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The Siberian moth, Dendrolimus sibiricus Tschetv., 1908 (Lepidoptera: Lasiocampidae) is a conifer pest that causes unprecedented forest mortality in Northern Asia, leading to enormous ecological and economic losses. This is the first study summarizing data on the parasitoid diversity and parasitism of this pest over the last 118 years (1905–2022). Based on 860 specimens of freshly reared and archival parasitoids, 16 species from two orders (Hymenoptera and Diptera) were identified morphologically and/or with the use of DNA barcoding. For all of them, data on distribution and hosts and images of parasitoid adults are provided. Among them, the braconid species, Meteorus versicolor (Wesmael, 1835), was documented as a parasitoid of D. sibiricus for the first time. The eastern Palaearctic form, Aleiodes esenbeckii (Hartig, 1838) dendrolimi (Matsumura, 1926), status nov., was resurrected from synonymy as a valid subspecies, and a key for its differentiation from the western Palaearctic subspecies Aleiodes esenbeckii ssp. esenbecki is provided. DNA barcodes of 11 parasitoid species from Siberia, i.e., nine hymenopterans and two dipterans, represented novel records and can be used for accurate molecular genetic identification of species. An exhaustive checklist of parasitoids accounting for 93 species associated with D. sibirisus in northern Asia was compiled. Finally, the literature and original data on parasitism in D. sibiricus populations for the last 83 years (1940–2022) were analysed taking into account the pest population dynamics (i.e., growth, outbreak, decline, and depression phases). A gradual time-lagged increase in egg and pupal parasitism in D. sibiricus populations was detected, with a peak in the pest decline phase. According to long-term observations, the following species are able to cause significant mortality of D. sibiricus in Northern Asia: the hymenopteran egg parasitoids Telenomus tetratomus and Ooencyrtus pinicolus; the larval parasitoids Aleiodes esenbeckii sp. dendrolimi, Cotesia spp., and Glyptapanteles liparidis; and the dipteran pupal parasitoids Masicera sphingivora, Tachina sp., and Blepharipa sp. Their potential should be further explored in order to develop biocontrol programs for this important forest pest.

ABSTRACT Now more than ever researchers provide more and more evidence that it is necessary to develop an ecologically friendly approach to pest control. This is reflected in a sharp increase in the value of the biological insecticide market in recent decades. In our study, we found a virus strain belonging to the genus Cypovirus (Reoviridae); the strain was isolated from Dendrolimus sibiricus, possessing attractive features as a candidate for mass production of biological agents for lepidopteran-pest control. We describe the morphological, molecular, and ecological features of the new Cypovirus strain. This strain was found to be highly virulent to D. sibiricus (the half-lethal dose is 25 occlusion bodies per second-instar larva) and to have a relatively wide host range (infecting representatives of five families of Lepidoptera: Erebidae, Sphingidae, Pieridae, Noctuidae, and Lasiocampidae). The virus strain showed a strong interaction with a nontoxic adjuvant (optical brightener), which decreased the lethal dose for both main and alternative hosts, decreased lethal time, and may expand the host range. Moreover, we demonstrated that the insecticidal features were preserved after passaging through the most economically suitable host. By providing strong arguments for the possible use of this strain in pest control, we call on virologists, pest control specialists, and molecular biologists to give more attention to the Cypovirus genus, which may lead to new insights in the field of pest control research and may provide significant advantages to compare with baculoviruses and Bacillus thuringiensis products which are nowadays main source of bioinsecticides. IMPORTANCE In this article, we describe a newly discovered cypovirus strain that displays features ideally suited for the development of a modern biological insecticide: high potency, relatively broad host range, true regulating effect, flexible production (possibility to choose host species for production), interaction with enhancing adjuvants, and ecologically friendly. Based on an alignment of CPV genomes, we suggest that the enhanced host range of this new strain is the sequence of evolutionary events that occurred after coinfections involving different CPV species within the same host. These findings suggest that we need to positively reconsider CPVs as prospective agents as biocontrol products.