: Zea mays; Sitophilus zeamais; Varietal screening; Maize; Stored-product pests; West Africa

1. Introduction

Sitophilus zeamais Motschulsky is a major insect pest of stored maize in the pantropics. Thehigh cost of pesticides and the risk of their contaminating users, food and ground water, togetherwith problems of supply of recommended formulations in developing countries, and theincreasing occurrence of insecticide resistance, mean that alternative control methods arerequired. The use of resistant varieties, in conjunction with other control methods to form anintegrated pest management programme, would provide a long-lasting system to maintain insectpopulations in stored maize in the tropics at an acceptably low level.The main focus of improvements in maize breeding programmes has traditionally been

concerned with increased yield and field pest and disease resistance. Since varieties are rarelyassessed for resistance to stored-product pests, the introduction of improved varieties has in thepast often been accompanied by reports of increased susceptibility to stored-product pests(Fortier and Arnason, 1982; Kossou et al., 1992, 1993).Resistance in stored maize to insect attack has been attributed to a number of factors including

kernel hardness (Eden, 1952a, b; Dobie, 1974; Serratos et al., 1987), husk protection (Eden, 1952a, b;Dobie, 1977; Kossou et al., 1993), obstruction from adjacent kernels (Kossou et al., 1992), kernelsize and texture (Kossou et al., 1993), starchy amylose content (Dobie, 1973), phenolic content(Classen et al., 1990, Arnason et al., 1994) or the presence of anti-feedants and ferulic andcoumaric acids (Serratos et al., 1987; Classen et al., 1990). These studies are important for theidentification of heritable resistance factors and the potential for their incorporation into newmaize lines by traditional breeding programmes or genetic manipulation.The comparison of the findings of a number of researchers is made difficult by the variation in

assessment techniques used. A number of different methods to assess the susceptibility of maize toattack by S. zeamais have been proposed (Howe, 1971; Dobie, 1974; Urrelo et al., 1990). In thecurrent study, 52 maize varieties were screened for susceptibility to attack by S. zeamais by themethods recommended by Dobie (1974) and Urrelo et al. (1990) (from hereon referred to simply asDobie and Urrelo, respectively) to assess relative susceptibility of the varieties and to compare thetwo screening methods. The effects of kernel size and hardness were also assessed as potentialindicators of resistance and to obtain an understanding of some of the possible mechanisms involved.

2. Materials and methods

2.1. Maize varieties

Maize varieties were obtained from the breeding programme at the International Institute ofTropical Agriculture (IITA), Ibadan, Nigeria. The hybrids and improved open pollinated varieties

I. Gudrups et al. / Journal of Stored Products Research 37 (2001) 287–302288

had been produced by the IITA breeding programme as possessing potentially suitableproduction characteristics. The selection of the local varieties was based on availability. Allmaize accessions were cultivated at the same time in a field at IITA to ensure similarenvironmental conditions and to obtain sufficient seed for evaluation. Post-harvest treatmentsincluded drying to 12% moisture content, shelling, and fumigation with phosphine. All varietieswere frozen upon receipt and kept at ÿ48C until required for evaluation (typically 2–3 months).Prior to use, the varieties were conditioned at 278C and 70% relative humidity (r.h.) for at least 6months. Fifty-two varieties were screened; of these, 30 varieties were improved open pollinated(IOP), 11 were improved hybrids and 11 were local. The same susceptible control variety (8321-18) was included in each experiment to confirm that screening conditions and methodologies wereconsistent throughout the investigation. Table 1 presents information on the accession type, graincolour, grain type, kernel number per 16 g and kernel weight.

2.2. Insects

A strain of S. zeamais collected in Lome, Togo in 1990 was used for the study. Cultures weremaintained on maize at 27 (� 1)8C and 70 (� 5)% r.h. in a constant environment room at theNatural Resources Institute.

2.3. Screening methods

A slight modification to the original Dobie method was used in this study. Eight replicates of16� 0.1 g per variety were weighed into round plastic pots (67� 32mm) with screw-top lids. A5-mm diameter hole was punched into each lid and covered with micropore (3M, USA) tape. Dueto the large number of varieties being screened there were four experimental periods with tworeplicates of each variety tested in each period. Pots were placed in sealed plastic tubs inincubators maintained at 278C, 70% r.h., the latter provided by 600ml of glycerol solution ofspecific gravity 1.168 placed in the bottom of the tub (Braun and Braun, 1958).Adult, 0–7 day-old S. zeamais were sexed by their rostral characteristics (Halstead, 1963) and

an appropriate number placed on each of the varieties for seven days. This conditioning periodwas to eliminate any possible short-term behavioural changes associated with an alteredenvironment. After this period, four female and two male 7–14 day-old adults were placed on theexperimental maize for 7 days. Following removal of the adults, the experimental maize wastreated with acid fuchsin stain to reveal the egg plugs, following a method modified afterFrankenfield (1950). The maize was stained for 5 s, rinsed with tap water for 5 s before being driedon tissue paper for 30 s. This adaptation, which reduced the staining period, was adopted tominimise the degree of water uptake by the maize kernels; thus reducing problems of mouldgrowth and artificially high moisture content (m.c.) of the maize for weevil development. The totalnumber of egg plugs in each kernel in each experimental pot was then observed using a binocularmicroscope. This also produced a total kernel count for each replicate and variety. The m.c. of themaize before staining was in the region of 13–14%; as a result of staining, the mean percentageweight increase of the maize was found to be in the region of 1.3%, giving a range of final m.c.’s of14.1–15.1%. Following examination, kernels were returned to the pots to monitor F1 weevilemergence.

I. Gudrups et al. / Journal of Stored Products Research 37 (2001) 287–302 289

Table 1

Maize accession type, grain colour, grain type, kernel number per 16 g and kernel weighta

Variety Colour Grain type Kernel number (per 16 g) Kernel weight (g)

Local varietiesGbogbe local W F 85 0.1882Toule Houdjii W Mix 79 0.2025

Kwadaso local W F 80 0.2000Cameroun local W F 81 0.1975Volta Region local W F 74 0.2162

Zouzouvou W Mix 84 0.1905Benin local W Floury 65 0.2319BSR (Bafia) Cameroun W F 69 0.2426

Togo local W F 49 0.3265Local TCHI W Mix 83 0.1928Cross River local W F 67 0.2388

Improved open pollinated varieties

Western Yellow Y FD 55 0.2909TZESR-W-SE W Floury(C) 72 0.2286Across 87 Pool 16 SR W D 52 0.3077

Across 87 TZUTSR-W W FD 56 0.2875Mokwa 87 TZPB-SR W D 52 0.3077ICW-SR BC4 F2 W F 56 0.2875

EV 8728-SR BC6 Y FD 51 0.2667Across 8363-SR W D,QPM 56 0.2875DMR-ESR-W W FD 60 0.2667Ikenne 8749 SR BC6 W D 50 0.3200

SAM(1) 8629- SR BC4 W D 44 0.3636Gusau-X G.Pool W FD 56 0.2875Across 85 TZSR-W W FD 56 0.2875

TZUTSR-W SGY YC FD 52 0.3077Across 86 TZESR-W W F 65 0.2462EV 8843 DMR SR W D 51 0.3137

Staha-SR BC4 F2 W FD 49 0.3265EV 8725-SR W F 52 0.3077Suwan 1 SR Y F 52 0.3077Across 85 TZSR-Y1 Y FD 55 0.2909

Ev 8731-SR BC6 Y FD 57 0.2807EV 8766-SR BC6F4 Y D,QPM 59 0.2712Ev 8444-SR BC4 W D 50 0.3200

TZB-SR-SE W Floury(C) 64 0.2500TZB-SR SGY YC FD 53 0.3019TZB-SR W FD 54 0.2963

Ndock 8701 W FD 55 0.2909EV 8722-SR BC6 W FD 54 0.2963EV 8762-SR BC6 W F,QPM 59 0.2712

EV 8730 SR BC6 W F 55 0.2909

Hybrids8321-21 W D 56 0.28578705-6 W SF 47 0.3404


Each pot was checked regularly before the expected time of emergence of the first filial (F1)generation. The date of the first emerging F1 (DFE) was noted and the number of F1 individualsemerging from each pot was recorded daily during the working week until all the F1 adults hademerged. The criteria for determining the end of the F1 generation was the absence of emergencefor at least three consecutive days, excluding the first week of emergence.

2.4. Indices of susceptibility

Susceptibility indices were used as a measure of the susceptibility of maize varieties toinfestation by S. zeamais. The higher the index, the greater the susceptibility of the maize. Theseindices are defined by the following formulae:

Dobie Index : SI ¼ Ln F � 100

DME;

where SI is the susceptibility index, Ln the natural logarithm, F the total number of F1 adults, andDME the date of median emergence of F1 (days).

Urrelo Index : SI ¼ LnE � 100

DFE;

where E is the total number of egg plugs and DFE the date of first emergence of F1 (days).A correction factor was applied to the calculated development period to allow for the fact that

eggs may have been laid at any time during the week-long oviposition period that the adultsremained on the maize, and that the number of F1 emerging each day was recorded only onceevery 24 h. The DME and DFE were calculated from the first day that the adults were placed onthe maize and then adjusted by subtracting 4 days from the calculated development period.Mortality rates were measured by the percentage of eggs that failed to result in offspring. The

percentage of infested kernels was calculated from the number of kernels in a sample that held oneor more eggs. These data were analysed after logit transformation (log p/1ÿp), where p is theproportion infested.

Table 1 (continued)

Variety Colour Grain type Kernel number (per 16 g) Kernel weight (g)

8644-32 Y DF 56 0.28758522-2 Y F 57 0.28078644-27 Y F 50 0.32008425-8 Y DF 65 0.2464

8644-31 Y D 52 0.30228321-18 W SF 55 0.29098338-1 W DF 52 0.3022

8705-4 W SF 54 0.29638505-5 W DF 52 0.30228321-18 (Control) W SF 54 0.2963

aKey. Colour: W - white; Y - yellow; YC - yellow composite.Grain type: D - dent; F - flint; DF - dent/flint; FD - flint/dent; C - composite; SF - semi-flint; QPM - quality protein

maize.


2.5. Determination of maize kernel hardness

Hardness was determined by a milling method using a Glen Creston mill fitted with a screenwith 2mm-diameter apertures and a plastic tube attached to the mill exit. A warm-up procedurewas required in order to achieve consistent milling characteristics. This was done by passing four20-g samples through the mill in rapid succession. The mill was thoroughly cleaned after the lastsample. This warm-up procedure was repeated if the time between tests exceeded 15min.All maize varieties had been equilibrated at 278C and 70% r.h. Twenty grams (� 0.1 g) of the

maize sample to be tested was placed into the mill hopper and the mill closed. The mill speed wasset at 6000 rpm and allowed to run for 20 s to attain a constant speed. The hopper slide was thenopened to admit the sample into the grinding chamber and grinding allowed to continue for 20 s.The material emitted from the grinding chamber was collected in the plastic tube. The partiallyground material retained within the grinding chamber was also collected and sieved for oneminute using a 710mm aperture sieve. The weight of the material retained by the sieve and that ofthe flour, which passed through the sieve, was weighed. Each sample was replicated twice. Anhardness index (HI) was then calculated according to the formula:

HI ¼Weight of retained part of flour ðgÞWeight of sieved part of flour ðgÞ :

This procedure was performed twice for each of 31 varieties (the number of varieties for whichsufficient sample size was available), and the mean value used as the hardness index. The decreasein hardness associated with an increase in m.c. of the maize grain after the staining procedure wasnot taken into account in the assessment of hardness.Data on hardness and kernel size (as indicated by the number of kernels per unit weight) were

categorised to facilitate interpretation of results. The range in hardness among the 31 varieties was0.59–1.79, which was divided into three classes of approximately equal intervals: class 1: hardness50.95; class 2: =0.95 and 51.38; class 3: hardness>1.38. The range in mean number of kernelsper 16 g for the same varieties was 42–88, which was divided to give the following classes: class 1:number of kernels 558 (a weight of 0.276 g per kernel); class 2: kernel number=58 and 574 (arange between 0.276 and 0.216 g per kernel); class 3: kernel number=74 (0.216 g per kernel).

2.6. Statistical analyses

Analyses of variance were performed on the susceptibility indices in order to investigatedifferences among varieties for their susceptibility to S. zeamais. The statistical packageGENSTAT was used for all analyses. Separate analyses were performed for each of the twoindices of susceptibility, i.e. Dobie and Urrelo. In order to use an analysis of variance the maizevarieties were split into two groups according to susceptibility rating, in order to meet theassumptions underlying the test (see discussion below).Kernel number per 16 g sample (a parameter of kernel size) and kernel hardness were

categorised in order to investigate their effect on varietal resistance. Logistic regression was usedto investigate whether SI and the percentage of the total number of kernels that were infested wasrelated to kernel number or kernel hardness. These data were analysed using the logittransformation (log p/1ÿp), where p is the proportion infested.


3. Results

3.1. Varietal differences for number of F1 weevils, number of eggs and mortality

Table 2 presents the Dobie and Urrelo susceptibility indices, as well as the results relating tonumber of F1 weevils and days to mean emergence (for Dobie) and the number of egg plugs, daysto first emergence and percentage kernel infestation (for Urrelo). The mean total number of F1

weevils produced by a single variety ranged from 11 to 44. Local varieties produced significantly( p50:001) higher numbers of F1 weevils than IOPs or hybrid. The mean number of F1 weevils thatemerged from the local varieties was 36, in contrast to 18 for both the IOPs and hybrids, with thestandard error of the difference between two means equal to 1.3 for the comparison of IOP’s withlocal varieties, and 1.5 for the comparison of hybrids with local varieties. The greatest number ofF1 weevils (44) was produced by the local varieties Cross River Local and Kwadaso: these varietiesalso had the highest Dobie SI (susceptibility index). The lowest number was produced by the IOPvariety, Suwan-1 SR, which also had the lowest Dobie SI among the 52 varieties tested.The mean number of egg plugs ranged from 47 to 120, with the greatest found on Cross River

Local. Significantly ( p50:001) greater numbers of eggs were laid on the local than on the IOPvarieties or hybrids. The mean number of eggs laid (as shown by number of egg plugs) on the localvarieties was 99, in contrast to 62 and 63 laid on the IOPs and hybrid, respectively. The standarderror of the difference between the two means was 2.9 for the comparison of local and IOPvarieties, while it was 3.4 for the comparison of local varieties and hybrids.Mortality rates, as indicated by the difference between the numbers of eggs laid and the number

of emerging F1, were relatively high. There were significant ( p50:001) differences among thevarieties for mortality levels, with the mean being 64% for the local varieties versus 72 and 71%for the IOPs and hybrids, respectively. The standard error of the difference between the twomeans was 1.5 for the comparison of local and IOP varieties, while it was 1.7 for the comparisonof local varieties and hybrids.

3.2. Varietal differences for kernel size

In general, local varieties had smaller kernels than IOPs or hybrids, with a mean kernel numberof 74, 55 and 54 per 16 g, respectively, for these three groups. There were no significant differencesin kernel size between the IOPs and hybrids, however, the difference between local varieties andboth IOPs and hybrids was highly significant (p50.001). The standard error of the differencebetween the two means was 1.0 for the comparison of local varieties and hybrids, while it was 0.9for the comparison of local with IOP varieties.

3.3. Varietal differences in susceptibility

Dobie index: The analysis of variance of susceptibility indices of all 52 varieties revealed that theplot of residuals against the fitted values showed that varieties with a mean Dobie SI of greaterthan 7 were considerably less variable in their performance than varieties with a mean SI of lessthan or equal to 7. This meant that one of the assumptions of the analysis of variance } that alltreatments have the same variance } was not met, which in turn meant that significance tests for


Table 2

Mean values for the Dobie and Urrelo indices, number F1 weevils and days to mean emergence (for Dobie) and thenumber of egg plugs, days to first emergence and percentage kernel infestation (for Urrelo), for each of the ‘lesssusceptible’ varieties (in rank order according to Dobie index)

Variety Dobie Urrelo Percent infested kernelsa

No. F1 DMEc SIb Egg DFEc SIb

Suwan 1-SR 11 45 4.93 49 38 10.13 50 (ÿ0.019)EV 8766-SR BC6 F4 12 46 5.02 47 40 9.66 41 (ÿ0.381)8644-31 12 47 5.04 47 36 10.67 38 (ÿ0.470)N ‘Dock 8701 12 45 5.06 61 38 11.08 51 (0.055)Across 85 TZSR-Y1 12 45 5.07 54 40 9.76 42 (ÿ0.317)EV 8725-SR 12 45 5.33 56 38 10.62 51 (0.043)8705-4 13 45 5.33 57 39 10.37 47 (ÿ0.107)EV8731-SR BC6 17 46 5.34 61 38 10.20 44 (ÿ0.250)Ikenne 8749-SR BC6 15 46 5.42 59 40 10.13 46 (ÿ0.167)8505-5 14 46 5.43 60 38 10.60 51 (0.043)Mokwa 87 TZPB-SR 14 46 5.47 49 38 10.12 37 (ÿ0.548)14ICWSR BC4 F2 17 46 5.53 58 40 10.31 48 (ÿ0.084)Samaru(1)8629-SRBC6 16 46 5.68 58 40 10.13 58 (0.313)TZUT-SR-W-SGY 15 45 5.71 55 39 10.23 46 (ÿ0.180)DMR-E-SR-W 17 47 5.74 57 39 10.23 43 (ÿ0.268)Across 86 TZESR-W 17 45 5.75 57 38 10.28 44 (ÿ0.229)TZB-SR SGY 16 46 5.77 57 38 10.57 50 (0.000)EV 8728-SR BC6 17 46 5.80 63 38 10.78 48 (ÿ0.064)EV 8762-SR BC6 15 42 5.84 58 37 11.07 45 (ÿ0.218)EV 8722-SR BC6 16 45 5.87 49 38 10.15 44 (ÿ0.241)Across 87 Pool 16 SR 16 46 5.91 64 39 10.59 55 (0.193)8644-27 15 45 5.93 56 39 10.36 48 (ÿ0.075)EV 8444-SR BC4 22 46 5.97 71 38 11.12 60 (0.395)EV 8730-SR BC6 17 46 6.12 58 38 10.56 51 (0.036)Western Yellow 17 44 6.17 63 37 11.21 50 (0.018)

8705-6 16 43 6.18 81 36 12.12 68 (0.742)8644-32 17 46 6.18 70 38 11.02 53 (0.138)8425-8 20 45 6.26 68 37 11.36 44 (ÿ0.228)Across 85 TZSR-W 19 45 6.30 67 38 11.15 56 (0.255)8321-18 (Control) 17 44 6.30 59 37 11.22 47 (ÿ0.133)TZB-SR 18 45 6.30 56 37 10.69 53 (0.117)EV 8843 DMR-SR 19 45 6.42 60 36 11.55 56 (0.258)

8321-21 20 45 6.57 61 38 10.79 48 (ÿ0.094)8522-2 22 45 6.62 54 38 10.53 41 (ÿ0.367)Local TCHI 24 44 6.73 87 37 12.22 54 (0.168)

Gusau x G. Pool 20 44 6.75 64 37 11.21 48 (ÿ0.086)Across 87 TZUT-SR-W 24 46 6.79 65 38 11.11 53 (0.126)

aAverage SED value for percentage of kernels infested=0.137 (average value given as exact value is different for

every pairwise comparison). Statistical analysis of percentage of kernels infested was undertaken on logit transformeddata (shown in parentheses), SED applies to values on transformed scale.

bThe higher the index, the greater the susceptibility level. Standard error of the difference (SED) between two means:

for Dobie Index: 0.869, except for comparisons involving the control, SED=0.753; for Urrelo Index: 0.831, except forcomparisons involving the control, SED=0.720; df=254.

cDME - days to mean emergence; DFE - days to first emergence.


differences among varieties were not valid. Therefore, it was decided to divide the varieties intotwo groups: those with a mean SI of greater than 7 (15 varieties) and those with a meansusceptibility of less than or equal to 7 (37 varieties). From now on these two groups will bereferred to as ‘more susceptible’ and ‘less susceptible’, respectively.A separate analysis of variation was undertaken for each of the two groups, and in both of them

the assumptions were satisfied. There were no significant differences in SI among the ‘lesssusceptible’ varieties (Table 2), but differences in SI among the ‘more susceptible’ varieties weresignificant at the 5% level (Table 3).Urrelo index: The assumptions underlying the analysis of variance of the Urrelo index were met

when all 52 varieties were analysed together } there was no evidence that variability decreased asthe mean susceptibility increased. However, in order to be consistent, two analyses of variancewere done, as for the Dobie index. For neither the 37 ‘less susceptible’ varieties, nor the 15 ‘moresusceptible’ varieties was there any evidence of differences within the groups for the value of theUrrelo index (Tables 2 and 3, respectively).

3.4. Relationship between the two indices

The correlation between the Dobie and the Urrelo SIs was strong for both groups: 0.76 for the‘less susceptible’ group and 0.78 for the ‘more susceptible’ group, indicating a close associationbetween the two indices.

Table 3

Mean values for the Dobie and Urrelo indices, and for the percentage of the total number of kernels that were infested,for each of the ‘more susceptible’ varieties (in rank order according to Dobie index)

Variety Dobie Urrelo Percent infested kernelsa

No. F1 DME SIb Egg SME SIb

Staha-SR BC4 F2 27 45 7.17 68 37 11.38 56 (0.256)

Across 8363-SR 20 41 7.32 73 35 12.44 56 (0.259)Toule Houdjii 31 46 7.33 92 38 11.93 58 (0.310)Volta Region local 31 45 7.45 88 38 11.81 57 (0.302)

Togo local 29 44 7.56 79 36 12.09 62 (0.478)TZESR-W-SE 27 43 7.69 93 36 12.77 60 (0.412)8338-1 35 46 7.72 85 35 12.23 64 (0.570)

Cameroun local 32 44 7.81 95 37 12.38 58 (0.320)TZB-SR-SE 30 43 7.92 100 35 13.16 62 (0.503)Gbogbe local 38 44 8.11 100 37 12.50 63 (0.519)Benin local 40 44 8.30 97 37 12.48 66 (0.659)

BSR (Bafia) Cameroun 36 43 8.30 108 37 12.77 61 (0.463)Zouzouvou 41 44 8.49 106 37 12.71 63 (0.553)Kwadaso local 44 44 8.54 118 36 13.48 67 (0.710)

Cross River local 44 45 8.71 120 36 13.51 73 (0.996)

aAverage SE value for percentage of kernels infested=0.137 (average value given as exact value is different for every

pairwise comparison). Statistical analysis of percentage of kernels infested was undertaken on logit transformed data(shown in parentheses), SED applies to values on transformed scale.

bStandard error (SE) of the difference between two means: for Dobie Index: 0.565; for Urrelo Index: SE=0.764;

df=101.


3.5. Differences in susceptibility due to kernel size and hardness

Data on kernel hardness were not available for all 52 varieties, so this analysis was done usingthe subset of the 31 varieties for which a measure of hardness was made. Plots of each of theDobie or Urrelo SI against hardness did not indicate any evidence of a relationship betweensusceptibility and kernel hardness.However, plots of the two indices against kernel size indicated that susceptibility increased as

kernel size decreased (Figs. 1 and 2). For both the Dobie and Urrelo SIs, data analysis indicated arelationship between differences in susceptibility to weevils and kernel size (p5 0.001). There wasalso an indication that differences associated with kernel size varied depending on kernel hardness(p50.001).For varieties in hardness classes 1 and 3, weevil susceptibility was significantly (p50.05) less for

kernel size class 1 than for classes 2 and 3. For varieties in hardness class 2, there was nosignificant difference in Dobie SI between kernel size classes 1 and 2. Both the Dobie and UrreloSIs were significantly higher for varieties in kernel size class 3 than class 1 for all three hardnesscategories (Tables 4 and 5).In order to verify the conclusions about kernel hardness and kernel size, the 31 varieties for

which there were data on both of these characteristics were divided into the ‘less susceptible’ and‘more susceptible’ categories mentioned earlier. Of the 31 varieties, 11 were ‘more susceptible’ and20 ‘less susceptible’. Of these, 80% of the varieties classified as ‘less susceptible’ were in kernel sizeclass 1 (large kernels) and only one (5%) was in kernel size class 3 (small kernels). For the ‘moresusceptible’ varieties, five (45%) were in kernel size class 3, while only two (18%) were in kernelsize class 1. Of the varieties classified as ‘more susceptible’, those in kernel size classes 2 and 3 weresignificantly more susceptible than those in kernel size class 1.

Fig. 1. Relationship between Dobie Susceptibility Index and kernel size (expressed as kernel number per 16-g sample).


Although no clear relationship between susceptibility and kernel hardness was evident from theabove analysis of the 31 varieties it is worth noting that only 10% of the ‘less susceptible’ varietieswere in the softest class (hardness class 1), whereas 55% of the ‘more susceptible’ varieties were inthis class.

3.6. Percentage of the total number of kernels that were infested

There were significant differences (p50.05) among the 52 varieties for the percentage ofinfested kernels (Tables 2 and 3). An analysis of whether the percentage of infested kernels was

Table 4

Mean value of Dobie index of susceptibility for each combination of hardness and kernel size

Kernel size Hardness Mean

Class 1a Class 2a Class 3a

Class 1b 5.43 (0.526)c 6.10 (0.203) 5.67 (0.163) 5.73

Class 2b 8.31 (0.310) 5.55 (0.310) 8.30 (0.563) 7.39Class 3b 7.97 (0.272) 8.11 (0.526) 7.45 (0.526) 7.84Mean 7.24 6.59 7.14

aClass 1 (softest): hardness 50.95; class 2: hardness=95 and 51.38; class 3: hardness41.38.bMean (n=8) kernel number per 16 g; the greater the number of kernels, the smaller the size. Class 1: kernel number

558; class 2: kernel number=58 and 574; class 3: kernel number474.cStandard errors (SE) for the difference between two means are different depending on which two means are being

compared, thus, SE of each individual mean is shown in parentheses, df=235.

Fig. 2. Relationship between Urrelo Susceptibility Index and kernel size (expressed as kernel number per 16-g sample).


related to kernel hardness and size was performed using transformed values. As with the analysisof SIs, differences due to kernel size were significant (p50.05). There was no clear trend inrelation to kernel hardness, but there was an indication that differences due to kernel size varieddepending on the hardness of the kernel. Varieties in kernel size class 1 (large kernels) hadproportionally less infested kernels than those in classes 2 and 3 (Table 6). However, for varietiesin hardness class 2, those in kernel size class 2 were significantly less infested than varieties inkernel size class 1 (large kernels).As expected, the more susceptible a variety according to the Dobie and Urrelo Indices, the

greater the percentage of infested kernels. The correlation coefficient between the Dobie Index andthe percentage-infested kernels was 0.55, while for the Urrelo Index it was 0.64.

Table 5Mean value of Urrelo index of susceptibility for each combination of hardness and kernel size

Kernel size Hardness Mean

Class 1a Class 2a Class 3a

Classb 10.60 (0.534)c 11.15 (0.206) 10.46 (0.166) 10.74Class 2b 13.17 (0.315) 10.35 (0.315) 12.48 (0.571) 12.00Class 3b 12.73 (0.276) 12.50 (0.534) 11.81 (0.534) 12.35

Mean 12.17 11.33 11.58

aClass 1 (softest): hardness 50.95; class 2: hardness=0.95 and 51.38; class 3: hardness=1.38.bMean (n=8) kernel number per 16 g; the greater the number of kernels, the smaller the size. Class 1: kernel number

558; class 2: kernel number=58 and 574; class 3: kernel number=74.cStandard errors (SE) for the difference between two means are different depending on which two means are being

compared, thus, SE of each individual mean is shown in parentheses, df=235.

Table 6Percentage of total number of kernels infested with maize weevils and standard error for each combination of hardnessand kernel sizea

Kernel size Hardness Hardness HardnessClass 1b Class 2b Class 3b

% SE % SE % SE

Class 1c 51 (0.043)a 0.098 51 (0.052) 0.037 48 (ÿ0.066) 0.031

Class 2c 66 (0.656) 0.098 43 (ÿ0.285) 0.055 66 (0.659) 0.096Class 3c 61 (0.450) 0.041 63 (0.519) 0.079 57 (0.302) 0.083Mean 57 (0.286) 52 (0.093) 55 (0.193)

aValues are backtransformed; values on the transformed scale are shown in parentheses. Standard errors apply totransformed values; df=235.

bClass 1 (softest): hardness 50.95; class 2: hardness=0.95 and 51.38; class 3: hardness41.38.cMean (n=8) kernel number per 16 g; the greater the number of kernels, the smaller the size. Class 1: kernel number

558; class 2: kernel number=58 and 574; class 3: kernel number474.


3.7. Anomalies in susceptibility given kernel hardness and kernel size

Residual values from the analyses of variance investigating the effect of kernel hardness and sizewere plotted against variety to determine if some varieties were more or less susceptible thanwould be expected given the hardness and size of their kernels. Any variety for which 75% ormore of its residuals were either negative or positive for at least one of the three analyses ofvariance, for Dobie Index, Urrelo Index, and percentage of infested kernels, was examined moreclosely. A negative residual indicates that the observed value for that experimental unit was lessthan the value expected given its kernel hardness and size, and therefore less susceptible thanexpected; a positive residual indicates the converse. This examination of residuals suggested thatvariety Across 85 TZSR-Y1 was less susceptible than expected given its kernel size and hardness.Additionally, varieties Ikenne 8749-SR BC6, BSR (Bafia) Cameroun, EV 8766-SR BC6 F4, and8321-18 were less susceptible than expected, while hybrid 8338-1 was more susceptible thanexpected given its kernel size and hardness.

4. Discussion

The maize varieties studied were divided into two groups in order to conduct an analysis ofvariance on the SIs; it was hoped that, by analysing these two groups separately, the assumptionsunderlying the analysis of variance would be met, and that tests for differences among varietieswithin the same group would be valid.Ten of the 11 local varieties tested (91%) fall into the ‘more susceptible’ group of varieties. In

contrast, 87 and 91% of the IOPs and hybrids, respectively, were classified as being ‘lesssusceptible’. This suggests that improved maize varieties are not necessarily more susceptible topost-harvest insect attack than local ones, when susceptibility is evaluated using the shelled maizegrain method as in our study. Although it is often found that improved varieties are less resistantto stored-product pests than local ones (Dobie, 1973, 1977; Kossou et al., 1992, 1993), this willvary according to the methods used for testing and the varieties in question. In some cases lowersusceptibility to weevils has been obtained with the utilization of improved varieties. For example,Kirk and Manwiller (1964) reported a substantial decrease over a period of two decades from themid 1940s, in both the number of ears infested by Sitophilus sp. and the percentage damage, dueto the uptake of improved varieties in the USA. It is possible that for the varieties we studied,increased kernel size combined with other kernel characteristics such as texture, has resulted indecreasing levels of susceptibility to weevils. None of the IOPs or hybrids belonged to the smallestkernel size class (class 3), and only 4 out of 41 belonged to the intermediate kernel size, class 2. Incontrast, 64% of the local varieties belonged to class 3, and 9% to class 1. Kossou et al. (1993)suggested that the longer median development period of weevils in improved varieties theyscreened as ears could be due to their larger kernel size. According to these authors (Kossou et al.,1992) larger size could result in the grain embryo being further away from the crown and thus, agreater kernel area for the emerging F1 adult weevil to search before finding an emergence site.Although this mechanism would not be entirely operational when insects are developing in shelledgrain, it may partly explain the differences we observed between small and large-seeded varieties.While this may be a component of resistance to weevils, Kossou et al. (1993) pointed out that


large kernel size could only be used successfully in areas where there is no strong preferenceamong consumers for small kernels.It is important to recognize that farmers in many parts of Africa often store their maize

unshelled with the husk leaves intact. Screening shelled grain to detect varietal differences insusceptibility to maize weevils as in this study is only useful for preliminary testing of diversegermplasm. Kossou et al. (1993) suggested the use of no-choice infestation of ears with husks onas a screening method for elite candidate varieties when the traditional farmer’s storage system isears with husks on. Husk cover quality is also a major component of resistance to weevils andneeds to be assessed. Indeed, due to their superior husk cover quality, local varieties were foundby Kossou et al. (1993) to be significantly more resistant to weevils when infested in the traditionalstorage form (ears with husks) than improved varieties. We did not observe a clear effect of kernelhardness on the susceptibility of maize to attack by S. zeamais, although there was an indicationthat the effect of kernel size was different depending on the hardness of the kernel. Unfortunately,because it was not possible to determine kernel hardness for all the varieties tested importantdifferences among the varieties may have been missed. Other authors have found a relationshipbetween susceptibility and kernel hardness. Dobie (1973) found a positive correlation between thesoftness of maize varieties and their susceptibility to S. zeamais. Serratos et al. (1987) found thatthe two less susceptible maize populations out of four they studied were the hardest, as measuredby the mean force at peak compression using an Instron fitted with an electronic force transducer.The flinty, local varieties studied by Kossou et al. (1993) consistently showed a significantlysmaller number of F1 weevils than improved varieties with softer grain. Vowotor et al. (1995)found that in general, larvae developed significantly slower on varieties with large hard kernels,than in small-seeded, soft ones. Of the varieties tested by Vowotor et al. (1994), Volta Local, avariety with floury endosperm, had the shortest median development period of weevils and thehighest Dobie index of susceptibility among storage forms.The analysis of varieties which were more or less susceptible than expected, given their kernel

size and hardness, showed that variety Across 85 TZSR-Y1 was less susceptible than expected. Itis possible there is some other factor(s) in this variety which confers an extra level of protectionagainst attack by S. zeamais. It is often considered that resistance of maize to stored-product pestsis multi-faceted, with a number of factors contributing to the overall level of resistance (Dobie,1977; Pandey and Pandey, 1983; Serratos et al., 1987).The strong positive correlation observed between the Dobie and Urrelo SIs indicates that these

two indices provide similar assessments of maize susceptibility to S. zeamais, thus the relativeperformance of the 52 varieties is expected to be similar when using either the Dobie or Urrelo SIs.The use of the Dobie Index is preferred, however, as it does not require time-consuming, labourintensive egg counts. The staining procedure (Frankenfield, 1950) employed in the Urrelo methodwas found to raise the moisture content of maize making it difficult to prevent subsequent mouldgrowth. A modified method was therefore adopted, involving no pre-soaking and a shorterstaining period, which minimised water uptake. As noted by Holloway (1985), an increase in themoisture content causes a reduction in weevil developmental period and this point is relevantwhen comparing the parameters of resistance measured in this study with others. We assume thatany decrease in developmental period will be constant for the different varieties, and that anychange in moisture content during the period between the first emergent adult and the last, doesnot significantly affect developmental period. However, it is possible, that there are differences in


moisture uptake between varieties even with short periods because of differences in seed coatcharacteristics. If the above assumptions do not hold it may be an additional disadvantage of theUrrelo method.Results from this study demonstrate that the conversion of two improved varieties, TZB-SR

and TZESR-W, to soft endosperm (SE) carried out by IITA maize breeders have resulted ingreater susceptibility to weevils in the soft endosperm versions of these varieties, TZB-SR-SE andTZESR-W-SE. Additionally, it was found that in general the local varieties tested are moresusceptible to weevils than improved ones. Many of the local varieties examined were soft flourymaizes, which may explain their greater susceptibility to weevils relative to the flinty local varietiesstudied by other researchers. Floury maizes are preferred in many parts of West Africa for theirease in milling and flour characteristics for cooking, but their greater susceptibility to weevilscannot be ignored. In this region of Africa many small-scale maize farmers continue to cultivatethese local varieties because they have a long protective husk cover, and maize is stored with thehusks on. In this way they reduce the chances of depredation by insects as it will take a muchlonger period for weevil populations to build up. Any changes in storage practices and varietiesfor this region have to take into consideration the many interacting factors that come into play inthe local storage environment.

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Documents

A comparison of two methods of assessment of maize varietal resistance to the maize weevil, Sitophilus zeamais Motschulsky, and the influence of kernel hardness and size on susceptibility