Effects of host density, host age, temperature and gamma irradiation on the mass production of Nesolynx thymus (Hymenoptera: Eulophidae), an endoparasitoid of Uzi fly, Exorista sorbillans (Diptera: Tachinidae)

Abstract

The Uzifly, Exorista sorbillans Weidemann, is an endoparasitoid of the the silkworm moth, Bombyx mori L., and can impact commercial sericulture. Effects of Uzifly age, density, temperature and gamma radiation (60Co) on the mass-production of the hyperparasitoid Nysolynx thymus (Girault) were investigated. There was a direct relationship between progeny production and an increase in number of host puparia presented, and a significant decline in the number of progeny with increased host-age. Maximum progeny production was obtained by maintaining a 1:5 to 1:8 parasitoid-to-host ratio for all the age groups. Two to 4-day-old host puparia were most suitable for obtaining the maximum progeny production of N. thymus for all the host-densities. The intrinsic rate of increase (r m day−1) increased with the increased host density for all the host-age groups. The values of net reproductive rate (R 0) and gross reproductive rate (GRR) increased for all the density levels and host age groups. On the other hand, the values for the doubling time (D) and finite capacity of increase (λ) gradually decreased with increased host density for all the host age groups. Host density and host-age significantly influenced the sex-ratio of progeny in N. thymus. Higher proportions of females were observed at higher host density levels and for younger host age groups. The progeny production and sex ratio of the parasitoid varied significantly with temperature. The maximum mean number of progeny was recorded at 25°C, while the minimum was at 30°C. The trend of progeny production at different temperatures was on the order 25 > 20 > 30°C. The highest values for the net reproductive rate (R 0) and GRR for the progeny production were recorded at 25°C compared to 20 and 30°C. Both the values for doubling time of capacity (D) and finite capacity (λ) increased with the trend of 25, 30 and 20°C. The highest value for the intrinsic rate of increase (r m day−1) of progeny production was recorded at 25°C, while the lowest value was at 20°C. The sex-ratios were always female-biased at all the temperatures. Temperature had a significant effect on the longevity of adult N. thymus. The longevity of the adults decreased with an increase of temperature for both sexes. The highest rate of parasitism was observed at 20°C followed by 25 and 30°C. More than 95% parasitism was observed at all temperatures. Gamma irradiation significantly increased the progeny production of N. thymus when reared either on early or late irradiated host puparia, particularly in the parental generation, but irradiated early host pupae were more suitable for mass production of N. thymus than the irradiated late pupae. The sex ratio of parasitoids developing from gamma irradiated host pupae varied significantly. Higher proportions of females were observed for all the dose and host-age groups.