Resistance of dried chips of yam (Dioscorea cayenensis-D. rotundata complex) landraces to Dinoderus porcellus Lesne (Coleoptera: Bostrichidae)

Yams (Dioscorea spp.) are one of the food crops that contribute to food security and poverty alleviation in Benin and, like several other products, the post-harvest phase is threatened by the rotting of fresh tubers. To overcome this constraint, yam tubers are traditionally processed into dried chips which unfortunately are severely attacked by Dinoderus porcellus Lesne (Coleoptera: Bostrichidae). Research studies on relative resistance of dried chips of 24 yam landraces to the attack of D. porcellus during storage were carried out using free-choice tests (antixenosis) and non-choice tests under laboratory conditions. Attractiveness of yam chips, mortality of initial pest populations, weight loss, and also the Dobie’s index of susceptibility were considered as indicators of resistance. The results revealed that the landraces Boniwouré and Wonmangou were significantly less attractive and consumed by D. porcellus. The highest mortality rate of D. porcellus was observed on the landrace Boniwouré followed by Alahina. Based on the Dobie index of susceptibility five yam landraces (Gaboubaba, Boniwouré, Alahina, Yakanougo, and Wonmangou) were scored as resistant to D. porcellus. All the remaining landraces were categorized as moderately resistant. Based on the present study, these five resistant landraces deserve special consideration and may be recommended for relatively longer storage to achieve the goal of sustainable management of D. porcellus. *Corresponding author: Laura Estelle Yêyinou Loko, Laboratory of Applied Entomology (LEnA), Faculty of Sciences and Technology of Dassa, National University of Sciences, Technologies, Engineering and Mathematics of Abomey (UNSTIM), BP 14 Dassa, Benin E-mail: lokoestelle@yahoo.fr Reviewing editor: Manuel Tejada Moral, University of Sevillem, Spain Additional information is available at the end of the article ABOUT THE AUTHORS Laura Estelle Yêyinou Loko is a lecturer-researcher at the Faculty of Sciences and Technology of Dassa of the National University of Sciences, Technology, Engineering and Mathematics of Abomey (Benin republic). Her research interests lie in entomology, plant genetic resources and crop protection. Her current research interests include the development and implementation of Pest Management practices for stored food product. PUBLIC INTEREST STATEMENT Yam is an important tuber crop that highly contributes to food security and poverty alleviation in Benin. However, yam tubers are highly perishable with 30 to 85 % post-harvest losses. In Benin, dehydrated yam chips are the only form in which fresh yam tubers are preserved throughout the year. Unfortunately, dried yam chips in traditional storage systems are severely attacked by insects which rapidly reduces yam chips into powder within few days of storage. Synthetic insecticides are currently used by farmers to control these pests, leading to many cases of food poisoning. This study proposed the use of resistant varieties as an economically feasible, technically easy and environmentally friendly alternative to minimize losses to storage insect pests. Five resistant landraces to yam chips insect storage pests were identified and can be included in the production system of yam in Benin meant for chips production. Received: 25 October 2017 Accepted: 27 November 2017 First Published: 02 December 2017 © 2017 The Author(s). This open access article is distributed under a Creative Commons Attribution (CC-BY) 4.0 license.


PUBLIC INTEREST STATEMENT
Yam is an important tuber crop that highly contributes to food security and poverty alleviation in Benin. However, yam tubers are highly perishable with 30 to 85 % post-harvest losses. In Benin, dehydrated yam chips are the only form in which fresh yam tubers are preserved throughout the year. Unfortunately, dried yam chips in traditional storage systems are severely attacked by insects which rapidly reduces yam chips into powder within few days of storage. Synthetic insecticides are currently used by farmers to control these pests, leading to many cases of food poisoning. This study proposed the use of resistant varieties as an economically feasible, technically easy and environmentally friendly alternative to minimize losses to storage insect pests. Five resistant landraces to yam chips insect storage pests were identified and can be included in the production system of yam in Benin meant for chips production.

Background
Yams (Dioscorea spp.) serve as an important source of carbohydrates for many people of the sub-Sahara region, especially in the yam zone of West Africa (Sartie, Franco, & Asiedu, 2012). In Benin, the fourth largest yam producer next to Nigeria, Ivory Coast and Ghana, yam is among the most important food crops with an annual production of 3.22 million tonnes (FAO, 2014). It is generally grown for its tubers, which represent the staple food of more than half of the population of Benin (Djoï & Monhouanou, 2003). In Benin, yam production is fully part of the customs and traditions of the people, to a point where we can speak of yam civilization (Baco, Tostain, Mongbo, Dainou, & Agbangla, 2004). More than a subsistence crop, yam has become a commercial crop and plays a key role at both the economical as well as the socio-cultural levels (Baco, Tostain, Mongbo, Biaou, & Lescure, 2007).
However, yam remains a seasonal crop and the fresh tubers are very perishable with an estimated post-harvest losses ranging from 30 to 85% of the total production (Babajide, Atanda, Ibrahim, Majolagbe, & Akinbayode, 2008). To minimize the post-harvest loss and its seasonal supply, yams are traditionally processed into chips (Akissoé et al., 2001;Hounhouigan, Kayodé, Bricas, & Nago, 2003). Unfortunately, yam chips are often severely attacked by storage insect pests, which can devastate the entire stocks into powder within a few months after storage (Ategbo et al., 1998;Babarinde, Babarinde, Odewole, & Alagbe, 2013;Oni & Omoniyi, 2012;Vernier, Goergen, Dossou, Letourmy, & Chaume, 2005). Previous studies undertaken in Benin  and in Nigeria (Oni & Omoniyi, 2012) revealed that the beetle Dinoderus porcellus Lesne (Coleoptera: Bostrichidae) is the most important pest of yam chips. In order to protect stored yam chips against this pest, some producers used chemical insecticides recommended for insect pests of cotton  which caused many cases of food poisoning on humans (Adedoyin, Ojuawo, Adesiyun, Mark, & Anigilage, 2008;Adeleke, 2009). It is therefore utmost important to find alternative management methods to protect the yam chips, and consequently to preserve both human health and the environment.
Thus, the search for easily available, eco-friendly, and cost effective insect pest management options are of paramount importance. One of the alternative management options is the use of resistant yam landraces against insect pests of yam including D. porcellus. The use of resistant varieties is economically feasible, technically easy and environmentally friendly alternative to minimize losses to storage insect pests . Previous studies also indicated that farmers have a strong preference for yam varieties whose chips are resistant to storage insect pests . Owing to the large diversity of yam germplasms in Benin, a participatory evaluation of yam landraces was conducted in 51 major yam production villages of the country and assessed 64 landraces whose chips were identified as being resistant to storage insect pests (Loko, Adjatin, Dansi, Vodouhè, & Sanni, 2015). This study evaluates chips of yam landraces of Benin for their resistance to D. porcellus based on a susceptibility index.

Yam landraces
The plant material consisted of 24 yam landraces (D. cayenensis-D. rotundata complex) belonging to the "Kokoro" variety group. The choice of these 24 yam landraces within the pool of the 64 landraces previously identified , was made on a basis of their agronomic (productivity, number of tubers), culinary (quality of pounded and boiled yam) and technological (quality of yam chips, ease of pounding) characteristics. The tubers of these 24 landraces were collected from farmers across northern and central Benin (Table 1) and processed into yam chips.

Processing of yam chips
Yam chips were obtained using the method described by Babajide, Oyewole, Henshaw, Babajide, and Olasantan (2006). To remove soil and other unwanted material yam tubers were washed with water and peeled with knife, and subsequently cut into slices of 2 to 3 cm of thickness. The yam slices were pre-cooked in water at 50°C for 2 h and allowed to macerate in the cooking water for 24 h to soften them. The slices were drained and dried in an oven at 60°C for 5 days to give a final moisture content of about 12 to 14%. The obtained dry chips were stored in polyethylene bags at room temperature in the laboratory. Prior to their use, chips were sterilized in an oven at 105°C for 2 h to kill hidden insects and their eggs, and left to cool down at room temperature for 1 h (Oni & Omoniyi, 2012).

Rearing of Dinoderus porcellus
Rearing of D. porcellus was done on healthy yam chips as described by Onzo, Biaou, Loko, Tamo, and Dansi (2015). Insect rearing was done in plastic containers (19.5 cm height, 6.5 cm diameter) kept in a wire-netted and gauzed shelf to prevent attack from other pests and promote a stable condition for reproduction (Babarinde, Sosina, & Oyeyiola, 2008). Insect colonies were raised by infesting 500 g of sterilized yam chips in plastic containers with 200 mixed sex adults of D. porcellus obtained from the infested yam chips from Dantokpa market located in Cotonou. Each plastic container was covered with a transparent muslin cloth held in place by a rubber band, in order to prevent the insects from escaping. The plastic containers were kept in the laboratory at 25 ± 2°C, 70 ± 5% R.H. and 12L:12D photoperiod (Oni & Omoniyi, 2012). Every two weeks, adult D. porcellus were removed in order to synchronize the F1 progeny that was used for the experiments (Isah, Ayertey, & Boateng, 2009).

Choice experiment
Preference of D. porcellus adults by the various yam landraces was evaluated in a choice experiment for delimiting an arena of a circular shape 18 cm in diameter on a cardboard, as described by Babajide et al. (2008). A 5 cm-radius circle was drawn at the center of the arena. The ring between the two circles was then divided into 24 equal parts ( Figure 1). The cardboard was then placed in a tray of 18 cm diameter and glued to the floor of tray. In each compartment, a sample of 5 g of yam chips of each landrace was deposited equidistant from the center of the arena. Later, 25 starved (1 h of starvation) D. porcellus adults(48 h old) were released at the center of the arena, which was immediately covered with a transparent muslin cloth, in order to prevent the insects from escaping (Isah et al., 2009). The experiment was replicated at 3 different times with 3 replications (a total of 9 repetitions), each time with fresh individuals of D. porcellus and new yam chips (Onzo et al., 2015). The distribution of the insects in the arena was recorded at 1, 24 and 48 h after infestation (Babarinde et al., 2008).

Adult mortality and yam chips weight loss
In this experiment, 20 g of yam chips of each landrace were placed in circular plastic containers (4.5 cm height × 3 cm diameter) according to Issa, Afun, Mochiah, Owusu-Akyaw, and Braimah (2011) and Onzo et al. (2015). In each container, the yam chips were infested with 5 adults (48 h old) of mixed sex D. porcellus. These containers were covered with a perforated plastic lid with a hole covered by a muslin cloth to allow aeration and prevent escape of the insects. The plastic containers were placed in completely randomized design in the laboratory for 10, 20, 30, 60 and 90 days (Chukwulobe & Echezona, 2014;Isah, Sylvester, & Zakka, 2012). For each landrace and time, the treatment was replicated 3 times. At the end of each experimental period, the percentage of weight loss was calculated using the formula used by Chukwulobe and Echezona (2014), Zakka, Lale, Duru, and Ehisianya (2013) and Atijegbe, Lale, Zakka, Atuakpoho, and Ehisanya (2014): The mortality rate of D. porcellus adults was calculated with data from observations of 10, 20 and 30 days after infestation, in order to avoid counting possible progeny of D. porcellus, using the formula: Percentage weight loss = Initial weight − Final weight Initial weight × 100

Susceptibility index
For each landrace, 20 g of yam chips were packed in plastic containers (4.5 cm height × 3 cm diameter) and closed with muslin cloth to prevent escape. Samples in each container were infested with ten 2-day-old D. porcellus adults, in the proportion five females and five males. Insects were sexed by their genitalia according to Halstead (1963). Adults of D. porcellus were allowed to feed and oviposit for 7 days after which the insects were removed. The plastic containers were kept in the laboratory at 25 ± 2°C, 70 ± 5% R.H. and 12L/12D photoperiod (Oni & Omoniyi, 2012). Yam chips were checked for adult emergence 35 days after exposure (Oni & Omoniyi, 2012), and every 2 days thereafter. In order to avoid a second generation, the sample inspections continued until no more adults emerged during three consecutive inspections (Torres, Saavedra, Zanuncio, & Waquil, 1996). Treatment was replicated 4 times for each landrace. The index of susceptibility was calculated as given by the formula of Dobie (1974): where the median development time was calculated as the time (days) from the middle of the oviposition period to the emergence of 50% of the F1 progeny (Dobie, 1977). A susceptibility index ranges from 0 to 11 was used to classify the yam chips from 24 landraces, where: 0 to 3 = resistant, 4 to 7 = moderately resistant, 8 to 10 = susceptible, and ≥11 = highly susceptible (Dobie, 1974).

Statistical analyses
Data on proportion of D. porcellus attracted by each yam landrace, percent adult mortality, weight loss were arcsine transformed, while data recorded on number of F1 progeny were log-transformed, in order to homogenize their variance. The transformed data were analysed using one-way ANOVA using the SPSS analysis software Version 17.0. Significant differences between means were separated using Student Newman Keuls test (p < 0.05). Back-transformed (original) data are presented in tables and figures.

Choice of D. porcellus among various yam chips
The choice of D. porcellus among the different yam chips varied from one landrace to another. The percentage of D. porcellus that made a choice increased with exposure time from 81.33 to 93.78% between 1 and 48 h, respectively ( Figure 1). However, there was no significant difference (df = 71; F = 0.422; p = 0.657) among the number of D. porcellus that made a choice between 1, 24 and 48 h. Similarly, the percent mortality number of D. porcellus did not change significantly (df = 26; F = 0.029; p = 0.972) with the duration of exposure.
In the choice experiment, the average number of D. porcellus found on each yam landrace varied from one landrace to another depending on the duration of the test (Table 2). After 1 h (df = 215; F = 2.776; p = 0.000) and 24 h (df = 215; F = 2.235; p = 0.002), D. porcellus displayed a strong preference for the landrace Kinkérekou (Table 2). After 48 h, the landraces Baniouré souan and Kokoro agbalé were significantly (df = 215; F = 2.129; p = 0.003) more preferred (Table 2). In general, significant differences (df = 647; F = 5.810; p = 0.000) in the distribution of D. porcellus on the yam chips from the 24 landraces were observed, with strongest preferences for the landraces Kinkérekou and Baniouré souan, while Ayé, Boniwouré and Wonmangou were the least preferred (Figure 2).

Table 4. Percentage weight loss (±SE) of the yam chips of the 24 landraces consumed by D. porcellus
Notes: Means within columns followed by the same lower-case letter, and means within rows followed by the same upper-case letter, are not significantly different (p ≥ 0.05).

Landraces
Mean proportion

Discussion
In the free choice condition, D. porcellus was more attracted by the chips from the landraces Baniouré souan, and Kinkérékou than those of the others. These results coincide with findings by Onzo et al. (2015) who showed that there is a differential attraction of D. porcellus with respect to chips from four different yam landraces (Otoukpanan, Singor, Kprakpra and Portchahabim) from Benin. This difference in choice of chips by the pest could be explained by the fact that, although dried, different landraces influence D. porcellus through physicochemical characteristics such as colour, smell or texture, that may be repulsive or attractive to the pest. Indeed, colour and physical properties of dried yam may be affected by the landrace (Akissoé, Hounhouigan, Mestres, & Nago, 2003) and by the drying method (Hsu, Chen, Weng, & Tseng, 2003).
Observed D. porcellus mortality in chips of the different yam landraces were significantly different. Boniwouré landrace showed significant mortality of D. porcellus. Pest mortality can be explained by the fact that several chemical compounds found in plants can exert insecticidal and repellent effects inhibiting the development of storage insects (Hatil, 2009). Studies conducted by Lale (2002) showed that, the soluble tannins and phenols contained, for example in corn, sorghum and millet cause adult mortality of storage pest such as Sitophilus oryzae L., S. zeamais Motschulsky and Sitotroga cerealella Olivier. Given that, resistant varieties are known to affect a cumulative reduction of pest and form a key component in all pest management programs (Lale, 2002), the Boniwouré landrace remains promising with respect to resistance to D. porcellus.
Despite the preference of D. porcellus for the landraces Baniouré souan and Kinkérékou, the ones that had the most significant losses were the landraces Singor and Agada bangahi. The composition of anti-nutritional factors would influence the consumption of each landrace by D. porcellus because, there is a difference between the yam landraces in terms of quantities of polyphenolic compounds (e.g. tannins), alkaloids (e.g. dioscorine), steroid derivatives (e.g. diosgenin), crystals of calcium oxalate and phytic acid (Polycarp, Afoakwa, Budu, & Otoo, 2012). Studies conducted by  Osipitan, Sangowusi, Lawal, and Popoola (2015) and Popoola, Opayele, and Nkpondion (2015) on the resistance of cassava chips Prostephanus truncatus (Horn), pest belonging to the same family of D. porcellus, revealed that the differential resistance of landraces to this pest was conferred by secondary metabolites such as tannins, saponins, alkaloids, and the hydrocyanic acid content. According to previous researches, chips loss rate due to storage insects also depends on a number of factors including: chips texture (Campbell & Runnion, 2003), a partial starch gelatinization following the precooking, which causes hardening of chips (Rajamma et al., 1996) the biochemical composition of chips (nutrients and anti-nutrients), which could encourage or discourage survival and multiplication of pests (Wong & Lee, 2011), in addition to environmental conditions (temperature, humidity, etc.) (Chukwulobe & Echezona, 2014). The biochemical composition of the Boniwouré and Wonmangou landraces should finally be investigated to determine which anti-nutritional factors are behind its low attractiveness and low loss rate with respect to D. porcellus.
A resistant yam landrace is expected to have relatively lower number of adult progenies than a susceptible landrace (Onzo et al., 2015). Lowest number of emerged D. porcellus adults, indicating that Gaboubaba landrace was less susceptible to D. porcellus attack. The higher mean of insect development period in landrace Yakanougo indicate the expression of antibiosis, characterized by a delayed larval cycle or larval development interruption (Lara, 1991). According to Zakka et al. (2013) the presence of phenolic compounds and saponins (secondary metabolites) in yams may confer antibiosis against developing stages of storage insect.
Among the chips of the 24 yam landraces evaluated, only Gaboubaba, Boniwouré, Alahina, Yakanougo, Wonmangou were regarded as resistant, while all the remaining landraces were categorized as moderately resistant to D. porcellus attack. These variations in the differential susceptibility of chips from the different yam landraces show the innate capacity of a particular landrace to resist D. porcellus attack. Resistant yam landraces exhibited low attractiveness of pest, minimum weight loss, high mortality of pest, reduced multiplication of F1 progeny, longer median developmental period and lower score of susceptibility index. These resistant yam landraces can reduce the cost of D. porcellus management and the resistance can also be utilized as an environmental friendly way to reduce damage by this pest.

Conclusion
This study indicated varietal differences in response of yam chips to damage by D. porcellus. From our results that chips from the Baniouré souan and Kinkérékou landraces exert a strong attraction on D. porcellus than chips from other landraces. Losses caused by D. porcellus were higher on chips from the Singor and Agada bangahi landraces. With less attractiveness, lower weight loss, lower F1 progenies emergency, lower indices of susceptibility, higher adult mortality and longer median development period, the Boniwouré, Gaboubaba, Alahina, Yakanougo and Wonmangou landraces were resistant to D. porcellus attack, while others were categorized as moderately resistant. The chips of these resistant yam landraces can be stored for longer periods of time under traditional storage system of small-scale farmers with reduced threat of D. porcellus, reduced cost of pest management and no adverse effect on the environment. Thus, it is important to include these resistant landraces in the production system of yam meant for chips production.

Funding
This work was supported by L'Oréal-UNESCO for Women in Science in Sub Saharan Africa through the 2014 postdoctoral fellowship won by the first author.