Some Nutritional Indices of the Khapra Beetle, Trogoderma granarium (Everts) (Coleoptera: Dermestidae) on Grain of Various Wheat Genotypes

Trogoderma granarium (Everts) is an economic insect pest of stored grains in Iran and many tropical and subtropical regions. If stored products contaminated with this insect are consumed by humans, it would cause serious digestive diseases. To control T. granarium in storage, chemical insecticides and fumigants are often used, but due to their extreme toxicity to humans and other dangerous side-effects, their application has been limited. Therefore, in order to develop alternative control methods with minimal harmful effects to humans and other non-target organisms, there is a need to understand feeding performances of the pest on different cultivars. The use of insect-resistant plants is one of the effective methods in integrated pest management programs. Study of nutritional indices of stored product insects is one of the important methods of measuring the resistance in plant cultivars.

Nutritional indices of T. granarium larvae were investigated using the gravimetric method provided by Wadbauer (1968). The grains of wheat genotypes including N-91-9, Heidari, Aftab, Tirgan, N-92-19, Gaskojen and Kouhdasht were crushed using a grinder. Then, first instar larvae were reared on the grains of these genotypes under 33 ± 1°C, 65 ± 5% R.H., and 14:10 (L:D) h. After emergence of fifth instar, the weight of larvae, before and after feeding, and the amount of grain eaten by the larvae were measured. The data were analyzed using one-way ANOVA using SPSS ver.16. Then, Tukey's HSD test, with a probability level of 5%, was used to investigate the statistical differences among the means.

There were significant differences in the nutritional indices of T. granarium fifth instar larvae on grain of different wheat genotypes. The larvae raised on line N-91-9 had the lowest weight of food consumption (24.33 ± 3.38 mg/10 larvae) compared to other genotypes tested. Larvae fed with line N-91-9 (5.33 ± 0.33 mg/10 larvae) and cultivars Aftab (3.00 ± 0.57 mg/10 larvae) and Heidari (3.00 ± 0.57 mg/10 larvae) showed lower weight gain than those fed with other genotypes. The highest efficiency of conversion of ingested food (ECI) was on cultivars Gascojen (36.56 ± 1.46 %) and Tirgan (33.42 ± 0.60 %), and the lowest was observed on cultivars Aftab (7.32 ± 1.51 %) and Heidari (7.86 ± 1.38 %). The highest relative consumption rate (RCR) was found on cultivar Gascojen (1.08 ± 0.05 mg/mg/day) and line N-92-19 (0.95 ± 0.02 mg/mg/day), and the lowest was seen on line N-91-9 (0.16 ± 0.03 mg/mg/day), and cultivars Heidari (0.19 ± 0.02 mg/mg/day) and Aftab (0.24 ± 0.02 mg/mg/day). As compared with the other examined genotypes, the relative growth rate (RGR) was the highest on cultivar Gascojen (0.394 ± 0.022 mg/mg/day). Larvae feeding from cultivar Gascojen (5.61 ± 0.17 mg/day) had the highest growth rate (GR), and larvae feeding from line N-91-9 (0.76 ± 0.04 mg/day) and cultivars Heidari (0.42 ± 0.08 mg/day) and Aftab (0.42 ± 0.08 mg/day) had the lowest GR value. The change in the nutritional indices of T. granarium larvae can be due to the differences in physical and chemical characteristics of the grains of tested wheat genotypes. The ECI index represents the ability of an insect to use eaten food to increase its body size and weight. In other words, the increase in ECI value on cultivars Gaskojen and Tirgan indicates the high ability of fifth instar larvae to convert the eaten grain into body biomass. This finding may be due to the high protein content and low grain hardness of these cultivars. The low weight of food eaten and body weight in larvae fed with cultivar Heidari and line N-91-9 may be due to the high grain hardness of these genotypes.

Our results showed that line N-91-9 and cultivars Heidari and Aftab had lower nutritional value than the other tested genotypes for T. granarium larvae, because the amount of food eaten, larval weight gain and nutritional indices of larvae fed with these genotypes were lower than other genotypes tested. So, these two genotypes can be used in genetic engineering programs to minimize the damage of T. granarium. The presence of defensive chemical compounds, grain hardness or the lack of essential nutrients for the development of the pest may be the reason why some of the tested cultivars are unsuitable for T. granarium feeding.