Comparison of improved and local maize varieties in the Republic of Benin with emphasis on susceptibility to Sitophilus zeamais Motschulsky

J. stored

Prod.

Res.

Vol.

29, No.

4, pp.

333-343,

1993

Printed in Great Britain. All rights resewed

Copyright 0

0022474X193 MM + 0.00 1993 Pergamon Press Ltd

COMPARISON OF IMPROVED AND LOCAL MAIZE VARIETIES IN THE REPUBLIC OF BENIN WITH EMPHASIS ON SUSCEPTIBILITY TO SITOPHILUS ZEAMAIS MOTSCHULSKY D. K. Kossou’, J. H. MARECK*and N. A. BOSQUE-*REZ’* ‘Facultk des Sciences Agronomiques, Universiti National du Benin, BP 526, Rbublique du Benin and 21ntemational Institute of Tropical Agriculture, PM9 5320, Ibadan, Nigeria (Received for publication 16 June 1993) AbstracG-Experiments were conducted to verify differences in agronomic traits and susceptibility to the maize weevil, Sirophitus zeamais, in the field and in traditional maize storage systems between two international improved varieties, which have not been readily accepted by farmers in the Republic of Benin, one partially improved local variety and a local variety obtained from farmers. Factors responsible for differences in susceptibility and suitable screening methods for resistance to the maize weevil were also studied. In the first part of this study the varieties were compared in the field, during field drying and when stored using traditional techniques. The international improved varieties had significantly higher grain yield (60%) but had significantly poorer husk cover. The percentage of damaged ears following field drying, and the numbers of storage pests and grain weight loss after one month of storage were significantly higher in the international improved varieties presumably as a result of poorer husk cover. In the second part of this study the varieties were compared in a factorial arrangement of three storage forms (undehusked, dehusked and shelled grain) and two infestation methods (free and no-choice). Due to their superior husk cover quality, Benin varieties were significantly more resistant to weevils when infested in the traditional, undehusked storage form than international improved varieties. The smallergrain, flinty, Benin varieties showed a significantly smaller number of F, generation weevils. A significant lengthening of the median development period (MDP) of weevils was observed when maize was stored as dehusked ears compared to shelled grain, particularly on the international improved varieties. No improved vs Benin variety comparisons interacted with free vs no-choice comparisons except for index of susceptibility in the undehusked storage form. Free choice infestations resulted in increased experimental error for number of F, weevils, presumably due to a clumped distribution of female weevils during oviposition. When the target fanner’s storage system is undehusked ears, it is suggested to use no-choice infestation of undehusked ears as a screening method for elite candidate varieties. An assessment of husk quality and no-choice infestation of shelled grain and/or dehusked maize ears is suggested as more appropriate for preliminary screening of diverse germplasm. Key words-Storage systems, maize, varietal susceptibility, screening methods, maize weevil, husk cover, breeding for resistance.

INTRODUCTION

Maize is an important staple food crop in the coastal savanna region of southern Benin, which in the 1950’s had the highest concentration of maize production in sub-Saharan Africa (Miracle, 1966). Today, for the country as a whole, maize constitutes 73% of the total cereal crop and annual maize consumption is 83 kg per person (CIMMYT, 1990). Maize productivity is low, estimated at 0.8 tons/hectare. There have been attempts to introduce improved, disease-resistant and high-yielding maize varieties to farmers in southern Benin, but adoption has been limited. Maize prices show a very wide seasonal variation in southern Benin with prices as much as 34 times higher 8-10 months after harvest than at harvest time. Thus, investments in increased maize productivity should help stabilize prices and show substantial returns if the produce can be safely stored, and if the improved technology is easily adapted to the farmers current technology. A major problem with adoption has been the high level of damage due to the maize weevil, Sitophilus zeamais Motschulsky, sustained by the improved varieties in storage. *To whom correspondence should be addressed. 333

334

D. K. Kosou et al.

The maize weevil is considered one of the most destructive maize pests in tropical and sub-tropical regions. Warnings have been given in various African countries that many of the new, higher-yielding maize varieties which lack weevil resistance characteristics would result in greater storage losses if farmers harvest and store improved maize using traditional practices. Reports from Kenya (Giles and Ashman, 1971), Malawi (Golob, 1981, 1984; Kydd, 1989) Nigeria (FAO, 1980) Ghana (Ghana Grains Development Project, 1987; Badu-Apraku et al., 1992), Cameroon (Almy and Asanga, 1988) and Benin Republic (Miracle, 1966; Anon., 1989a, b) have indicated that improved maize is more susceptible to storage losses than local maize. The presence of long tight husks is known to reduce weevil infestation in the field (Eden, 1952a, b; Wiseman et al., 1970; Giles and Ashman, 1971; Schulten, 1976; Dobie, 1977; Golob, 1984). However not everyone recognizes that the protective function of husks can carry over into the maize storage period (FAO, 1980). The most commonly cited kernel resistance factor is kernel hardness (Eden, 1952b; Dobie, 1974; Golob, 1984). The study of weevil resistance factors in maize kernels was considerably advanced when Dobie (1974) proposed the use of an “index of susceptibility” as a combined measure of numbers of F, generation weevils and the time required for their development. The utility of screening shelled grain to detect varietal differences in susceptibility to maize weevils, however, is questionable if the target storage system is ears with the husks on, which is the traditional storage method in many African countries including the Republic of Benin. Thus, there is a need to develop methods to screen maize ears for resistance to the maize weevil. Since the adoption of improved, high-yielding, disease-resistant maize varieties in Benin Republic is being hampered by their susceptibility to the maize weevil, a series of experiments was initiated in 1988 by the International Institute of Tropical Agriculture (IITA) in cooperation with the Universitt National du Benin. The objectives of the first part of this study were to: (1) document the agronomic and postharvest differences of Benin maize varieties and international improved ones; (2) identify varietal characteristics which affect crop losses in the field and in the store; (3) determine if natural maize weevil infestations could be used to screen maize varieties stored with the husks on and (4) select more appropriate breeding goals. The objectives of the second part of this study were to: (1) verify differences in susceptibility to weevils; (2) determine which factors are responsible for the differences and (3) compare various weevil infestation methods. This information can be utilized to determine the most efficient screening techniques to develop improved, high-yielding varieties with levels of resistance to weevils which are acceptable to farmers.

MATERIALS

AND

METHODS

Origin and pedigree of varieties Studies were conducted at the IITA station at Abomey-Calavi, near Cotonou in the Republic of Benin. Four maize varieties were compared: two international improved (Sekou 85 TZSR-W-l and EV 8443~SR C,), one partially improved local variety (NH2) and a local variety obtained from farmers in Mono Province (Gbogbe). Sekou 85 TZSR-W was selected from the international population TZSR-W in full-sib progeny trials conducted by Direction de Recherche Agronomique (DRA) at Sekou, Atlantique Province, in 1985. The TZSR-W population, developed by IITA in 1980, was one of the first maize streak virus (MSV) resistant populations formed. Sekou 85 TZSR-W is two cycles of selection more advanced than the TZSR-W variety tested in the 1986-87 on-farm trials in Mono Province. EV 8443~SR C, was derived by backcrossing MSV resistance into the international population number 43, or “La Posta”, developed at CIMMYT (International Maize and Wheat Improvement Center), in Mexico. It differs from EV 8343SR, which was extended to farmers through CARDER Atlantique in 198485, by an additional backcross to the recurrent parent, selection for MSV resistance, and four cycles of modified half-sib family selection primarily for improved husk coverage. NH2 (IRAT 42) was developed from a cross between 28 local breeding lines (28-Synthetique-1) and the introduction Costeiio and released in the early 1970’s (Marchand, 1990).

Susceptibility of maize varieties to

Sitophifus zeumais

335

Agronomic practice

Planting was done at the onset of rains on 24 April 1988, in a randomized complete block design with seven replications. Plots consisted of twelve 11 m long rows spaced at 0.75 m. Three seeds per hill were planted at 50 cm spacing and plants thinned to two per hill 2 weeks after planting (WAP). A compound fertilizer providing 60 kg N, 60 kg PZOSand 60 kg KzO ha-’ was applied during land preparation and supplemented 4 WAP by urea at the rate of 60 kg N ha-‘. Two hand weeding operations were performed and no insecticide was applied. Bird scarers were employed to reduce damage caused by birds. Yield and agronomic characteristics

The number of days to 50% silk was measured as the number of days from planting until half of the ears in the plot showed silks (female flowering). Plant and ear height were measured from the soil to the base of the tassel and the node giving rise to the uppermost ear, respectively. Husk cover rating was done using a newly devised scale from 1 to 5 where the rating is done by placing the hand around the husk leaves as they extend beyond the ear tip and making a fist such that the base of the hand rests on the tip of the ear. If the husk leaves are longer than four fingers the rating is one, longer than three fingers the rating is two, etc. until when the husk leaves are not longer than one finger the ear tip is exposed and the rating is five. The husk rating is reduced by a value of one when, by squeezing the husk leaves in the fist, they appear to be loose or easily compressed. From the 1Zrow plot the two external border rows were discarded, ears harvested from the 6 central rows were used for the artificial infestation experiment discussed below and one of the four remaining rows was used for each of the following aspects of the first part of this study: (1) variety yield potential and agronomic characteristics; (2) losses due to field drying; (3) samples for one-month storage and (4) samples for 6-month storage. In all cases, an internal 10 m section of the row was sampled to eliminate border effects. The number of root- and stalk-lodged plants, grain moisture, and grain yield potential were determined 17 WAP. Root-lodged plants are plants leaning from the roots and stalk-lodged plants are bending in the stalk at an angle of 45” or more. Grain moisture was determined using a hand-held Dickey-John moisture tester and grain yield was adjusted to a constant 15% moisture level. Field drying losses

In southern Benin maize is allowed to become well dried in the field before harvesting and storing the ears with husks intact. In order to assess potential losses due to field drying, harvesting of one of the rows from each replication was delayed until the field drying stage was completed 19 WAP. At this point at least 50% of the ears hung downwards from their attachment point on the stalk. Husk tip extension, number of husk leaves, number of damaged ears, and percent weight loss were determined from ears harvested at this time. Husk extension was measured as the length (cm) of the husk extending beyond the tip of the ear. Ears were considered damaged if a single kernel on the ear showed damage from any source. Weight loss of grains in the field was estimated using the procedure described by De Breve et al. (1982, cited by Boxall, 1986). Infestation during storage

Before storing their maize, farmers in southern Benin often practice some degree of selection so as to avoid badly damaged ears. Thus, for our tests, twenty apparently undamaged ears with husk leaves intact were collected 19 WAP from the appropriate rows for 1 and 6 months storage experiments. The ears were placed in labelled cotton cloth bags (35 x 60 cm) which were then put in traditional cribs built at the station by a farmer. One crib was used for each of the storage experiments. Cribs (1.5 m high, 2 m dia) were round and raised 1 m above the ground. The wood used for the structural components was from Fagara xanthonyloides (Rutaceae), the walls were woven using stems from Mallotus oppositzjiolius(Euphorbiaceae), the floor and roof were formed from Elaeis guineensis (Palmae), and the roof was thatched with leaves of Imperata cylindrica

D. K. KOSSOUet al.

336

(Gramineae). One-month-storage bags were maintained closed to serve as a measure of field infestation, while those of 6 months were left open to allow new infestations. At the end of each storage period, samples were fumigated with phostoxin to prevent the escape of live insects. Numbers of lepidopterous larvae and stored product insects were determined. Grain moisture was measured and dry matter loss calculated using the count and weigh method of Adams and Schulten (1977). Data were analysed using analysis of variance procedure and a preplanned contrast of Benin local vs international improved varieties. Data involving numbers of insects were transformed using square root prior to analysis and data ratios were transformed using angular transformation prior to analyses (Snedecor and Cochran, 1967). Infestability

by S. zeamais

At harvest, 20 ears per row were taken from the six center rows of each plot. Ears were placed in 35 x 60 cm cotton cloth bags (20 ears per bag) and placed in a freezer for 3 weeks to eliminate insects present at the time of harvest. Moisture content readings were taken with a Dickey-John moisture tester after a 5-6 week conditioning period. S. zeamais were obtained from the Faculte des Sciences Agronomiques, Universite National du Benin where they were cultured on the maize variety Pirsabak. Insects were transferred to bulk grain to ensure an abundant supply of young individuals. A split-plot design was used for the study. The six main plot treatments were a factorial arrangement of three storage forms: undehusked ears ( = husk leaves undamaged and covering the ear), dehusked ears (= husk leaves removed but grains remained on the cob), and shelled grain (=maize kernels with husk and cob removed) and two infestation methods (free choice and no-choice). The sub-plots were the four maize varieties. Wood cages (60 x 90 x 60 cm with three 30 cm diam aeration openings covered with fine wire mesh) were used as infestation units (one per main plot treatment). The test was replicated seven times (field replications = test replications). Infestation of maize samples was done using 3-day-old unsexed weevils. For the no-choice method, 100 insects per bag were used to infest each variety/storage treatment combination and each bag was closed after adding the weevils to it. For the free-choice infestation, each of the varieties remained in a bag and the four bags were randomized and arranged radially within the cage; bags were left open to allow free movement of weevils towards any variety and 400 insects were placed in an open vial in the center of the circle. In all cases weevils were allowed a seven-day oviposition period, the standard length of time when screening shelled grain (Dobie, 1974). At the end of the oviposition period, weevils were removed and the bags were closed and removed from the cages but kept in the same experiment room. In order to remove the weevils from the undehusked ears, husk leaves were pulled back and returned as much as possible to their original position after the removal of weevils. Temperature and relative humidity in the experimental room were 23 + 3°C and 75 + 5% over the infestation and storage period. F, generation adult weevils emerging were counted and removed daily from each bag. Median developmental period (MDP), defined as the time, in days, from the middle of the oviposition period until the emergence of 50% of the F, generation, and index of susceptibility, defined as [(log, F)/D 1100, where F = number of F, weevils and D = MDP, were determined using Dobie’s (1974) method. However, in this experiment, MDP measured on undehusked ears is a measure of more than just the time from oviposition to emergence of the new adult. In this case, the observed MDP includes a relative measure of the time required to pass through the husk leaves covering the ear. Standard methods of data analysis for split plot design (Snedecor and Cochran, 1967) were followed where possible, however, it was noted that errors for number of F, weevils and MDP were not independently distributed. Transformations could not be found that restored independence, therefore the following alterations in the analysis were done: main plot treatments were tested by their respective main plot by replication interaction. Within the sub-plots, mean comparisons and interactions were tested by pooling the relevant error estimates and obtaining the estimates of error degrees of freedom by methods developed by Satterthwaite (1946). Mean comparisons and interactions tested and reported were based on pre-planned questions and are not necessarily orthogonal nor all inclusive.

Susceptibility of maize varieties to Sifophilus zeamais

337

There are potential problems for a standard analysis of index of susceptibility as it is a ratio of two variables which, in this experiment, were correlated. However an analysis of residuals indicated that errors were normally, but not independently distributed, therefore the variable was analyzed using the methods described above. RESULTS

Yield and agronomic characteristics The results of the field investigations comparing Benin varieties to the international improved varieties are shown in Table 1. The international improved varieties showed a significant and substantial (60%) grain yield advantage over the Benin varieties under the relatively high soil fertility conditions in this trial. The international improved varieties were also significantly shorter in plant and ear height than the local Gbogbe. The highest level of stalk lodging recorded was less than 1% and there were no differences among varieties for this trait. The Benin varieties, especially Gbogbe, had significantly better husk coverage of the ear, as measured by the husk rating scale and by the length of the extension of the husk beyond the ear tip. There were no significant differences among varieties for number of husk leaves. Field drying losses For ears sampled from the field following field drying, the international improved varieties had nearly twice as many damaged ears as the Benin varieties, and this difference was highly significant (Table 1). Confounding effects, such as rotted ears from root-lodged plants, resulted in non-significant differences among varieties for percentage of grain weight loss at harvest. Infestation during storage The results of storing the varieties for 1 and 6 month periods are shown in Table 2. Differences among varieties for grain moisture content were not significant and declined rapidly to 15.7% at 1 month and to 13.2% in 6 months. The local variety Gbogbe had less infestation of stored grain pests and significantly lower grain weight loss after 6 months storage than the other varieties. In general the number of stored grain pests per 20 maize ears was low after both 1 and 6 months of storage. However, after 1 month storage, there were significantly more lepidopterous larvae (mostly Mussidia nigrivenella Ragonot) and significantly more Tribolium spp. on the international improved varieties than on the Benin varieties. This resulted in significantly greater grain weight loss for the international improved varieties after one month storage, Znfestability by S. zeamais The use of free choice in our studies affected estimates of experimental error for number of F, weevils (Table 3), limited the use of standard methods of data analysis and made it more difficult to detect differences between treatment means. Free choice appears to have resulted in clumped distributions of female weevils during the oviposition period. Clumped distribution of maize weevils has been reported to occur in natural field infestations (Dix and All, 1985), likely due to the Table 1. Agronomic performance of Benin and international improved maize varieties, Cotonou, Republic of Benin. 1988

Variety Benin Gbogbe NH2 Inlemational improved Sekou 85 TZSR-W-I EV. 8443-SR C4 F test probability Amona varieties Local vs Improved

Grain yield (t/ha)

Days to 50% silk

Plant height (cm)

Ear height (cm)

Grain moist. (%)

Root lodge (%)

Husk rating (I-5)b’

Husk tip extension (cm)b

3.4 4.0

5-l 55

237 209

138 107

16.5 17.5

20 I2

1.0 1.8

9.0 7.4

9.1 10.1

32 32

7.8 3.0

5.9 6.0

56 55

205 203

103 96

19.5 20.1

I8 9

2.2 2.1

6.2 6.5

10.3 IO.1

54 66

6.9 7.6

to.001

<0.001
<0.001 <0.001



ns ns


“s “S


‘Angular transformation prior to analysis; means retransformed. bMeans of 20 maize ears. ‘Rating scale of I-5, where 1= excellent ear tip coverage and 5 = exposed ear tips. dDamaged ear = an ear with one or more kernels damaged irrespective of the causal agent SPR 29/+-E

Husk Damaged leaves ears (no.)b (%yb,d

Weight IOSS

(%)“b

D.

338

K. Kossouer al.

Table 2. Insect pest incidence and percent grain weight loss of maize ears stored with husks-on in a traditional crib after 1 and 6 months storage periods, Cotonou, Republic of Benin, 1988 Variety

Lepidopterous larvae (No.)“.~

Sitopbiius

Tribo&un

spp. (No.)~

spp. (No.)”

Grain weight loss (%)’

1.8 1.8

0.0 0.02

0.7 1.5

0.4 0.6

3.6 4.0

0.02 0.0

5.1 3.9

1.8 2.8

to.05
ns ns

ns

<0.05


-

4.0 17.1

5.2 24.0

0.8 4.7

-

18.3 19.9

25.5 17.5

3.5 4.5

-

ns

ns

ns

ns

After one month storage

Benin Gbogbe NH2 International improved Sekou 85 TZSR-W-I EV 8443-SR F test probability Among varieties Local vs Improved After six months storage

Benin Gbogbe NH2 International improved Sekou 85 TZSR-W-I EV 8443-SR F test probability Among varieties Local vs Improved

<0.05 ns

‘Means of 20 maize ears. Square root transformation prior to analysis; means retransformed. bMostly Mussidia nigrivenella. ‘Means of 20 maize ears. Angular transformation prior to analysis; means retransformed.

production of an aggregation pheromone by male weevils (Walgenbach et al., 1983). The different storage forms (undehusked, dehusked, and shelled grain) also influenced the error mean square (Table 3) for the observed median development period. Because of the effects of treatments on error mean squares, a standard analysis of the data was not possible. Therefore, only the meaningful pre-planned mean comparisons were analyzed and are reported (Table 4). In spite of the clumped distribution of weevils in the free choice tests there were no significant interactions between infestation methods (free vs no-choice) and varietal groups (international improved vs Benin varieties) with the exception of a significant (P < 0.01) interaction for index of susceptibility measured on undehusked ears (Table 4). Therefore, mean comparisons for treatments are reported across infestation methods except where interaction occurred. The planned comparison to confirm if the international improved varieties are more susceptible to weevils than the Benin varieties in the undehusked ear storage form (across infestation methods) showed the former to have 2.2 times more F, weevils than the latter (Table 5), but differences in observed MDP were not significant (Table 6). The index of susceptibility of international improved varieties was significantly greater than that of Benin varieties (Table 7). Table 3. Estimates of error mean square and observed Fvalues for varieties from analyses of variance of number of F, weevils, weevil Median Development Period (MDP) and index of susceptibility for four maize varieties analyzed within main plot treatment combinations of storage form (undehusked ear, dehusked ear, and shelled grain) and artificial infestation method (free and no choice) Mean square erro$

Observed F value for varietiesb

Free choice

No choice

Free choice

No choice

Undehusked ear Dehusked ear Shelled grain

57,374 19,438 58.038

I 1,202 21,122 26,800

5.48” 8.23” 2.68

9.36.’ 7.95” I.00

MDP Undehusked ear Dehusked ear Shelled grain

10.15 6.53 1.12

14.87 4.18 I.31

0.76 6.40** 4.391

0.53 9.59** 0.62

4.24 I .23 1.90

I .62 0.79 0.64

7.57’1 0.63 3.04

I .41 4.87’ 2.85

No. of F, weevils

Index of susceptibility Undehusked ear Dehusked ear Shelled grain

‘Error mean square (replication x variety interaction) has 18 #except for husked ear/free choice treatment combination for MDP and index of susceptibility, which has I7 dl: “The probability of differences among varieties at the 0.05 and 0.01 levels are indicated by * and **, respectively.

Susceptibility of maize varieties to Sitophilus zeamais

339

Table 4. Tests for statistical significance for pre-planned comparisons of various treatment main effects and interactions for number of F, weevils, Median Development Period (MDP) and index of susceptibility Statistical

Treatment

comparison

s1orage form effects Undehusked vs dehusked Dehusked vs shelled grain Varietal effects Improved vs Improved vs Improved vs

MDP (days)

**

**

l

**

**

l l

Varietal interactions (Imp. 0s local) with storage form: vs undehusked vs dehusked YS dehusked vs shelled grain with infestation method: vs free vs no choice/undehusked vs free vs no choice/dehusked YS free vs no choice/shelled grain

tests”

No. F, weevils

“S

b local/undehusked localidehusked local/shelled grain

significance

* *

ns *t

l

ns

ns ns

“S **

ns ns ns

“S ns ns

Index of suscept. *

*t “S *

* *

l

** ns ns

*The symbols ** and * indicate significance at P < 0.01 and P < 0.05 levels, respectively; ns = not significant. bA “/” means that the comparison is made within one level of another treatment factor. When a treatment factor is not specified in the comparison, then the comparison is across all levels of that factor.

When varieties in the undehusked ear storage form were compared using the free choice method, the international improved had 3.5 times more F, weevils than the Benin ones (Table 5) and had a higher index of susceptibility (Table 7). Compared to undehusked ears, dehusked ones showed a significant increase in the number of F, weevils (Table 5), a significant reduction in observed MDP (Table 6) and a significant increase in the index of susceptibility (Table 7). The variety by husk removal interaction can be used to determine if differences in the quality of husk cover explain differences in susceptibility of these varieties when evaluated as undehusked ears. This interaction was not significant for number of F, weevils, was nearly significant at P = 0.05for observed MDP, and was significant for index of susceptibility. The dehusked ears of international improved varieties had 1.5 times more F, weevils than the Benin varieties (Table 5) but in the former, weevils had a significantly longer MDP (Table 6). The average effect of shelling maize grains was a non-significant increase in numbers of F, weevils (Table 5), a significant decrease in MDP (Table 6) and a significant increase in the index of susceptibility (Table 7). When the international improved and Benin varieties were infested in the shelled grain form, their differences in susceptibility to weevils were diminished. Table 5. Mean numbeP of F, weevils emerging from two international improved and two Benin maize varieties infested as undehusked ears, dehusked ears and shelled grain using free and no choice methods Maize variety Storage form and method of infestation

International 8443

TZSR

Mean

NH2

Undehusked ears No choice Free choice Mean

552 439 496

429 529 419

491 484 487

273 219 246

Dehusked cars No choice Free choice Mean

196 716 756

117 531 627

757 627 692

Shelled grain No choice Free choice Mean

823 755 789

741 539 643

785 647 716

“Means of 20 ears per replicate

improved

per treatment,

Benin Mean

Mean

331 61 196

140 221

396 312 354

439 463 451

601 357 479

520 410 465

638 518 578

672 442 557

731 435 586

705 439 572

745 543 644

seven replicates,

Gbogbe

D. K. Kassou

340

et al.

Table 6. Length, in days, of the median development period for we&Is emerging from two international improved and two Benin maize varieties infested as undehusked ears, dehusked ears and shelled grain usinn free and no choice methods* Maize variety Internat. improved

Storage form and method of infestation

8443

TZSR

Mean

NH2

Gboabe

Mean

Mean

Undehusked ears No choice Free choice Mean

46.4 44.9 45.6

44.4 46.0 45.2

45.4 45.4 45.4

44.1 43.4 43.8

44.4 44.1 44.6

44.3 44.1 44.2

44.9 44.8 44.8

Dehusked ears No choice Free choice Mean

41.0 44.0 42.5

43.0 43.1 43.4

42.0 43.9 42.9

38.9 42.1 40.8

37.6 38.1 38. I

38.2 40.7 39.5

40.1 42.3 41.2

Shelled grain No choice Free choice Mean

32.6 33.1 32.9

33.1 34.6 33.9

32.9 33.9 33.4

33.3 34.0 33.6

32.7 32.1 32.7

33.0 33.4 33.2

32.9 33.6 33.3

Benin

‘Means of 20 ears per replicate per treatment, seven replicates.

DISCUSSION Feedback from farmers suggest that the levels of grain loss they have experienced when storing improved maize varieties are much greater than the ones we observed in the first part of our studies. This difference may be due, in part, to the low levels of natural weevil infestation in our experiments and the selection of undamaged ears with husk leaves intact for the tests. Additionally, the two international improved varieties we used had been subjected to selection for better husk cover and will be likely to perform better (i.e. be less susceptible to weevils) than the earlier versions of these varieties tested by farmers in Mono and Atlantique Provinces. Following field drying, the percentage of damaged ears was twice as large in the international improved varieties compared to the Benin varieties. These differences were very likely due to the poorer husk cover of the improved varieties. However, since damage to a single grain on an ear constituted a damaged ear, the percentage damaged ears was not closely associated with percent grain weight loss at harvest. A scoring method that takes into consideration levels of ear damage may be more realistic for assessing this parameter. Such a method might have revealed a relationship between damaged ears and percent grain weight loss. Although the number of grain pests detected on the cobs was low after 1 and 6 months of storage, the variety Gbogbe clearly sustained lower insect numbers and less damage than the other varieties. The superior husk cover of this variety is believed to be responsible for this.

Table 7. Index of susceptibility for two international improved and two Benin maize varieties infested as undehusked ears, dehusked ears and shelled grain using free and no choice methods” Maize variety Storage form and method of infestation

Internat. improved 8443

I-ZSR

Mean

NH2

Gbogbe

Mean

Mean

Undehusked ears No choice Free choice Mean

13.7 13.3 13.5

13.6 13.5 13.6

13.7 13.4 13.5

12.5 11.7 12.1

12.9 8.8 10.9

12.7 10.3 11.5

13.2 11.8 12.5

Dchusked ears No choice Free choice Mean

16.2 14.9 15.6

15.4 14.4 14.9

15.8 14.6 15.2

15.6 14.3 15.0

17.0 15.0 16.0

16.3 14.6 15.5

16.1 14.6 15.3

Shelled grain No choice Free choice Mean

20.5 19.7 20.1

19.8 17.9 18.9

20.2 18.8 19.5

19.3 17.7 18.5

20.0 18.5 19.2

19.7 18.1 18.9

19.9 IS.5 19.2

Benin

‘Means of 20 ears per replicate per treatment, 7 replicates

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The lepidopterous ear borer M. nigrivenella was detected after 1 month of storage, but not after 6 months. This insect has been reported to be a field to storage pest but it does not reproduce in storage (Moyal and Tran, 1991). In the tests using natural infestations we observed a substantial variation in number of weevils among replicates of the same variety (i.e. a range of 9-92 for EV 8443-SR and O-20 for Gbogbe). This variation makes it difficult to identify varietal differences in susceptibility to weevils. The maize weevil has been reported to have a clumped distribution (Dix and All, 1985). Our results suggest that natural infestations are not sufficiently reliable to detect differences when screening is done using cobs with the husks on. Thus, from the breeding stand-point, artificial infestation with uniform numbers of maize weevils will greatly facilitate the screening of maize varieties for resistance to S. zeamais. In the artificial infestation studies the international improved varieties had substantially more F, weevils developing on their grains. Thus, with respect to the varieties compared in this experiment, the improved varieties are more susceptible to weevils than the Benin varieties when stored in the traditional manner. The use of free choice infestation with husks on magnified the difference between international improved and Benin varieties. The magnitude of difference better reflects farmer’s experience when they store improved varieties. Dehusked ears had a significantly greater index of susceptibility than ears with the husks on. This is consistent with the notion that husk cover is a major factor in determining susceptibility to weevils in the traditional storage system. The dehusked ears of international improved varieties had more F, weevils than the Benin varieties, but in the former, weevils had a significantly longer MDP. Kernel quality factors such as hardness and size probably explain these differences. The longer MDP counteracts the greater number of F, weevils in international improved varieties and resulted in a non-significant difference between varietal groups for index of susceptibility. Since the international improved varieties did not have a greater index of susceptibility than the local varieties when husks are removed, their greater susceptibility with husks on must be due to their inadequate husk cover. The observed reduction in MDP due to shelling maize grains may be of considerable value in understanding and/or developing suitable weevil control strategies since the difference in MDP between storage forms is larger than the maximum difference observed between varieties (within the same storage form) in this experiment and in the original experiments of Dobie (1974). The longer MDP of weevils on dehusked maize ears compared to shelled grain was subsequently verified and explained (Kossou et al., 1992) as due to the less suitable endosperm diet of first instar larvae which hatch from eggs laid on or near the kernel’s crown and the difficulty of finding a site on the kernel where the F, generation adults can emerge. There was a significant varietal group by shelling interaction for MDP and for index of susceptibility, but not for number of F, weevils. This varietal group by shelling interaction for MDP, combined with significant varietal group differences for MDP of dehusked ears clearly shows that there is genetic variability for the longer MDP of dehusked maize ears. The longer MDP of weevils in improved varieties in the dehusked ear form may be due to their larger kernel size. As we have previously explained (Kossou et al., 1992) larger kernel size would likely result in the embryo being further away from the crown and there would be a greater kernel area for the emerging F, adult weevil to explore before finding a suitable exit site. This may be of value in a weevil resistance breeding program because it raises the possibility that the low number of F, weevils found on dehusked ears of the Benin varieties can be combined with the longer MDP of the improved varieties. However, large kernel size could only be used successfully in areas where there is no strong consumer preference for small kernels. Before attempting to design a selection strategy for developing high-yielding varieties which have acceptable levels of resistance to weevils, a review of the three main resistance factors which are present in the varieties studied and relevant to current farmer’s storage practices in Benin Republic would clarify the issues. First, adequate husk cover appears to be the key factor in developing resistant varieties, as it was responsible for differences in susceptibility between Benin and international improved varieties. Indeed, given the rate of population increase of weevils on the most resistant variety, it appears that deterring access to the maize grains through adequate husk cover is essential to preserving stored maize for periods longer than a few months.

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Secondly, the Benin varieties consistently had fewer F, weevils emerging from their grains than the international improved varieties across all storage methods. The experiment did not allow us to separate effects of numbers of eggs laid or egg/larval mortality, therefore the exact cause of this difference is not clear. Nevertheless, the Benin varieties have a harder, flinty endosperm while the international improved varieties have dent (EV 8443-SR) or intermediate dent-flint (TZSR-W-l) texture, and it is likely that the main factor operating here is differences in kernel texture. Thirdly, there was an unexpectedly large increase in weevil MDP across all varieties when they were infested as dehusked ears compared to shelled grain. Weevils had a longer MDP on dehusked ears of the international improved varieties than on the Benin varieties, but the varietal groups did not differ for MDP when infested as shelled grain. When developing a selection strategy, one should also recognize that germplasm goes through two selection phases, a preliminary selection phase, where large numbers of individuals or progeny are screened and a final phase, where a few, elite, candidate varieties are screened in-depth. Screening of maize germplasm in an undehusked form would seem to be the most appropriate method, since it incorporates all the important effects and directly corresponds to farmer’s storage methods. However, given the large experimental errors involved in this form of screening, it would require replicated experiments and thus only be useful for final screening of elite germplasm where multiple ears of a variety would be available. In such final screening prior to release of new varieties, it would seem wise to use a no-choice infestation method. Since evaluation of the individual factors responsible for resistance can often result in more efficient selection, screening of shelled grain and/or dehusked ears may be more desirable for preliminary screening. An independent assessment of the quality of husk cover would then need to be taken prior to de-husking. We believe preliminary screening of germplasm for resistance to weevils using shelled grain offers little value for maize improvement when the target storage system is undehusked ears. Differences between varieties for both number of F, weevils and MDP were small and difficult to detect. In contrast, differences between varieties were large and easily detected when screening was done using dehusked ears. The only advantages of the shelled grain method is that it is the established, standard method of screening, it requires only a small portion of the kernels on an ear and it provides an estimate of relative varietal susceptibility when shelled grain is stored. Determination of infestation method (free or no-choice) for shelled grain or for dehusked ears depends primarily on which method would be logistically efficient. However, given the large experimental errors observed in the free choice tests, again we suggest utilizing a no-choice method. In our experiments we observed that weevils can and do penetrate the husk at sites other than the silk channel at the tip of the ear. Following our findings a re-examination of the factors influencing the quality of the husks (e.g. husk extension beyond the ear tip, husk tightness, numbers of husk leaves, thickness of husk leaves, etc.) with respect to their deterrence to weevils was undertaken and results of these tests are reported elsewhere. The greater susceptibility of improved maize varieties to weevils would suggest that resistance to weevils has not been a high priority in international maize research in the past. Competitive international pressures exist which bring about an over-emphasis on yield improvement. Expectations also exist that germplasm with large increases in yield potential can induce farmers to modify their traditional production and storage techniques. Yet resistance breeding has long been the hallmark of IITA maize research (Fajemisin et al., 1985). Recognition of the importance of husk cover for resistance to birds, ear rots, etc. in addition to its role in weevil control has also been present, and “good” husk cover has been an important selection criterion, as part of an overall resistance breeding strategy at IITA. The problem then becomes inadequate recognition of how good “good” husk cover needs to be to adequately resist weevils, which in turn, implies that there has been inadequate communication between international researchers and the farmers they are ultimately meant to serve. The results of the on-farm research in Benin (Anon., 1989a,b) and the results presented here demonstrate that the quality of husk cover in existing international improved varieties of maize is inadequate for weevil resistance in traditional storage systems. These results should stimulate maize breeders across Africa to pay more attention to all weevil resistance factors, particularly husk cover wherever the established on-farm storage system depends upon this either as a pre-harvest or both pre- and post-harvest weevil deterrent method.

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