Insects
S. granarius were reared for several generations on whole durum wheat kernels (var. Simeto) in cylindrical glass containers (Ø 15 × 15 cm) covered with a fine mesh net (0.5 mm). Colonies were maintained in the dark at 25±2°C and 60±5% relative humidity. Unsexed 30-day-old adult beetles were used for intact kernel susceptibility tests and flour disks bioassays.
Plant materials and grain quality assessment
Three wheat genotypes were chosen for this study, including the bread wheat (Triticum aestivum L. subsp. aestivum) variety “Mec” (Marzotto/Combine) and two durum wheat [(Triticum turgidum L. subsp. durum (Desf.)] genotypes: “Ofanto” (Adamello/Appulo), an élite variety with high grain yield and wide adaptability to Mediterranean basin (De Leonardis et al. 2007), and a purple durum wheat genotype, “T1303” (USDA code PI 352395) with high levels of anthocyanins in the grain. The three wheat genotypes were grown simultaneously in a replicated (n = 10) field trial carried out at the CREA-CI of Foggia, Italy (41°28′N, 15°32′E; 75 m a.s.l.), on a clay-loam soil (Typic Chromoxerert) during the 2019-20 growing season, using standard agronomic practices. The harvested wheat genotypes were analysed to determine the main qualitative and technological parameters and stored at low temperature (4±1°C) until needed for biological tests. Moisture of whole grains was determined using the single-stage air oven method (ASAE 2003). This method utilizes whole grain dried for 18 h at 130°C to determine the moisture content. Thousand-kernel weight (TKW) was calculated from the mean weight of three sets of 500 grains per each wheat genotype. Protein content (PC) was determined by nitrogen combustion analysis according to Approved Method 46-30 (AACC International 2010) using a Dumas nitrogen analyzer (Leco Corp, St. Joseph, MI). The Single Kernel Characterization System 4100 (SKCS) (Perten Instruments, North America, Inc., Springfield, IL, USA) was used to characterize kernels hardness (Ha) using a sample of 300 kernels (Method 55-31) (AACC, 2010).
Total anthocyanin content (TAC) was evaluated using a colorimetric method with different pH solutions as reported by Ficco et al. (2014). Briefly, two aliquots of the supernatants extracted (750 μL) were put into different tubes and diluted (1:2, v/v) with either potassium chloride buffer (0.03 M KCl), for pH 1.00, or sodium acetate buffer (0.4 M CH3CO2Na·3H2O), for pH 4.50. The resulting samples were incubated for 30 min at room temperature in the dark and then filtered with 0.45 μm regenerated cellulose syringe filters. The absorbances of the samples at 520 nm were measured against distilled water as the blank. Total anthocyanin content was corrected for the dry matter and is expressed as Cy-3-Glc equivalents as micrograms per gram dry matter.
In order to ensure the absence of live insects inside the wheat kernels to be used for biological bioassays, samples were frozen at -20 °C for 72 h before the experiments.
Susceptibility tests
Susceptibility tests with intact kernels were performed on wheat samples conditioned for 7 days at 25±2°C, 60±5% r.h. after frozen treatment. For each wheat genotype, not infested kernel samples (60 g) placed in cylindrical glass containers (Ø 9 x 14.5 cm) were infested with 12 S. granarius adults of mixed sexes. Containers were closed by screw caps and maintained in the dark at 25±2°C and 60±5% r.h. For each wheat genotype, there were 5 replicates. After 15 days exposure, insects were removed, sexed and the number of dead insects in each replicate was recorded. The F1 progeny was monitored by removing and counting newly emerged adults every 3 days. The experiment was terminated when no adults emerged for five consecutive days (Shazali 1987).
For each wheat genotype, the following parameters were calculated: (1) total number of F1 progeny; (2) median development period (D), estimated as the time, expressed in days, from the middle of the oviposition period to the emergence of 50% of the F1 generation (Dobie 1974); (3) percentage of mortality during the oviposition period; (4) number of adult offspring per female; (5) percentage of weight loss = Wi–Wf/Wi*100 were Wi = Initial dry weight and Wf = Final dry weight (Reed, 1987); (6) food consumption by an insect.
Flour disks bioassays
For each genotype, a whole wheat flour was prepared by milling kernel samples (20 g) using a Tecator Cyclotec 1093 (International PBI, Milano, Italy) laboratory mill (1 mm screen-60 mesh). A sample (2.5 g) of each wheat flour was uniformly suspended in distilled water (8 mL) and stirred by a magnetic stirrer (MS-H280-PRO, DLAB Scientific Co., Beijing, China).
To obtain flour disks to be used in feeding bioassays, aliquots (200 μL) of suspension were dropped onto holes (Ø 1 cm, height 3 mm) of a rectangular support (15 cm x 15 cm) designed for this purpose and manufactured using a 3D printer (Zortrax S.A., Olsztyn, Poland). Then, the support was placed in a fume cupboard for 7 h until solid flour disks were obtained. Initial humidity of flour disks was stabilized overnight at 25±2 °C in an airtight glass desiccator using a NaCl solution (Greenspan 1977) that generated 60±5% r.h.
In a pre-weighed glass vial (22 mL) two flour disks and 5 group-weighed weevil adults were introduced. Each vial was then re-weighed and maintained in the dark at 25±2°C and 60±5 % r.h. for 6 days. For each genotype, 5 replicates were set up. After 6 days, the glass vials with flour disks and alive insects were weighed again and the number of dead insects were recorded.
Six days after the experiment start, the adult weevil mortality rate (%) was calculated. Moreover, the following nutritional indices were calculated: Relative Consumption Rate (RCR) = D/(B×day), where D = biomass ingested (mg)/No. of live insects on the sixth day; Relative Growth Rate (RGR) = (A−B)/(B×day), where A = mean weight (mg) of live insects on sixth day, B = original mean weight (mg) of insects; efficiency Conversion of Ingested Food (ECI) = (RGR/RCR)×100; Feeding Deterrence Index (FDI) (%) = [(C−T)/C]×100, where C = consumption of control disks (Mec or Ofanto) and T = consumption of disks of the genotype considered (T1303) (Farrar et al. 1989; Huang and Ho 1998).
Statistical analysis
Data were submitted to analysis of variance (ANOVA) followed by Tukey’s HSD test for mean comparisons. Before ANOVA, data were submitted to Shapiro-Wilk’s test to verify the normal distribution of data and to Levene’s test to assess the homogeneity of variances. Statistical analyses were performed with SPSS (Statistical Package for the Social Sciences) v.18 for Windows (SPSS Inc., Chicago, IL).