Investigation on the resistance of different genotypes of sugar beet to the Beet Armyworm, Spodoptera exigua (Hübner) in north Khuzestan

Background and Objectives
Beet armyworm, Spodoptera exigua (Hübner) is a polyphagous species and an economically important pest that primarily damages various crops, including sugar beet, cotton, corn, and vegetables in Iran. The beet armyworm is known for developing resistance to various insecticides, making management increasingly challenging. The beet armyworm poses a considerable threat to agriculture, requiring ongoing research and adaptive management strategies to mitigate its impact on crops such as sugar beet. In recent years, the damage of the mentioned moth has been observed and reported in the north of Khuzestan. Due to the wide host range of this pest, less attention has been paid to the resistance of cultivars to this pest in sugar beet breeding programs.
Materials and Methods
In order to evaluate the resistance of 12 sugar beet cultivars (Ratna, Callas, Algar, Bifort, Portal, Sentinel, Brandon, Motor, Homa, Dena, Sharif and 061) to sugar beet armyworm, experiments were carried out in laboratory and experimental fields located at Agriculture and Natural Resources Research and Education Center of Safiabad, Dezful, during 2021-2022. Calculating percentage of preference of beet armyworm larvae to different sugar beet cultivars in the laboratory, and determining damage index of beet armyworm to different cultivars in the field with natural infestation (choice access), with artificial infestation (no-choice access), and percentage of yield (Kg/m²) reduction of the pest in the experimental field conditions included in this study. Damage index of larvae was defined in five groups, namely very sensitive, sensitive, semi-resistant, resistant, and very resistant.
Results
In the evaluations carried out in the field, significant difference was seen between the beet armyworm damage index of experimental cultivars by natural damage. In this experiment, the highest damage index was seen in Ratna cultivar and the lowest in Brandon, Callas, Sentinel, Algar and Homa cultivars. The yield reduction percentage was not significant at the 5% level. Damage compensation in low larval densities and production of new leaves was one of the reasons for the non-significance of yield reduction due to larvae feeding. In laboratory evaluations, the preference percentage of beet armyworm larvae was significant for different sugar beet cultivars. The highest percentage of larvae prevalence was seen in Kalas, Portal, Brandon and Ratna cultivars and the lowest in Dena cultivar. Sentinel, Bifort and Homa cultivars also attracted less larvae after Dena. No significant difference was seen between sugar beet cultivars in terms of decreasing or increasing the survival period of the larvae, but the index of larval feeding on the leaves of the cultivars was significant in the second laboratory tests. The lowest amount of larval feeding was seen in Motor, Dena and Sharif cultivars and the highest amount was seen in 061 and Homa cultivars. The resistance indices/coefficients were calculated based on total evaluations, and according to that, Motor, Kalas and Sentinel cultivars are in the resistant group, Dena and Sharif cultivars are in the semi-resistant group, Algar, Bifort, Portal, Ratna and 061 cultivars are in the semi-resistant group. Brandon and Homa cultivars were grouped in the sensitive group. The calculated resistance index had a significant negative correlation with the damage index of beet armyworms in the field (no-choice test), the percentage of yield reduction by larvae, larval survival period, and the amount of larval feeding on leaves in the laboratory.
Discussion
The results obtained indicated that, although the sugar beet armyworm is a pest with a very broad host range, the feeding rates and survival percentages of its larvae vary across different sugar beet cultivars. These variations can be utilized to enhance and develop resistant varieties of sugar beet. In further studies, the Sentinel, Motor, and Kalas cultivars, which showed relative resistance in the experiments, along with the Brandon and Homa cultivars, identified as sensitive, can serve as control groups. Additional studies related to the mechanisms of resistance and morphological and biochemical aspects which are effective in the resistance of sugar beet cultivars to the beet armyworm are recommended for use in breeding programs. The use of sugar beet cultivars resistant to the beet armyworm can be effective in reducing the use of pesticides against this pest, managing the resistance of the beet armyworm to pesticides and the integrated management of this pest.