Biological Characteristics of Trichospilus diatraeae (Hymenoptera: Eulophidae) are Influenced by the Number of Females Exposed Per Pupa of Tenebrio molitor (Coleoptera: Tenebrionidae)

Different numbers of parasitoid females confined with a host can variously affect the number, sex ratio and other characteristics of the parasitoid's progeny. The objective of this study was to elucidate the effects of various ratios of Trichospilus diatraeae Cherian & Margabandhu (Hymenoptera: Eulophidae) females to pupae of one its hosts, Tenebrio molitor L. (Coleoptera: Tenebrionidae), primarily on the number of this parasitoid's progeny and their sex ratio. Both the parasitoid and the host used in this study were taken from cultures reared in the laboratory by standard methods. In order to minimize the effects of variations in host weight, 24 h-old T. molitor pupae weighing between 0.110 and 0.140 g were held as a single individuals in glass tubes (2.5 cm Ø × 14 cm L) with 48 h-old T. diatraeae females for 72 h to allow parasitization. After this period, female wasps were removed and the host pupa were transferred, one per glass tube, to a climate-controlled room at 25 ± 2 °C, 70 ± 10% RH and 12:12 h L:D. The experimental design was completely randomized with six treatments (parasitoid-host ratios) and 12 replicates per treatment. The ratios of T. diatraeae females per host used were: 1:1, 7:1, 14:1, 21:1, 28:1 and 32:1. The percentage parasitism of T. diatraeae on T. molitor pupae was 33.33, 82.00 and 83.33% at ratios of 1:1, 7:1 and 14:1, respectively, and 100% at all other ratios. The emergence of T. diatraeae adults from parasitized pupae was 75% at a parasitoid-host ratio of 1:1 and 100% at ratios of 21:1, 28:1 and 32:1. The duration of the parasitoid's life cycle ranged from 21.00 ± 2.22 to 24.00 ± 2.00 days at parasitoid-host ratios of 32:1 and 1:1, respectively. The number of T. diatraeae progeny per T. molitor pupa was highest at a ratio of 21:1 (246.50 ± 50.18). The proportion of T. diatraeae females in the offspring decreased as the parasitoid-host ratio increased, varying between 0.82 ± 0.06 and 0.97 ± 0.01. A parasitoid-host ratio of 21:1 T. diatraeae females per T. molitor pupa is considered the most adequate and appropriate for mass-rearing of this parasitoid. Tenebrio molitor appears to be a suitable alternate host for efficient mass-rearing of T. diatraeae for biological control of lepidopteran pests. At the parasitoid-host ratio of 21:1, each T. molitor pupa supported the production of 246.5 parsitoids of which 88% were females, i.e., 216.9 females and 29.6 males; each T. diatraeae female produced 9.55 ± 0.48 female progeny, and the developmental time from egg to adult was 20.4 days.