Background

Cassava, Manihot esculenta Crantz, is the fifth root crop in the world and considered the most important food crop behind maize, rice, wheat, and potatoes (FAO, 2017). Cassava plays a major role in the food security of the poorest people in Africa, who produce more than half of the world cassava production (FAO, 2017). In sub-Saharan Africa, cassava is cultivated under a wide range of ecological and agronomic conditions due to its adaptability to relatively marginal soils and erratic rainfall conditions and to its high productivity per unit of land and labor (Spencer and Ezedinma, 2017). Its roots are rich in calories, and its leaves are a good source of protein and minerals (Burns et al. 2012). In Benin, cassava is one of the main priority crops identified by the government for the development of the agricultural sector. This speculation is of great importance in the diet of people in Benin, where its production is estimated at 4,341,848 tonnes in 2017 (FAO, 2017). In Benin, cassava is consumed under various forms and therefore has become a commodity crop by marketing many its derivatives, thereby contributing to poverty reduction (Hongbété et al. 2011).

However, despite its nutritional and economic importance, the cultivation of cassava in Benin remains subject to many abiotic and biotic constraints (Agre et al. 2015). The high perishability of fresh cassava roots remains the main constraint for their post-harvest use (Chijindu and Boateng, 2008) and accounts among the main reasons for the loss of diversity in Benin (Agre et al. 2015). Indeed, after harvest, cassava roots cannot be stored for more than 4 days because of post-harvest physiological deterioration (Buschmann et al. 2000). To overcome these constraints, harvested cassava roots are traditionally processed into many derivatives including chips. Unfortunately, cassava chips are often severely attacked by some stored product insects that can reduce whole stocks of chips into powder in a few months, thereby rendering it unfit for human consumption and sale (Chijindu and Boateng, 2008). Among the diversity of insect pests of stored cassava chips, the larger grain borer Protephanus truncatus (Horn) (Coleoptera: Bostrichidae) is the most widespread in Benin and the most important in terms of damage caused (Gnonlonfin et al. 2008). Losses due to P. truncatus are significant and can reach 40 to 50% of stored cassava chips after just 3 months of storage (Hell et al. 2006). To preserve stored cassava chips, farmers usually use synthetic insecticides that apply to cotton (Hell et al. 2006). As a result, farmers and consumers are often poisoned, leading to death as reported by Ton et al. (2000). It urges, therefore, to seek/for alternative control methods that preserve the environment and protect farmers’ and consumers’ health.

Biological control through the use of predators is widely recognized as an effective control method for regulating insect pest populations (Eneh, 2011). Since P. truncatus is an exotic invasive pest originating from Latin America, there have been attempts in West Africa to introduce biological control agents from the area of origin, notably the predator Teretrius nigrescens Lewis (Borgemeister et al. 2003). However, the introduced predator seemed to be more adapted to forest habitats than storage structures and thus was subsequently unable to disperse and control P. truncatus by itself (Borgemeister et al. 1997), unless used in augmentative releases (Hell et al. 2006). A biological control agent of P. truncatus better adapted to storage structures may well be the generalist predator Alloeocranum biannulipes Montrouzier and Signoret (Hemiptera: Reduviidae). Several studies have shown that A. biannulipes is an important predator of stored commodity insect pests such as Dinoderus porcellus Lesne (Loko et al. 2017) and Tribolium confusum du Val (Awadallah et al. 1984) that also occur in stored cassava chips (Schäfer et al. 2000; Gnonlonfin et al. 2008). It would therefore be interesting to evaluate the potential of A. biannulipes as a biological control agent for P. truncatus. One such evaluation could be through the study of its functional and numerical responses (Lester and Harmsen, 2002; Rahman et al. 2012) to P. truncatus populations in cassava chips. Indeed, functional and numerical responses are used to determine whether a predator can be a good biocontrol agent. The functional response of a predator describes the relationship between the numbers of prey attacked at different prey densities, while the numerical response is defined as the change in the reproductive capacity of a predator at different densities of the prey (Rahman et al. 2012).

The present study aimed, therefore, at provide such quantitative information on the ability of A. biannulipes to control P. truncatus in stored cassava chips in Benin. The results herein may assist to develop an integrated pest management, IPM, program to control P. truncatus larvae, and pupae in infected cassava chips.

Materials and methods

Rearing technique

Adults of the prey pest, P. truncatus, were obtained from samples of cassava chips collected in farmers’ storage structures at Lamougo village in the Municipality of Dassa-Zoumé (latitude 7° 41′ 33″ N and longitude 2° 13′ 25″ E), Benin. The collected adult insects were raised in one-ended plastic boxes (9.5 × 6.5 cm2, height per diameter) containing sterilized cassava chips. The open-end of the rearing plastic boxes was covered with muslin and held in place by elastic strings to allow adequate ventilation and prevent the insects from escaping. The plastic boxes containing the insects were shelved in the laboratory under ambient conditions at 25 ± 2 °C, 45–65% RH, and 12L:12D cycle. After 7 days, the parent-adult P. truncatus was removed from the rearing boxes, and newly emerged adults were collected to be used in the bioassay study.

Adults of the predator A. biannulipes were collected from stored rice at Magoumi (latitude 8° 10′ 26″ N and longitude 2° 13′ 59″ E), a village in the Municipality of Glazoué, Benin. They were reared in experimental plastic boxes (9.5 × 6.5 cm2 height per diameter) containing cassava chips infested with P. truncatus. Five hundred grams of cassava chips was placed in each plastic box in which 100 adult P. truncatus of undetermined age and sex were transferred. Two weeks later, 10 adults A. biannulipes of undetermined age and sex were added to plastic boxes containing the infested cassava chips. Every 2 weeks thereafter, adult female predators were removed from the rearing boxes and used in the experiments.

Functional response of A. biannulipes to P. truncatus

Evaluation of the functional response of the predator A. biannulipes feeding on larvae and pupae of P. truncatus was made according to the methodology described by Loko et al. (2017). The larvae and pupae of P. truncatus were collected by breaking infested cassava chips with a handheld mortar and placed in plastic boxes (3.5 cm diameter × 3 cm height). Adults of both sexes of the predator (2–4 days old) were starved for 24 h (Sing and Arbogast, 2008; Rahman et al. 2009) and transferred individually into plastic boxes, where P. truncatus larvae or pupae have been introduced 2 h ago, (Atlıhan et al. 2010). Plastic boxes used for the control treatment were maintained without any predators to record the natural mortality of P. truncatus. Experiments were conducted with P. truncatus at various densities, i.e., 1, 2, 4, 6, 8, and 10 for larvae (Rahman et al. 2009; Farhadi et al. 2015) and 1, 2, 3, 4, 5, and 6 for pupae (Rahman et al. 2009; Sahayaraj et al. 2015). The experiment was repeated 6 times for each prey density. After 24 h, the number of prey killed by the predator was recorded daily for 7 consecutive days, while replacing prey killed at the target density (Rahman et al. 2009).

Numerical response of A. biannulipes to P. truncatus

The numerical response of A. biannulipes to P. truncatus larvae or pupae was evaluated according to the methodology described by Sabaghi et al. (2011), with slight modifications. Five females of A. biannulipes (2–4 days old) were individually paired with an adult male in Petri dishes (9 cm diameter × 2 cm height) to allow mating (Omkar and Pervez, 2004). The ejection of spermatophore capsules by mated females confirmed successful copulation (Ambrose et al. 2009). After mating, the females were isolated in Petri dishes and starved for 24 h. Subsequently, they were exposed to 6 different densities of P. truncatus larvae (1, 2, 4, 6, 8, and 10) and of pupae (1, 2, 3, 4, 5, and 6) (Rahman et al. 2009). After 24 h, the females were removed from the Petri dishes, and the number of eggs laid and prey consumed were recorded. Observations were made for 7 consecutive days with the same prey densities (Rahman et al. 2012). The number of replicates was 6 at each prey density.

Data analysis

To determine the type of functional response exhibited by A. biannulipes to P. truncatus, the data were analyzed in 2 steps as suggested by Allahyari et al. (2004) and using the SAS 9.3 Analysis Software (SAS, 2009). In a first step, a logistic regression of the proportion of P. truncatus larvae or pupae killed (Na/N0) as a function of initial density (N0) was used to determine the type of response of A. biannulipes. For this purpose, the cubic model in the logistic regression analysis was used (Juliano, 2001; Xue et al. 2009; Butt and Xaaceph, 2015), according to the following formula:

$$ \frac{N_a}{N_0}=\frac{\exp\ \left({P}_0+{P}_1{N}_0+{P}_2{N}_0^2+{P}_3{N}_0^3\right)}{\left[1+\exp\ \Big({P}_0+{P}_1{N}_0+{P}_2{N}_0^2+{P}_3{N}_0^3\right]} $$

where P0, P1, P2, and P3 are the constants of linear, quadratic, and cubic coefficients, respectively. A value of P1 that does not differ significantly from zero indicates type I functional response (Juliano, 2001). A significant negative P1 value describes type II functional response, while a positive P1 value describes a type III functional response (Butt and Xaaceph, 2015).

The second step consisted of modeling the relationship between the number of prey consumed (Na) and the initial prey density (N0) in order to estimate the instantaneous searching time or attack rate (a) and the handling time (Th). Estimations of these two parameters of a functional response were made using the Holling (1959) type I model and the type III model of Hassell et al. (1977):

  • Type I functional response Na = aTN0

  • Type III functional response Na = N0 {1 −  exp [−a (T − Th N0)]}

where a is the instantaneous search time or attack rate, Th is the handling time by prey density, T is the total exposure time of P. truncatus larvae or pupae (24 h), Na represents the number of prey consumed, and N0 is the initial prey density.

Nonlinear regression was performed to estimate the parameters a and Th (Xue et al. 2009). For this, the values of a and Th required by the nonlinear regression method were found by linear regression of Na/N0. The resulting intercept is the initial estimate of Th, and conversely the regression coefficient (slope) is an estimate of a (Livdahl and Stiven, 1983). These first estimates were refined by the NLR method.

Variation in the number of prey killed by the predator with density was assessed by ANOVA using the IBM SPSS 25 statistical analysis software. The data submitted to ANOVA were log-transformed before by hand to ensure the homogenization of the variances. Significant differences between the means were separated using the Student Newman Keuls test (P ≤ 0.05).

The efficiency of conversion of ingested feed (ECI) to egg biomass at different prey densities was calculated using the following formula described by Omkar and Pervez (2004):

$$ \mathrm{ECI}=\frac{\mathrm{Number}\ \mathrm{of}\ \mathrm{eggs}\ \mathrm{laid}}{\mathrm{Number}\ \mathrm{of}\ \mathrm{prey}\ \mathrm{consumed}}\times 100 $$

Data on oviposition and ECI by A. biannulipes at different prey densities were fitted using regression analysis to determine the relationship between oviposition and prey density and ECI and prey density.

Results and discussion

Functional response of A. biannulipes to P. truncatus

The predator A. biannulipes feed on P. truncatus larvae and pupae as well as on other stored products insect pests such as Dinoderus porcellus (Loko et al. 2017), Tribolium confusum, Corcyra cephalonica, and Anagasta kuehniella (Awadallah et al. 1984). Predation on P. truncatus larvae by A. biannulipes differed significantly among larval prey densities (Table 1). Though the predation gradually increased along with the increase in prey density, it decreased beyond the density of 8 (Table 1). The relationship between the percentage of larvae consumed by A. biannulipes and the different larval densities offered was expressed by a polynomial equation with a regression coefficient of R2 = 0.858 (Fig. 1). Functional response of A. biannulipes to P. truncatus larvae fitted a Holling’s type I functional response (Table 3). The adults A. biannulipes consume a constant proportion of P. truncatus larvae with larval densities increasing until the predator’s killing ability is saturated (Ofuya and Akingbohungbe, 1988). Indeed, although the sign of the linear term was negative, it was non-significant (P > 0.05). A type I functional response was also exhibited by other Hemipterans such as Orius insidiosus and Nabis capsiformis fed on bollworm eggs (Parajulee et al. 2006). This type I functional response exhibited by A. biannulipes is surprising, however, because alike the majority of Reduviidae, when this predator feeds on D. porcellus larvae it had exhibited a type II functional response (Loko et al. 2017). However, the negative value of the linear (P1) parameter, which showed that the proportion of prey consumed declines monotonically with the initial prey density, indicated a tendency towards a type II functional response (Patel and Zhang, 2017). In fact, the transition from a type I functional response to type II can be influenced by environmental conditions, temperature and relative humidity, host plant, prey type, and density (Mohaghegh et al. 2001).

Table 1 Mean number of P. truncatus larvae killed in 24 h by A. biannulipes in function of density
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Fig. 1
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Mean proportion of P. truncatus larvae consumed by A. biannulipes at six different densities

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The present results report that A. biannulipes consumed more P. truncatus pupae than larvae. However, this was the reverse when this predator was fed on D. porcellus larvae and pupae (Loko et al. 2017). This preference for P. truncatus pupae could be due to its large size and high-energy content compared to larvae (Demianyk and Sinha, 1988). Indeed, Cogni et al. (2002) showed that for the predator Zelus longipes, a large prey item represented the most profitable prey in terms of energy gained. The number of P. truncatus pupae consumed by A. biannulipes did not differ significantly among prey densities (Table 2). Percentage of P. truncatus pupae consumed at varying densities was fitted using nonlinear regression of R2 = 0.3814 (Fig. 2). The percentage of prey consumed was positively density-dependent, and the positive linear parameter (P1) and the negative quadratic parameter (P2) were significant, thereby indicating that the type III functional response would provide the best description of A. biannulipes attack on P. truncatus pupae (Table 3). This type III functional response is characterized by significantly lower proportional prey mortality at low prey densities than at high prey densities. This indicates that in a biological control program, A. biannulipes may be more effective in controlling P. truncatus populations with high densities of pupae. In fact, according to Holling (1965), a type III functional response allows long-term persistence of the predator, which contributes to regulating the population of their prey. A type III functional response was also observed in other reduviid bugs such as Z. longipes feeding on adults and nymphs of Diaphorina citri (Navarrete et al. 2014).

Table 2 Mean number of P. truncatus pupae killed in 24 h by the predator A. biannulipes in function of density
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Fig. 2
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Mean proportion of P. truncatus pupae consumed by A. biannulipes at six different densities

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Table 3 Maximum likelihood estimates from logistic regression of the proportion of P. truncatus larvae and pupae eaten by A. biannulipes as a function of initial prey density
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The estimated parameters of functional response are good indicators of the predation rate, and predatory efficiency respectively are presented in Table 4. Handling time of A. biannulipes estimated was lower than that estimated, when fed on D. porcellus larvae (Loko et al.,2017). Moreover, it has been proven that the difference in handling time may be due to several abiotic factors that influence the time for capturing, paralyzing, killing, and digesting the prey (Cogni et al. 2002). The attack rate of A. biannulipes was lower when it fed on larvae than pupae of P. truncatus (Table 4). These attack rates of A. biannulipes were lower than those reported feeding on D. porcellus larvae and pupae (Loko et al. 2017).

Table 4 Estimated values for A. biannulipes feeding on P. truncatus larvae and pupae
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These comparisons revealed that A. biannulipes could be a better biological control agent against D. porcellus than P. truncatus. In the present study, the maximum level of attack by A. biannulipes was estimated at 68.18 P. truncatus pupae per individual predator within a 24-h period, giving an indication of the number of predators that have to be introduced in stored cassava chips to improve the control of this pest based on prey density.

Numerical response of A. biannulipes to P. truncatus

Oviposition by A. biannulipes increased with larval density, but a declining trend was noticed at higher densities (Table 5). In contrast, the number of egg hatched was 9.50 ± 2.95, and adult emerged were 7.33 ± 1.36 per female predator were higher at the lowest larval density. Although A. biannulipes significantly consumed more P. truncatus larvae with increasing of larvae density, the number of eggs laid by females, eggs hatched, and adult emerged did not significantly change among the different larval densities (Table 5). The nonlinear regression between the total number of eggs laid by A. biannulipes female and the different densities of P. truncatus larvae was not significant (R2 = 0.285, P > 0.05) (Fig. 3). The number of eggs laid by A. biannulipes feeding on P. truncatus larvae during 24 h reached its peak at the highest prey density (10 larvae/predator). The correlation coefficient between the number of P. truncatus larvae consumed and the number of eggs laid by A. biannulipes was non-significant (Fig. 4). This proves that there was no correlation between the number of P. truncatus larvae consumed and the number of eggs laid by A. biannulipes. The curve of the efficiency of food conversion into eggs by A. biannulipes female showed a significantly varying trend with prey densities (df = 35, F = 20.138, P < 0.000); it increased at low prey density, whereas it decreased at high density of P. truncatus larvae (Fig. 5). The present results showed that when A. biannulipes was fed on P. truncatus larvae there was a rapid and a significant increase in oviposition at low densities, but the oviposition rate declined gradually at high densities. This numerical response of A. biannulipes was also observed in Rhynocoris fuscipes (Ambrose and Claver, 1997). The lower numerical response at higher larval densities implies that A. bannulipes would be unable to keep up with the growth of a P. truncatus population if infestation should occur. Therefore, inoculative releases of A. biannulipes would be required to prevent P. truncatus development in the early cassava chips infestation.

Table 5 Numerical response of predator A. biannulipes feeding on P. truncatus larvae at various densities
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Fig. 3
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Total number of eggs laid by A. biannulipes females at different densities of P. truncatus larvae

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Fig. 4
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Average number of eggs laid per female A. biannulipes based on P. truncatus larvae consumed

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Fig. 5
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Relationship between efficiency of ingested feed conversion (ECI) to egg biomass of A. biannulipes at varying densities of P. truncatus larvae

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The maximum number of eggs laid by A. biannulipes female fed on P. truncatus pupae was observed at the highest pupal density. The maximum number of hatched eggs and the maximum number of emerged adults per female were recorded (Table 6). It was observed that A. bianulipes consumed significantly more P. truncatus pupae with increasing of pupal density. Likewise, the number of eggs laid and eggs hatched per A. biannulipes female were density-dependant (Table 6). The density-dependent fecundity was also observed in Dicrodiplosis manihoti that fed on eggs of the mealybug Planococcus citri (Al-Zubaidy and Al-Shammari, 2017). However, the number of young A. biannulipes adults that emerged did not significantly differ with increasing pupal density (Table 6). The relationship between number of pupae consumed (x) and number of eggs laid (y) by A. biannulipes was nonlinear and can be represented by the regression equation presented on Fig. 6. The average number of eggs laid per female per day was negatively correlated with the number of P. truncatus pupae consumed per day (Fig. 7). Moreover, the efficiency of food conversion into eggs for A. biannulipes female did not vary significantly with increasing prey density. The better digestion and assimilation of nutrients contained in P. truncatus pupae could explain the high conversion levels of ingested food into eggs by A. biannulipes (Fig. 8). In fact, it is known that ingested food conversion depends both on the predator’s ability to digest and assimilate the consumed food and on the allocation (maintenance, growth, reproduction) of assimilated nutrients (Hodek et al. 2012). The results showed that the functional and numerical responses of A. biannulipes female fed on P. truncatus pupae are inter-linked, in which prey biomass consumed by the predator is directly used for egg production. This also implies that P. truncatus pupae could be used for a long-term mass production of A. biannulipes.

Table 6 Numerical response of predator A. biannulipes feeding on P. truncatus pupae at various densities
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Fig. 6
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Total number of eggs laid by A. biannulipes females at different densities of P. truncatus pupae

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Fig. 7
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Average number of eggs laid per female A. biannulipes based on P. truncatus pupae consumed per day

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Fig. 8
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Relationship between efficiency of ingested feed conversion (ECI) to egg biomass of A. biannulipes at varying densities of P. truncatus pupae

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Conclusion

A. biannulipes predation on P. truncatus larvae and pupae in infested cassava chips were best described by a type I and III functional responses, respectively. A. biannulipes can effectively control P. truncatus larvae at low densities and pupae at low and high densities. The increased P. truncatus pupae consumption tends to enhance the numerical response of A. biannulipes by enhancement of its progeny production. The obtained results conclude that A. biannulipes can be an efficient biological control agent of P. truncatus in stored cassava chips. The predator/prey interaction could be used among the control solutions of large grain borer, as it is eco-environmental friendly method. However, further studies under on-farm storage conditions are needed to evaluate the suppressive effect of A. biannulipes on P. truncatus populations.