Interventions have limited effects on the population dynamics of Ips typographus and its natural enemies in the Western Carpathians (Central Europe)

Abstract Outbreaks of the European spruce bark beetle Ips typographus kill extensive areas of Norway spruce forests in Central Europe, affecting both protected areas and neighboring commercial forests. In protected areas, uncontrolled (non-intervention) management allows natural beetle-induced tree mortality, while in commercial forests infested trees are salvage-logged in order to lower I. typographus numbers and stabilize wood production. However, the effects of active pest controls on I. typographus population are often ambiguous, and little is known about how antagonists, beetle density, or intraspecific competition help terminate I. typographus outbreaks. To answer this question, we studied I. typographus population development, breeding performance, and natural antagonists under intervention and non-intervention management in the High Tatra Mts. from 2014 to 2017. We used sentinel logs and windthrown trees, following the current legislation in Tatranský narodný park (TANAP, Slovakia) and Tatrzanski Park Narodowy (TPN, Poland). Both parks are mountainous, are covered by Norway spruce forests that have been extensively affected by windthrows and extensive I. typographus outbreaks over the last 20 years, and are zoned under intervention and non-intervention types of management. We found that I. typographus was a dominant species and that its numbers gradually increased during the study, but that the numbers of maternal galleries in sentinel logs (from 50 to 300 per m2) and sex ratio (1 male:2.5 females) remained the same among years. The length of maternal galleries and egg production (per m2 of bark), however, gradually declined, and breeding performance decreased from 10 daughter beetles per female in 2014 to almost zero in 2017. These parameters did not significantly differ between intervention and non-intervention sites. Antagonists were not abundant and had limited effects on I. typographus numbers. Active intervention did not affect I. typographus population dynamics or rates of parasitism but increased numbers of Thanasimus beetles, which are predators of bark beetles. In other words, intervention increased the numbers of some natural enemies of I. typographus but did not affect I. typographus population dynamics. Because control measures did not reduce I. typographus numbers in the intervention zone, the natural ecological processes crucial for maintaining ecosystem functioning and biodiversity were evidently preserved.