Gamma irradiation for management of pulse beetle callosobruchus maculatus (Fabricius) (Coleoptera: Chrysomelidae

Abstract

The study entitled “Gamma irradiation for management of pulse beetle, Callosobruchus maculatus (Fabricius) (Coleoptera: Chrysomelidae)” was carried out from October 2019 to June 2020 at Pesticide Residue Laboratory, College of Horticulture, Vellanikkara. The objectives of the study were to assess the effectiveness of gamma irradiation for the management of pulse beetle, C. maculatus and to study the effect of gamma irradiation on the nutritional factors of cowpea grain. Susceptibility of different developmental stages - viz., egg, larva, pupa and adult – of C. maculatus to gamma radiation was studied by irradiating cowpea grains (variety- Kanakamany) with insect stages. Irradiation of one-day and three-day-old eggs at doses ranging from 16 Gray (Gy) to 24 Gy resulted in 100 per cent mortality of one-day-old eggs. While in three-day-old eggs, mortality varied from 52.50 per cent at 16 Gy to 65 per cent at 24 Gy. However, in control, the mortality was only 15 per cent. The mean egg period of three-day-old eggs significantly increased from 5.15 days in control to 5.36 days in treatment with the highest dose of 24 Gy. Although the irradiated eggs hatched out, the grubs did not develop further and reached the adult stage. One-day-old eggs were more radiosensitive than three-day-old eggs. Gamma irradiation on different grub stages revealed that early instar grub (7-day-old) was more sensitive to radiation than late instar grub (10-day-old). Exposure of seven-day-old grubs to gamma radiation at doses ranging from 16 Gy to 24 Gy showed a significant decrease in per cent pupation from 35 per cent at 16 Gy to 15 per cent at 24 Gy and were significantly different from that of control (82.5%). As the age of grubs advanced to 10 days, a higher dose of 50 Gy and 75 Gy caused 52.50 per cent mortality followed by 60, 65 and 70 per cent, respectively at 100, 125 and 150 Gy and was significantly higher compared to control (7.50%). Gamma radiation did not significantly alter mean grub period. However, none of the irradiated grubs were developed into adults. Exposure of four-day-old pupae to gamma radiation (100 Gy and above) completely inhibited adult emergence. One-day-old female and male adults were irradiated separately at doses of 100, 500, 750, 1000 and 1500 Gy to assess the effect of gamma radiation. Mean adult longevity of both female and male was decreased significantly with an increase in radiation dose. Cent per cent mortality of both sexes occurred on third day at 1500 Gy whereas, in control, the adult female and male survived for 13 and 11 days, respectively. Irradiation of one-day-old adults at doses ranging from 16 Gy to 24 Gy adversely affected progeny production of C. maculatus. The fecundity of irradiated adults did not differ significantly from control, but hatchability of the eggs reduced significantly. None of the eggs laid by the irradiated adults were developed into pupa. Irradiation at a dose of 24 Gy induced complete sterility in adults of C. maculatus. To study the effectiveness of gamma irradiation in protecting grains infested with different developmental stages of C. maculatus, a lot of 500 gram of infested cowpea grains was irradiated with a range of 20 to 100 Gy and recorded the adult emergence. After 17 days, in control treatment the mean adult emergence was 537.50 while it was only 25.20 beetles in 100 Gy irradiated lot. Nutritional studies of both irradiated and control treatments revealed that irradiation did not significantly alter the protein and carbohydrate content of cowpea grain at any of the irradiation doses. Hence, gamma radiation is a promising technology for the management of pulse beetle because of its effectiveness, fast and easy application, absence of residues, and minimal changes to the nutritional properties of the treated cowpea grain.