Insect Diversity in Pinus sylvestris Forest Stands Damaged by Lymantria monacha

Simple Summary: The nun moth (Lymantria monacha L.), a defoliator of conifers and broad-leaved trees, is expanding its range, and outbreaks are increasingly occurring in the forests of central and eastern Europe. The only way to control and eradicate mass outbreaks of the pest is aerial spraying with a biological insecticide, Foray 76B. However, knowledge of variations in non-target insect assemblages following a treatment is limited. The present study aimed to determine the effect of the treatment on the diversity of non-target insects in Pinus sylvestris stands in three regions of Lithuania in the year following a nun moth outbreak. The treatment was found to influence the diversity of psyllids, ants, and beetles. Treated pine forests near the Baltic Sea exhibited lower insect species richness and a decreased relative abundance of beetles on the forest floor. The spraying influenced a reduction in the relative abundance of Carabus arcensis in the forests in the southern part of the country. The treatment also influenced the movement of ants from the tree canopy to the forest floor at all studied locations. Outbreaks of Lymantria monacha are of great concern, as their occurrence is predicted to become more intense and frequent due to a warming climate. A frequent treatment to control mass outbreaks of the pest is with the bioinsecticide Foray 76B. However, knowledge of how this treatment affects non-target insect species is limited. We surveyed the assemblages of non-target epigeal and arboreal insects in Pinus sylvestris forests in the year following bioinsecticide application. A collection of insects using sweep nets and pitfall traps was carried out in L. monacha-infested pine stands, (i) treated with Foray 76B and (ii) untreated, in three regions of Lithuania from May to October 2021. The results revealed that, in Neringa forests, species richness of the epigeal insects was lower in treated than in untreated sampling plots, with 36 and 41 different insect species, respectively. The relative abundance of epigeal Coleoptera in treated plots was 3.6%, while in untreated it was 53.2%. There was a significant decrease in the relative abundance of Carabus arcencis in Kap & ccaron;iamiestis (by 7.4%) and Marcinkonys (by 16.7%). Treated plots were distinguished by lower relative abundance of arboreal Hymenoptera at all three study locations.