Impact of varietal diets on the effectiveness of modified atmospheric conditions against the life stages of bean beetle, Acanthoscelides obtectus (Say) (Coleoptera: Chrysomelidae)

Abstract

The bean beetle Acanthoscelides obtectus (Say) (Coleoptera: Chrysomelidae) is one of the most important pests of legumes in the field and storage, it causes losses in kidney bean and cowpea seeds exceeding 16% in one generation. The physical control method using modified atmospheres is an alternative method to toxic gases in storage. This study aimed to determine the sensitivity of A. obtectus stages that fed on two types of hosts (kidney bean seeds and cowpea seeds) to the modified atmosphere using three concentrations of carbon dioxide (20, 40, and 60%) and different exposure periods at 30 °C and 65% R.H. In general, the results showed that A. obtectus stages reared on cowpeas were more sensitive to CO₂ than those reared on kidney beans. Also, the stages are arranged according to their degree of sensitivity to carbon dioxide as follows: adults > eggs > larvae > pupae. According to the LT values, the sensitivity of the external stages of the seeds to carbon dioxide was slightly affected by the type of food, while the internal stages were clearly affected. Complete mortality was achieved after 72 h for adults in all treatments as well as after 7 and 6 days for eggs at 20 and 40% CO₂ in both hosts. The exposure periods required to achieve complete mortality were 12, 11, and 10 days for larvae and 12, 12, and 10 days for pupae produced from cowpea seeds, which increased to 14, 12 and 10 days for larvae and 18, 13, and 12 days for pupae produced from kidney bean seeds when treated with 20, 40, and 60% carbon dioxide, respectively. In conclusion, we recommend using the highest concentration of CO₂ to control all stages of A. obtectus for 10 days in the case of cowpea seeds and 12 days in the case of kidney bean seeds.