Bivoltinism in the northern bark beetle <i>Ips duplicatus</i> (Sahlberg, 1836) (Coleoptera: Curculionidae, Scolytinae) observed in Estonia

Abstract The European spruce bark beetle ( Ips typographus ), a primary concern for forest management in Europe, often co‐infests its host tree Norway spruce ( Picea abies ) with the northern bark beetle ( Ips duplicatus ). To achieve effective bark beetle control, it is crucial to differentiate between the two species. However up‐to‐date research on I. duplicatus in the hemiboreal region of Europe is lacking. In Estonia, I. duplicatus has been considered univoltine. To address previous suspicions, the current pilot study was conducted to explore the voltinism of I. duplicatus . The study confirmed the development of two generations during the activity period of 2024, with the majority of newly emerged adults of the second generation leaving the host tree by October. Calculation of accumulated degree days above the lowest temperature threshold of 6.3°C demonstrated a significant increase in the annual temperature sum over an 80‐year observation period, indicating the extension of the activity period of I. duplicatus . The shift to bivoltinism is significant for forest managers because it leads to greater forest damage across a larger area within a single activity season. In some cases, I. duplicatus can become the primary pest, requiring distinct monitoring and control measures compared to I. typographus . However, the details of overwintering habits and the exact temperature sum necessary to complete a full generation require further investigation.