After high population densities of insect defoliators there is often a dramatic decrease in their abundance due to various limiting factors. Here, we compared gypsy moth larvae (Lymantria dispar L.) reared singly and in crowded conditions. We compared a number of physiological parameters of these insects and the effect of L. dispar population density on the activation of covert baculovirus infections in the larvae. It was found that the population density of gypsy moth larvae did not affect the mortality due to the activation of the covert virus infection or the total mortality. On the other hand, solitary-reared larvae were heavier, took longer to develop, and showed a four-fold higher concentration of dopamine in their haemolymph than larvae reared in groups. Thus, we demonstrated that an increase in the population density of larvae per se facilitates some changes in fitness and innate immunity traits but is not related to the activation of covert baculovirus infection. We suggest that an increase in population density does not increase the risk of epizootics triggered by the activation of covert baculovirus infection and that researchers should pay more attention to studying density-associated factors, such as starvation.

Entomopathogenic fungi can be inhibited by different soil microorganisms, but the effect of a soil microbiota on fungal growth, survival, and infectivity toward insects is insufficiently understood. We investigated the level of fungistasis toward Metarhizium robertsii and Beauveria bassiana in soils of conventional potato fields and kitchen potato gardens. Agar diffusion methods, 16S rDNA metabarcoding, bacterial DNA quantification, and assays of Leptinotarsa decemlineata survival in soils inoculated with fungal conidia were used. Soils of kitchen gardens showed stronger fungistasis toward M. robertsii and B. bassiana and at the same time the highest density of the fungi compared to soils of conventional fields. The fungistasis level depended on the quantity of bacterial DNA and relative abundance of Bacillus, Streptomyces, and some Proteobacteria, whose abundance levels were the highest in kitchen garden soils. Cultivable isolates of bacilli exhibited antagonism to both fungi in vitro. Assays involving inoculation of nonsterile soils with B. bassiana conidia showed trends toward elevated mortality of L. decemlineata in highly fungistatic soils compared to low-fungistasis ones. Introduction of antagonistic bacilli into sterile soil did not significantly change infectivity of B. bassiana toward the insect. The results support the idea that entomopathogenic fungi can infect insects within a hypogean habitat despite high abundance and diversity of soil antagonistic bacteria.

The ability of parasitic wasps to manipulate a host's metabolism is under active investigation. Components of venom play a major role in this process. In the present work, we studied the effect of the venom of the ectoparasitic wasp Habrobracon hebetor on the metabolism of the greater wax moth host (Galleria mellonella). We identified and quantified 45 metabolites in the lymph (cell-free hemolymph) of wax moth larvae on the second day after H. hebetor venom injection, using NMR spectroscopy and liquid chromatography coupled with mass spectrometry. These metabolites included 22 amino acids, nine products of lipid metabolism (sugars, amines and alcohols) and four metabolic intermediates related to nitrogenous bases, nucleotides and nucleosides. An analysis of the larvae metabolome suggested that the venom causes suppression of the tricarboxylic acid cycle, an increase in the number of free amino acids in the lymph, an increase in the concentration of trehalose in the lymph simultaneously with a decrease in the amount of glucose, and destructive processes in the fat body tissue. Thus, this parasitoid venom not only immobilizes the prey but also modulates its metabolism, thereby providing optimal conditions for the development of larvae.

Habrobracon hebetor is a globally acknowledged larval ectoparasitoid that is widely used to control lepidopteran pests. Wolbachia is a natural endosymbiont that regulates various aspects of the insect host biology. The ability of H. hebetor to paralyze and develop on lepidopteran larvae from five families was tested under laboratory conditions. Two lines of the wasp were used, “W+” containing a naturally occurring Wolbachia from the supergroup B, and “W−”, with the endosymbiont eradicated by antibiotic treatment, followed by propagation of 20 subsequent generations. The proportions of larvae in which host paralysis, as well as parasitoid oviposition, larval, pupal, and adult development were observed, were usually higher in W+ compared to W−. In Loxostege sticticalis, differences in these indices were not statistically significant. In Galleria mellonella, Mamestra brassicae, and Ostrinia nubilalis, some of the parasitism indices were significantly higher in W+ than in W−. In Bombyx mori and Plutella xylostella, H. hebetor could not complete its life cycle, but parasitism levels at the initial steps (from paralysis symptoms to the presence of larvae/pupae of the parasitoid) were 2–5 times lower in W− compared to W+ (p < 0.01). It can be suggested that the presence of Wolbachia is advantageous for H. hebetor, as it increases the success of parasitism in a broad range of lepidopteran hosts.