The silkworm Bombyx mori (Lepidoptera: Bombycidae) is a lepidopteran model insect of great economic importance. The parasitoid Exorista sorbillans (Diptera, Tachinidae) is the major pest of B. mori and also a promising candidate for biological control. However, the molecular interactions between hosts and dipteran parasitoids have only partially been studied. Gene expression analysis by reverse-transcription quantitative real-time polymerase chain reaction (RT-qPCR) is indispensable to characterise their interactions. Accurate normalisation of RT-qPCR-based gene expression requires the use of reference genes that are constantly expressed irrespective of experimental conditions. In this study, the expression stability of 13 traditionally used reference genes was estimated by five statistical algorithms (ΔCt, geNorm, Normfinder, BestKeeper, and RefFinder) to determine the best reference genes for gene expression studies in different tissues of B. mori under E. sorbillans parasitism. Specifically, TATA-box-binding protein was the best reference gene in epidermis and testis, while elongation factor 1α was the most stable gene in prothoracic gland and midgut. Elongation factor 1γ, ribosomal protein L3, actin A1, ribosomal protein L40, glyceraldehyde-3-phosphate dehydrogenase and eukaryotic translation initiation factor 4A were the most suitable genes in head, silk gland, fat body, haemolymph, Malpighian tubule and ovary, respectively. Our study offers a set of suitable reference genes for gene expression normalisation in B. mori under the parasitic stress of E. sorbillans, which will benefit the in-depth exploration of host-dipteran parasitoid interactions, and also provide insights for further improvements of B. mori resistance against parasitoids and biocontrol efficacy of dipteran parasitoids.

In laboratory experiments, pupae of the uzi fly, Exorista sorbillans Wiedemann (Diptera: Tachinidae), an endoparasitoid of the silkworm, Bombyx mori L. (Lepidoptera: Bombycidae) were subjected to UV-irradiation for different durations. Upon adult emergence, irradiated and control flies were crossed in all possible combinations and allowed to parasitise healthy silkworms. Adult emergence, fecundity and longevity of the resulting progeny were recorded. Progeny of irradiated E. sorbillans showed significantly reduced adult emergence, oviposicion period and hatchability. The reduction in fecundity was negatively correlated with the duration of exposure to UV radiation. The longevity of uzi flies of both the sexes was also significantly reduced.

Larva of fifth instar silkworm, Bombyx mori L. (Lepidoptera: Bombycidae), parasitized by uji-fly, Exorista sorbillans (Weidemann) (Diptera: Tachinidae), consumed less food and egested less faecal matter than unparasitized one in the instar period. Uji-fly infested worm showed increased digestibility in the last 2 days while the unparasitized larva ingested excessive amounts of food than needed during these days. A reduction of 18.1% in the mean weight of parasitized worm at pupation was recorded. Further, parasitization resulted in 50.7% reduction in the mean cocoon weight and 51.12% reduction in the mean shell weight. Parasitized worms started spinning a day earlier than the healthy ones and the cocoon spun by the former was abnormal.