Investigation of distribution and the loss assessment of Helicoverpa armigera in bean fields of Markazi Province.

According to statistics released by the Ministry of Agricultural Jahad, the area under cultivation of beans in Iran is 106264 hectares, with an average yield of 2401 kg/ha. Helicoverpa armigera has recently caused extensive damage to bean fields. Young larvae feed on the leaf parenchyma and reproductive organs of chickpeas (flowers and buds) and then, at the same time as the pods form, medium and large larvae enter the pod and feed on its seeds. Farmers spray every year for fear of pests, and sometimes even in the absence of pests. In addition to polluting the environment, these sprays reduce natural enemies and cause other consequences, including the return of the pest, and imposing additional costs on farmers. Therefore, considering the key role of beans in the central province and the importance of this pest and the need to reduce and optimal use of agricultural pesticides, studying the amount of damage caused by pod worms is one of the requirements. Therefore, considering that the population of this pest has increased in bean fields in recent years and it has caused damage, and so far no research project has been conducted on this pest in this province, the distribution and damage rate of this pest was investigated.
 
The project was carried out in bean farms in Khomein, Arak, Milajerd, Khondab and Shazand cities for two years. In order to investigate the damage of bean pod worm in important areas of bean cultivation, five bean cultivation regions of Markazi province including Arak, Shazand, Khomein, Milajerd and Khondab that had relatively high density of this pest were selected. Twenty five farms, five farms from each region were selected and investigated and 5 farms and a total of each region. Twenty-five bean fields were surveyed at three times of mid-flowering and pod formation and before harvest. For sampling from each field and in all three stages of mid-flowering, pod formation and before harvest, 20 plants were randomly selected and while shaking the plants on a white paper, the number of larvae was counted and recorded. To estimate the amount of damage, before harvest, 20 plants were selected from each field and all healthy and damaged pods of each plant were identified and counted due to the pest and the percentage of field contamination was estimated.
 
The results of this study showed that in both two years of the project, Khomein and Khondab regions showed the highest and lowest pollution rates, respectively. The largest population of bean bud worm larvae is in the middle stage of flowering, so it is recommended to carry out chemical control at this stage if it is necessary. Due to the fact that among the studied local mass, red and white, respectively, were more sensitive to this pest. In order to reduce the percentage of infection, white and red local mass are preferable to cultivation in the mentioned areas. The results showed that the optimum time for chemical control of chickpea cocoons is simultaneously with the largest number of larvae which coincides with the middle of flower formation (flowering stage) and early cocooning of plants (Shafaghi et al., 2020). The results of current study are similar to the results of the present study in terms of having the largest population of larvae in the mid-flowering stage. In a study conducted in India on a weekly basis in different regions to investigate the distribution of Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), it was concluded that the average number of larvae per plant was 2.5 and the minimum number of larvae per plant was 1.4. The average number of larvae varied in different parts of the country (Wakil et al., 2010). Due to the differences in weather conditions and the product under study, the results of the present study are not similar to the results of this study. A research project found that first- and second-age larvae of soybean pod eaters were denser than older larvae, which may be due to the death of younger larvae that could not reach older ages (Barari et al., 2011). In the first year, the average percentage of damages in Arak, Shazand, Khomein, Milajerd and Khondab counties were 6.51, 7.87, 9.72, 5.01 and 4.43% in the second year, respectively 6.45, It was 8.68, 9.67, 5.55 and 4.26 percent, respectively. In a study it was concluded that Caboli type cultivars were more sensitive to this pest, but the compensation mechanism was more in these cultivars relative to the Deci cultivar. In this experiment, it was found that ILC537 and Pirooz cultivars showed the least infestation to this pest (Khanizad & Kanouni, 2006).
 
Due to the fact that among the studied cultivars, red and white, respectively, were more sensitive to this pest. In order to reduce the percentage of infection, white and red cultivars are preferable to cultivation in the mentioned areas.