Integrated Pest Management of the Yam Chip Beetle Dinoderus porcellus Lesne (Coleoptera: Bostrichidae): Current Status and Future Prospects

In West Africa, Dinoderus porcellus Lesne (Coleoptera: Bostrichidae) is a pest that attacks and spoils stored yam chips. Despite this fact, very little attention has been given to this pest, which could destroy up to 65% of stocks. In order to prevent any damages, farmers are widely using chemical substances for fighting against this pest despite their negative impacts on human health and environment. This chapter aims at proposing a solution approach and discussing the development of an integrated pest management strategy. The solution approach includes storage bags, varietal resistance, botanicals, and biological control. Further research should be done on the use of hermetic bags, essential oils, entomopathogens, insect growth regulators, pheromones, and their combined effects in the D. porcellus control.


Introduction
Yam (Dioscorea sp.) is a plant that produces edible tubers that contribute to food security and poverty reduction in West Africa. With a production of 67,312,076 tons in 2017, representing 96.3% of world production, West Africa is rising to first place in the production and consumption of yam (236 kcal) per capita per day [1]. Yam tubers that are used in the preparation of various culinary dishes in West Africa [2] are very rich in carbohydrates, vitamins, and minerals [3]. In addition to the important trade in yams in West Africa [4], it is anchored in the sociocultural life of the populations as evidenced by the many festivals organized for the release of new yams [5].
Unfortunately, because of their high water content [5], fresh yam tubers are highly perishable with postharvest losses of up to 85% [6]. An alternative to the perishability of yam tubers is the transformation into chips, stabilized product by peeling, precooking, and drying in the sun [7]. The manufacture of yam chips,

Description
Beetles of the Dinoderus genus are the smallest of all bostrichids [20]. According to Schäfer et al. [21], they are characterized by a very short forehead with a fronto-clypeal suture very little distinct; their antennas are composed of 9-11 segments with the second segment shorter than the first segment. Dinoderus spp. are also characterized by the last abdominal segment visible and curved; the pronotum having a perforated posterior surface is bordered anteriorly by a row of teeth; the subapical carina is absent in elytra [21]. The adult of D. porcellus is reddish-brown with elytra black and hard, shiny, appearing glazed, almost glabrous on their dorsal part (Figure 1). D. porcellus is different from the other Dinoderus by a pronotum without fovea but with a posterolateral carina reaching the first row of marginal teeth and a dorsal punctuation of the elytra consisting of large perforations [21].

Biology
D. porcellus digs holes on the yam chips in which it reproduces. The female lays her eggs inside the dug holes, which hatches after 6-8 days [19]. It was noted that at a temperature below 10°C and above 40°C, no oviposition was observed in females of D. porcellus [19]. The larva development time is an average of 24 days, while the nymphal stage lasts an average of 5. 25

Pest status
In West Africa, the beetle D. porcellus is a serious pest of stored chips of several roots and tubers such as cassava [21], yam [10], and cocoyam [22]. D. porcellus digs holes in the yam chips (Figure 2), drastically reducing their internal parts into powdery waste, which negatively affects their visual quality and decreases their commercial value. Under laboratory conditions, the weight loss due to this pest in  4.5 months was estimated to 29.5% in newly dried yams and 39.2% in stock that had already been in store for 6 months [11]. In addition, when yam chips were infested with D. porcellus and stored for 3 months, the reconstituted thick paste (amala or télibo) was unsuitable for human consumption and not accepted by consumers [9].

Physical methods
Physical control of stored product insects involves the manipulation of physical factors (temperature, relative humidity, atmospheric composition, etc.) to eliminate them or reduce their population to a tolerable level [23]. The control of stored product insect such as D. porcellus could be done by the use of heating or cooling of storage structures. Traditionally, infested yam chips are sun-dried by farmers [10]. Although sun-drying remains one of the oldest methods of control used in the protection of dried foods, it has several disadvantages such as exposure to waste and deterioration of vitamins [24]. In addition, this method is not very effective against D. porcellus, which feeds inside the chips so as not to be exposed to sunlight. Oni and Omoniyi studies [19] show that storage of yam chips at 20-30°C is optimal for the development and reproduction of D. porcellus. Lethal temperatures varied significantly with the species of yam used for making chips. In general, it is therefore recommended to store yam chips at temperatures below 20°C and above 35°C to control D. porcellus [19]. However, the manipulation of the temperature of storage structures requires infrastructures such as gas-tight containers, which are not accessible to smallholder farmers who are engaged in the processing of yam chips in West Africa. An alternative could be the use of triple-bagging consisting to seal dried food in a series of two heavy-grade polyethylene plastic bags which were expected to be as hermetic [25]. The use of the Purdue Improved Crop Storage (PICS) triple layer bag as an alternative to the use of the common polyethylene bags has been tested by Hell et al. [26] and have proven ineffective in protecting cassava chips against Dinoderus spp. and Prostephanus truncatus. Nevertheless, further research is needed to fill the knowledge gaps and provide adequate information needed to inform decision-maker for the use of PICS triple layer bag for yam chips protection against D. porcellus.

Botanical insecticides
Botanical insecticides are one of promising alternative to the use of chemical synthetic insecticides in pest control because of their minimal costs and ecological side effects [27]. For the protection of stored yam chips against insect pests, Vernier et al. [7] evaluated the level of protection provided by various biological products derived from neem (Azadirachta indica A. Juss.) and Crotalaria sericea L. compared with a synthetic pesticide, Sofagrain (1.5% deltamethrin +0.5% pirimiphos methyl). Among the tested organic products, oil, powdered leaves, and neem seeds gave the best level of protection in terms of reducing the damage caused by pests [7]. Eze et al. [28]  and could be a source of novel repellent against this pest. The propanol extract of B. ferruginea at 5% proved to be a good fumigant against D. porcellus, with 88.89% of pest mortality at 160 μL/L air, while the acetone extract of K. senegalensis could be used in the development of a contact insecticide against D. porcellus because of having a low LC 50 of 0.29 μL/insect. However, to increase the efficacy of medicinal plants identified, it is important to develop methods such as mixing with some fixative materials. Knowing that plant essential oils are promising alternatives to chemical insecticides [29] and have demonstrated their effectiveness against some Bostrichidae pests of stored products [30,31], it is important to conduct research on their use on the control of D. porcellus.

Resistant varieties
The use of resistant varieties is the cheapest, effective, and ecologically safe method of protecting stored products against insect pests [32]. In fact, among the solutions proposed by farmers to fight D. porcellus, resistant yams chips have been the most plebiscite [10]. To meet the expectations of farmers, a participatory assessment of yam landraces in Benin was conducted in 51 villages through the yam production zone of Benin, and 64 landraces whose chips are resistant to storage insect have been identified [33]. The agro-morphological [33] and molecular characterization [34] of these 64 landraces revealed the existence of many duplicates and classified them in 24 morphotypes [33]. Choosing based on their good agronomic and culinary characteristics, the susceptibility of 5 of the 64 landraces identified as given resistant chips were tested in the laboratory by Onzo et al. [35]. These authors demonstrated the existence of a differential susceptibility between the different tested landraces with respect to D. porcellus with Singor and Portchahabim landraces as the least vulnerable to the attacks of this pest. Varietal resistance of yam chips from 24 landraces (corresponding to the 24 morphotypes obtained during morphological characterization) to D. porcellus was evaluated by Loko et al.

Biological control
Biological control is an important integrated pest management component (IPM), which broadly includes all control types involving living organisms, and represents a good alternative to the use of pesticides. Concerning D. porcellus, two predators (Alloeocranum biannulipes Montrouzier & Signoret (Hemiptera: Reduviidae) and Teretrius nigrescens Lewis (Coleoptera: Histeridae)) have been found in yam chips infested with this pest [10]. The functional responses of T. nigrescens and A. biannulipes feeding on D. porcellus were compared, and the results showed that both predators have a potential as biological control agents of D. porcellus [17]. The suppressive effect of A. biannulipes on the population dynamics of D. porcellus and the yam chip losses caused by this pest was evaluated under laboratory and natural conditions [18]. Results showed that A. biannulipes has the potential to be an effective biological agent against D. porcellus in stored yam chips (Figure 4). In addition, Loko et al.
[18] provided detailed information on the biology, behavior, and life history of A. biannulipes, which are necessary for the mass rearing and use of this predator to control D. porcellus. However, A. biannulipes is a generalist predator which can consume several stored product insect pests such as Corcyra cephalonica (Stainton) (Lepidoptera: Pyralidae), Tribolium confusum du Val (Coleoptera: Tenebrionidae), and Anagasta kuehniella Zeller (Lepidoptera: Pyralidae) [36]. What could be affected is its effectiveness as a biological agent of D. porcellus in farmer storage conditions. Therefore, prior to the use of this predator in an IPM program against D. porcellus, it is important to do a molecular gut analysis for determining the part of D. porcellus in its diet. Moreover, it is important to evaluate the population dynamics of A. biannulipes and D. porcellus within multispecies and/or multitrophic systems.

Synthetic insecticides
Synthetic insecticides are the main control method used by farmers and traders, to protect stored yam chips against insect attacks [7,10]. In Benin, some recommended synthetic insecticides for storage insect pest control such as Sofagrain (1.5% deltamethrin +0.5% pirimiphos methyl) and Antouka (permethrin 3 g/ kg + pirimiphos 16 g/kg) showed a good level of stored yam chip protection against D. porcellus [7,10]. But in practice, most of the time, farmers use synthetic insecticides focusing other crops such as cotton in Benin [10] and cacao in Nigeria [37] to protect stored yam chips. This misuse of chemical insecticide leads to many cases  of poisoning [13,14]. Indeed, a study of Sosan et al. [37] revealed the presence of the organochlorine pesticide residues such as Dichloro-diphenyl-trichloro-ethane (DDT) and hexachlorohexane (HCH) at outrageous levels in dried yam chips obtained from Ile-Ife markets, southwestern Nigeria. These two organochlorine pesticide residues classified as dangerous by the World Health Organization [38] could cause serious health and environmental risks. Moreover, the use of aluminum phosphide marketed as Phostoxin, which is a highly toxic pesticide banned in several countries [39], was registered by Adesina et al. [40] as use by traders for yam chip protection. It is therefore important to sensitize farmers and traders to the use of chemical insecticides suitable for the protection of yam chips.

Integrated pest management
Integrated pest management (IPM) relies on managing insect populations through the combined use of several control methods in a way, which affords the highest priority to the protection of human health as well as the environment. The promising approaches toward effective IPM for D. porcellus are the use of resistant yam landraces combined with botanical powders of three medicinal plants (B. ferruginea, B. sapida, and K. senegalensis) [41]. Based on the few studies carried out in the context of the yam chip protection against D. porcellus, we can recommend to smallholder farmers the integration of the different methods in the following way: • Peel and cut fresh yam tuber in slice ranging from 2 to 3 cm of thickness for fast drying [16].
• Precook at around 40°C, and soak during minimum 12 h fresh yam tubers before drying for having the best quality yam flour [42], but also protect chips against insect attacks [43]. Indeed, Nwana and Azodeh [44] showed that the intensity of damage by A. fasciculatus to yam chips blanched and soaked before drying were low.
• Add plants such as leaves and sorghum straw during the parboiling process for red coloration and its insect repellent properties [10]. Indeed, it is known that color largely affects the acceptability of Amala (thick paste obtained after mixed boil water and yam chips flour) by consumers [45].
• Dry yam chips on clean surface to avoid insect infestation that could inadvertently be carried to storage; yam chip moisture content must be less than 13% to avoid fungi and insect attacks [46].
• Put dried yam chips in polythene-lined jute for lower insect damage and yam chip discoloration [28].
• Add leaf powders of B. ferruginea, B. sapida, and K. senegalensis at a concentration of 5% (weight/weight) in bagged yam chips for a short period of conservation (3 months) because of their high repellent activity against D. . However, for a long period of yam chip conservation (10 months or more), we recommend the use of neem leaves (100 g per kg) or neem seeds (20 g per kg) powders [7].
• Tightly seal bags and put them in clean and dried place • Inspection of stored bags should be done monthly • If stored yam chips are attacked by D. porcellus, we recommend heating infested yam chips to more than 35°C [19] or adding the predator A. biannulipes at a density of one predator for 10 preys [18] or applying the recommended chemical insecticides Antouka or Sofagrain.

Scope for future research and development of innovative management strategies
In order to develop a good strategy of integrated pest management for D. porcellus in West Africa, further research should be done in the development of alternative control measures and techniques. The alternative methods to be explored that could be adapted to D. porcellus control in stored yam chips are as follows:

Hermetic bags
Hermetic storage bags have proven to be a low-cost solution for preventing storage losses due to insects [47]. Storage systems based on the hermetic principle can be used to maintain stored product quality without the need for pesticide application [48]. Apart from testing the effectiveness of the Purdue Improved Crop Storage (PICS bags™) in D. porcellus control, it is important to evaluate the efficacy of other hermetic methods of storage marketed in West Africa such as AgroZ Bag™, and SuperGrain bags™ [49].

Essential oils
Essential oils can have various effects on stored insect pests (repellence, contact toxicity, antifeedant, growth inhibitory, fumigant, etc.) and can be applied as a part of integrated pest management programs for stored products protection. Indeed, plant essential oils can be used in combination with other control techniques for controlling storage insect pest [50]. Essential oil of plants found in the West African flora such as Citrus sinensis [31], Cymbopogon citratus [51], Ocimum basilicum [52], and Zingiber officinale [53] have proven their efficacy on control of several Bostrichidae of stored products such as P. truncatus and R. dominica. Research on the potential use of essential oils of these plants or other medicinal plants found in West Africa in control of D. porcellus in stored yam chips should be prospected.

Entomopathogens
Entomopathogens have an important place in the biological control because they have a wide host range and are harmless to the environment and human. These include entomopathogenic fungi, nematodes, bacteria, and viruses. These are all widespread in the natural environment and cause infections in many pest species. Entomopathogens contribute to the natural regulation of many populations of arthropods. Much of the research in this area concerns the causal agents of insect diseases and their exploitation for biological pest control. Many entomopathogens can be mass produced, formulated, and applied to pest populations in an analogous manner to chemical pesticides. Also, they can be used more against stored product pests with the development of new biotechnical methods. Indeed, the effectiveness of the formulation of the entomopathogenic fungi, Beauveria bassiana (Bal.) Vuillemin and Metarhizium anisopliae (Metch.), against the Bostrichids P. truncatus [54,55] and R. dominica [56,57] has been proven. It would therefore be interesting to evaluate the effect of entomopathogenic fungi in the context of D. porcellus control.

Parasitoids
The parasitoid wasp, Anisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae) found in stored yam chips by Vernier et al. [7], is an important biological agent of several Bostrichidae larvae of stored products such as R. dominica [58], and P. truncatus [59]. Similarly, Dinarmus basalis Rondani (Hymenoptera: Pteromalidae), which is present in all the different regions of West Africa, was also found in stored yam chips [10]. However, D. basalis is known as an efficient ectoparasitoid of bruchid pests [60,61]. Further research must be done to evaluate the potential of these two parasitoids as biological agents for the control of D. porcellus in stored yam chips.

Insect growth regulators
Insect growth regulators are insecticides that mimic hormones in young insects and can be a potential component in integrated pest management against D. porcellus. Several features of insect growth regulators make them attractive as alternatives to broad-spectrum insecticides. Indeed, insect growth regulators are more selective; they are less harmful to the environment and more compatible with pest management systems that include biological controls. In addition, insect growth regulators are generally low in toxicity to humans. Kavallieratos et al. [62] have proven the efficacy of insect growth regulators as grain protectants against P. truncatus in maize and R. dominica in wheat. Therefore, investigations must be done to assess the effects of insect growth regulators on the development of D. porcellus.

Pheromones
Methods for detecting and for monitoring D. porcellus are crucial components for the development of an integrated management strategy against this pest. Pheromones which are volatile organic molecules of low molecular weight that elicit a behavioral response from individuals of the same species can serve as a tool to detect infestation at an early stage and to determine the right time for control measures [63]. Thus, Campion et al. [64] and Hodges [65] have shown the efficiency of traps baited with the synthetic aggregation pheromones for detecting and monitoring P. truncatus in East and West Africa. Research should develop specific pheromones and attractants of D. porcellus to aid in its monitoring and trapping.

Conclusions
The pest status of D. porcellus is higher in West Africa. This pest causes both quantitative and qualitative damage to stored yam chips. Synthetic insecticides used by farmers and traders for fighting against this insect are very dangerous for human Entomology health and environment. The present chapter has emphasized utilization of resistant varieties, botanicals, parasitoids, and physical methods for D. porcellus control. Thus, more emphasis should be placed on the integration of these different methods of D. porcellus control. This chapter proposes an integrated pest management combining yam chip processing practices, physical methods, botanical insecticides, biological control, and resistant varieties for fighting D. porcellus in stored yam chips. However, detection and monitoring tools should be developed, and it appears that the use of hermetic bags, essential oils, entomopathogens, insect growth regulators, pheromones, and their combined effects should be further investigated in the D. porcellus control.

Conflict of interest
The authors declare that they have no competing interests.