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):
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.