Biology and population growth parameters of Trogoderma granarium Everts (Coleoptera: Dermestidae) on various rice cultivars

Background and Objectives
Rice (Oryza sativa L.) is one of the most important cereal grains and serves as a staple food for nearly half of the world’s population. Trogoderma granarium Everts (Coleoptera: Dermestidae), commonly known as the Khapra beetle, is a significant pest that infests stored grains and other stored products in various countries worldwide, including Iran. The pest's polyphagous nature and high ability to survive for extended periods without food necessitate using chemical fumigants to control T. granarium in storage facilities. However, the serious adverse effects of these insecticides on humans and the environment render it imperative to explore alternative pest control approaches. One of the best safe options is using resistant cultivars in integrated pest management programs. Insect-plant interactions and the resistance of various cultivars to herbivorous insects can be studied by insects’ life history and population parameters.
Materials and Methods
The resistance and susceptibility of six commercial rice cultivars (Tisa, Khazar, Shafagh, Sadri, Nemat, and Hashemi) to T. granarium were determined by analyzing the life history and population growth parameters of the pest. The seeds of tested rice cultivars, which have a large cultivation area in Iran, were obtained from the Agricultural and Natural Resources Research Center (Gilan province, Iran). The initial colony of T. granarium was obtained from the laboratory of Department of Plant Protection, University of Mohaghegh Ardabili (Ardabil, Iran), and was reared in groups on wheat seeds (as the main host). The adult insects obtained from the primary colony were transferred to the seeds of tested rice cultivars (20 pairs on each cultivar), and were reared as a group for two generations. Then, the eggs (1-day-old) laid on each rice cultivar were used for the experiments. The beetle rearing and all experiments were carried out in a growth chamber set at 33 ± 1°C, 65 ± 5% RH, and 24 h darkness. Subsequently, the raw data obtained from the life history of T. granarium on various rice cultivars were analyzed with TWOSEX-MSChart software using an age-stage, two-sex life table model.
Results
The study results indicated that immature survival rate on cultivar Sadri was higher than on the other tested cultivars. The incubation period was longer on cultivar Shafagh than on cultivars Sadri and Tisa. The shortest larval, pupal and the entire immature periods were observed on cultivars Sadri and Tisa, and longest on cultivar Khazar. The beetles raised on cultivar Khazar took approximately eight days longer to complete their development than those on cultivar Sadri. The fecundity of T. granarium (number of eggs laid by each female) on cultivars Sadri and Tisa was significantly higher than on cultivar Khazar. The longest oviposition period was observed on cultivars Sadri and Tisa, and the shortest on cultivar Khazar. The longevity of female and male adults was longest on cultivars Sadri and Tisa, and shortest on cultivar Khazar. The intrinsic rate of increase (r) was highest on cultivar Sadri and lowest on cultivars Hashemi, Shafagh, and Khazar. Additionally, the finite rate of increase (λ) was higher on cultivars Sadri and Tisa than on the other tested cultivars. The longest mean time generation was observed on cultivar Khazar and the shortest on cultivars Sadri and Tisa.
Discussion
One of the reasons for the rapid development and high survival of T. granarium on cultivar Sadri may be the high protein content of its seeds, which has provided sufficient energy for the immature stages' rapid growth. However, the amount of protein and starch in cultivar Khazar is lower than in other cultivars, which may be a factor in this cultivar's resistance to T. granarium. Our results indicate that Sadri and Tisa are the most susceptible cultivars to potential population increase of the pest because the highest values of r and λ are on these two cultivars. However, owing to the highest values of r and λ on cultivars Hashemi, Shafagh, and Khazar, they are the most resistant cultivars to T. granarium. Consequently, resistant cultivars can be used in the genetic engineering of plants to minimize the damage caused by this key pest.