Dynamics of Scots pine health after the release of Thanasimus formicarius in bark beetle foci

In Eastern Polissia, a severe decline of Scots pine started in 2015. It was caused by drought and aggravated by outbreaks of Ips acuminatus (Gyllenhal, 1827) followed by Ips sexdentatus (Börner, 1776). The sanitary felling could not mitigate the effects of the bark beetle outbreak. Therefore, there was an attempt to release a predatory beetle, Thanasimus formicarius (Linnaeus 1758) (Coleoptera: Cleridae) into bark beetle foci. In 2018, the rearing of this predator started in the State Specialized Forest Protection Enterprise “Kharkivlisozahyst”, and it was released into the foci of bark beetles in Chernihiv, Sumy, and Kharkiv regions. The study aimed to evaluate the effects of Th. formicarius introduction into the foci of bark beetles. For the purpose of analysing the situation, sample plots in the Forest zone, on the border of the Forest and Forest-Steppe, and in the Forest-Steppe zone have been selected. The analysis shows that the outbreak of bark beetles in north-eastern Ukraine started earliest in Chernihiv region (Forest zone) and latest in Kharkiv region (Forest-Steppe zone). Selyaninov's Hydrothermal index was less than the standard of the corresponding natural zones. This contributed to the development of bark beetle populations. In the year of the predator introduction (2018), the bark beetle outbreak began to decline in Chernihiv, culminated in Sumy, and started in Kharkiv. In all years, the diameter of recently died trees was smaller than the average diameter of the stand, that is, the tree mortality was not pathological, because bark beetles infested trees of different sizes. In 2018, the mortality of trees was the highest in Chernihiv region, where the outbreak began earlier. The health of the trees improved, and the proportion of recently died trees decreased in all the sample plots in 2019. The health of the trees worsened in all the sample plots in the following years, due to the new outbreak of bark beetles. The trend and level of tree health worsening were most pronounced in the sample plots in Sumy, where regular sanitary thinning was conducted. Thus, the release of Th. formicarius into the bark beetle foci contributed to tree health improvement the following year. However, a one-time predator release could not restrain the new outbreak and prevent further forest decline. It is possible that additional releases of predators can provide long-lasting and environmentally safe pest control.