Musa Abdulrasak Kannike, Ajayi Mary, Yusuf Sáadat Yetunde, and Lawal Mujidat Temidayo

Department of Crop Protection, University of Ilorin, Ilorin, PMB 1515, Nigeria

Abstract: A study was carried out to determine the relative susceptibility of six maize (Zea mays L.) varieties to maize weevil,Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae). It was conducted at Department of Crop Protection Laboratory,University of Ilorin, Nigeria at ambient temperature of 28±2℃ and 68%±3% relative humidity. Samples of the varieties were infested with five pairs of S. zeamais adults in their respective containers arranged in completely randomized design with three replications.Data were collected on moisture content, adult mortality, adult emergence, grain loss and grain damage. The index of susceptibility and means of damage parameters were used to determine susceptibility of the maize varieties. The results showed that 2008 SYNEE W DT STR was highly resistant and 2004 TZE-W POP STR C4 was moderately resistant, while 2004 TZE-Y-POP STR C4,99 TZEE-Y-STR, EV99QPM and TZE-Y-POP DT STR QPM C0S1 were susceptible. Under artificial infestation, the weevils had significantly longer survival period in the susceptible varieties than those in the resistant genotype. It could be concluded that the susceptible varieties already released to farmers should be protected from maize weevil damage.

Key words: grain pest, variety, susceptibility, damage index

Introduction

Cereals belong to the family Poaceae (Gramineae)of which wheat, rice, maize, barley, oats, rye, millet,sorghum, buckwheat and mixed grains are family members. Among the various cereals grown in Nigeria,maize has the greatest geographical spread both in production and usage. The order of importance of the top three cereals in the world is wheat, rice and maize.Sasson (2012) reported that though maize occupies less land area than either wheat or rice, it gives a greater average yield per unit area. The importance of maize in West Africa is well-studied and yet, receiving increased attention (Nwosu, 2014). The crop has been reported as one of the most important cereal crops for alleviating global food crises (Tongjura et al., 2010;Sasson, 2012).

Despite the importance of maize, post-harvest damage of maize grains is largely caused by abiotic factors, such as fluctuating temperature and relative humidity and biotic factors, such as invertebrates(insects, mites) and vertebrates (rodents, birds). The maize weevil, Sitophilus zeamais Motschulsky, has a high capacity to penetrate grain mass and can infest grains in the field as well as storage and accounts for about 50% of loss in stored maize (Udo et al., 2011).Declining food production, worsened by huge losses resulting from S. zeamais attack during maize storage exposes farmers to different magnitudes of food shocks(Nwosu and Nwosu, 2012). Maize weevils infest maize, while the crop is still in the field (Pendleton et al., 2005; Asawalam and Hassanali, 2006) and destroy the crop in storage (Musa, 2005). The use of chemical insecticides against S. zeamais has a number of shortcomings which include toxicity to non-target organisms, carcinogenicity, pesticide resistance, etc.These chemicals are not safe for the users and the environment, often require periodic repetition and are consequently not cost-effective (Gadzirayi et al.,2006). In improved varieties, different sources of resistance to insect pests of stored products, especially S. zeamais, have been incorporated (Mwololo et al.,2012), but another report has shown that S. zeamais still infests and damages elite maize varieties (Nwosu,2014). Therefore, a great number of researches still have to be carried out using improved maize varieties to develop resistance to S. zeamais. Most researches on improved maize genotypes have been carried out in the field (Demissie et al., 2008; Siwale et al., 2009). This study aimed at determining the relative susceptibility of six maize (Zea mays L.) varieties to maize weevil,S. zeamais in storage.

Materials and Methods

Laboratory assays

The study was carried out in the laboratory of the Department of Crop Protection, University of Ilorin,Ilorin, Nigeria at the average temperature of 28±2℃and relative humidity of 68%±3%.

Insect culture

Twenty-five pairs of adult weevils were collected from naturally infested maize grains in the laboratory and introduced into 500 mL capacity Kilner jar holding 200 g of maize grains to establish a fresh culture of S. zeamais. The jar was covered with muslin cloth fastened with a rubber band to secure the insect. To obtain individuals of standardized age for the study,the female adults were allowed to oviposit inside the grains for 7 days. All the weevils were removed with a camel`s hair brush and discarded. Freshly emerged weevils were used for the study.

Maize varieties

The maize varieties used for the study included: 2004 TZE-W-POP STR C4, 2004 TZE-Y-POP STR C4, 99 TZEE-Y-STR, 2008 SYN-EE-W-DT STR, EV99QPM and TZEE-Y-POP DT STR QPM C0S1 obtained from the International Institute for Tropical Agriculture(IITA), Ibadan, Nigeria. These varieties were already released to farmers for increasing production and consumption. Moisture contents of the maize varieties were determined by oven dry method using the formula:

Where,

w=wet weight

d=dry weight

MC=Moisture content on a percentage basis

The maize varieties were disinfested in separate air-tight polyethylene bags placed in the freezer compartment of a refrigerator for 7 days to kill any forms of insects and microbes (Kossou et al.,1993). The maize genotypes were conditioned in the laboratory for 7 days to attain prevailing temperature and relative humidity.

Experimental design

Five pairs of freshly emerged adult weevils (1-2 days old) were separated and counted with the aid of tweezer and tally counters and then introduced into each jar (9 cm in diameter and 14.2 cm in height)holding 100 g of grains (average number of 556 kernels) from each of the six maize varieties. The jars were covered with muslin cloth (18 cm×18 cm)fastened with rubber band to secure the weevils. Each treatment was arranged in completely randomized design with three replications.

Data collection

After an oviposition period of 7 days, the live and dead adult (parent) weevils were removed, counted and discarded from the samples. Insect pins were used to probe immobile weevils at the abdomen and any weevil that did not respond to the pin probe was considered dead.

On the 28th day after infestation the maize samples were sieved daily for adult emergence counts and this reading continued until the last weevil emerged. The time from oviposition to the emergence of 50% of adult emergence was also noted for each maize variety.Dobie (1974) index of susceptibility was calculated from the relation:

Where,

MDP=Mean of developmental period and this was the period (days from the middle of the oviposition period to the middle of the adult emergence (i.e., 50%emergence).

x=Developmental period of adult weevils in each replicate/Number of insects that emerged daily

SI=Index of susceptibility

loge=Sometimes written as logn=Natural logarithm

Dobie (1974) index of susceptibility and modified method of Osipitan and Odebiyi (2007) using damage parameters were used to classify the maize varieties.The damage parameters evaluated included percentage grain damage, percentage grain loss, mean number of emerged adults and mean adult mortality which were converted to scores (1-5) (Table 1) according to Osipitan and Odebiyi (2007).

Grain loss

The difference between the initial weight and thefinal weight of the maize grains was taken divided by the initial weight and then expressed as percentage. Grain weight loss was expressed as a proportion of the initial weight of grains and then expressed as percentage(Tefera et al., 2011).

Table 1 Damage parameters used in evaluating resistance and susceptibility of maize varieties to Sitophilus zeamais

Grain damage

Damage assessment was carried out on the infested maize varieties by separating the grains into damaged(grains with characteristic holes) and undamaged categories and computing percent grain damage(Osipitan and Odebiyi, 2007).

Data analysis

Data collected were subjected to analysis of variance and means were separated using Least Significant Difference (LSD) at 0.05 level of probability(Montgomery, 2001).

Results and Discussion

Table 2 showed the resistance rating of the six improved maize varieties using the mean scores from the damage parameters to classify the maize varieties into susceptible, moderately resistant and highly resistant.

Table 2 Resistance rating of six improved maize varieties using mean score of damage parameters

Table 3 showed the adult mortality of the weevil,adult emergence and grain damage and weight loss as a result of the feeding activities of the maize weevil,S. zeamais in the maize varieties. The adult mortality did not differ significantly (p＞0.05) among the maize varieties. 2008 SYN-EE W DT STR had the highest mean adult mortality of 6.00±1.73 while 99 TZEE-Y STR had the least mean adult mortality of 0.67±0.58.Since 2008 SYN-EE W DT STR had not been able to confer complete immunity against the maize weevil,there was therefore the need for more detailed studies on the variety especially on the genetic and physical chemical composition. Flinn et al. (2006) had earlier reported when avidin content in transgenic maize grains reached about 100 ppm or higher, it inhibited the development of almost all the insect pests which damaged grain during storage. The presence of this and other glycoproteins in the maize varieties might be responsible for weevil mortality in the maize varieties.

Table 3 Performance of six maize varieties screened for post-harvest resistance of Sitophilus zeamais

In this investigation, weevil emergence, grain damage and grain loss varied with the varieties. Parent weevil survival tended to be lower in the resistant varieties (2008 SYN-EE W DT STR) than that in the susceptible varieties. Thus, the most susceptible varieties had the highest mean number of weevil emergence (40.33±10.26) compared with other varieties. The action of shaking, pouring and sieving might have reduced the number of emerged adults. The higher numbers of emerged adults were a consequence of the larger numbers of parent survival.It could therefore be deduced that the susceptible varieties favoured the development of the larger numbers of immature stages. The results agreed with the previousfindings (Garcia-Lara et al., 2004)that progeny tended to be higher in the susceptible varieties than that in the resistant varieties. The shortest developmental period occurred on the 99TZEE-Y STR and produced the greatest numbers of emerged weevil with the greatest weight loss of 4.86%. Siwale et al. (2009) attributed the median developmental period between 39.67 and 99.30 days to the less than optimal relative humidity in the constant climate room. The longest developmental period occurred on 2008 SYN-EE W DT STR and produced the least number of emerged adults with the least grain loss of 1.84. Using the rating scale of damage parameters, 2004 TZE-Y POP STR C4; EV 99 QPM and TZEE-Y POP DT STR QPM COS1 with weighted mean ranging from 9.3 to 9.5, were rated susceptible,while 99 TZEE-YSTR with weighted mean of 15.6 was rated highly susceptible. Low numbers of emerged adults reduced low grain loss, which was a clear indication of the variety resistance to the weevils (2008 SYN-EE W DT STR) with a weighted mean of 2.5(Table 4). The behaviour and development of maize weevils was influenced by the nature of the maize varieties and the grain damage and losses were the consequences of emerged adults from the grains. This observation confirmed that chemical and nutritional composition of the grains were important primarily in resistance to insect attack and damage (Dobie, 1974).In another investigation, grain damage ranging from 1.94% to 28.57% had been reported on selected maize varieties screened for resistance to S. zeamais (Nwosu et al., 2015).

Table 4 Improved maize varieties as tools for determining the resistance and susceptibility of maize weevil, Sitophilus zeamais

In this study, the grain moisture content varied from 9.24% to 12.02%. The least grain moisture content of 9.24% recorded in 2008 SYN-EE W DT STR conferred resistance and reduced survival of maize weevil. Bergvinson (2001) reported that maize varieties with less than 12% moisture content conferred resistance against larger grain borer Prostephanus truncatus (Horn) and S. zeamais. This report showed that the rate of multiplication of the insect occurred irrespective of the moisture contents of the genotypes.Siwale et al. (2009) used maize varieties that had moisture contents ranging from 10.5% to 12.5%, while Arnason et al. (1994) used maize samples that had moisture contents ranging from 10.4% to 14.90%.Bergvinson (2001) stated that moisture contents above 16% rendered resistant maize varieties susceptible.Using Dobie's (1974) index of susceptibility, 2008 SYZN-EE W DT STR had a mean of 2.51±0.44 and was rated resistant while other varieties had mean between 10.66±1.76 and 17.36±16.33 and were rated susceptible (Table 4). The higher the susceptibility index the more susceptible the variety. Knowledge of the insect behaviour as it attacked the maize kernels would help in enhancing its effective management in storage. Nwosu et al. (2015) found that the bases of resistance were increased chemical attributes of phenolic acid, trypsin inhibitor and crudefibre while the bases of susceptibility were increased protein and starch.

The range of values of indices obtained in the study (2.51 to 17.36) was a deviation from those recorded by other researchers. Siwale et al. (2009)recorded a susceptibility index range of 0.77 to 7.11.The differences in susceptibility could be attributed to the variability in character of the varieties and the environmental effect. Duarte et al. (2005) reported that variety had a much larger influence on grain quality parameters than that of the environment. However, the study agreed with thefindings of Arnason et al. (1994)who obtained indices as high as 14.0 in the susceptible varieties. In this study, 2008 SYN-EE-W DT STR had significantly lower grain loss than other varieties.The most susceptible variety was 99 TZEE-YSTR with significantly lower adult mortality, high grain loss, grain damage, progeny emergence and index of susceptibility than other varieties.

The report confirmed that resistant maize cultivars could reduce losses, due to weevil infestation, but no maize grain was immuned to attack by the weevil(Ivbijaro, 2009). The use of resistant varieties alone might not provide a permanent solution to the problems of maize storage, but rather might contribute to integrated pest management (Gudrups et al., 2001;Credland et al., 2003). In this study, the susceptibility of maize varieties to maize weevil infestation was in agreement with the record of Enobakhare and Law-Ogbomo (2002) that improved maize varieties were highly prone to postharvest infestation by storage insects. Osipitan and Odebiyi (2007) concluded that resistance of improved maize varieties to P. truncatus was the consequence of antixenosis and antibiosis, but not tolerance. Painter (1951) defined tolerance as the ability of the plant to grow and reproduce itself or to repair injury to a marked degree in spite of supporting a population of insects approximately equal to damage a susceptible cultivar. From this definition, tolerance as a mechanism of resistance was not achievable in stored products.

Conclusions

This study showed that varietal resistance was a potentially useful option for the long-term control of maize weevil especially if combined with other methods (Integrated Pest Management). It could be concluded that the susceptible varieties already released to farmers should be protected from maize weevil damage. It is recommended that the resistant maize variety, 2008 SYN-EE W DT STR could be stored for more than two months without the use of synthetic pesticide if prevented from cross infestation.