Non-target catches of saproxylic beetles during bark-beetle mass-trapping

The stenographer bark beetle Ips sexdentatus Borner has been causing significant damage to the coastal pine stands of Tuscany. Here, after some isolated outbreaks in 2013, its population density dramatically increased exploiting the huge availability of pines weakened by other factors. Mass trapping, together with the prompt cutting of infested trees, is one of the few control methods available to contain the I. sexdentatus populations. However, it may negatively affect other saproxylic insects, which are attracted to bark beetle pheromones. In this study we addressed non-target catches (NTC) collected during four years (2016-2019) of control activity using 25 pheromone-baited Theysohn traps. Catches were collected every ten days from March to December, while the lure was renewed every 30-50 days according to the dispensers’ life-span and to the temperatures in the study area. NTC, beetles in particular, were collected and examined to assess the use of this kind of trap in the effort to maximize its effectiveness while limiting collateral damages. Special attention was reserved for two major bark beetle predators Thanasimus formicarius (L.) and Temnochila caerulea (Olivier). However, in 2018 and 2019 other beetle families were considered in our observations. As a result, we report observations regarding a list of saproxylic beetles, both beneficial and harmful ones. The Theysohn trap is described as one of the most selective types for monitoring and controlling bark beetles. However, T. formicarius and T. caerulea were caught in considerable numbers. More specifically, the prey/predator ratio was about 6:1, with about 50,000 I. sexdentatus catches estimated for the whole study period. This prey/predator ratio may be considered unsustainable, given the important contribution carried out by the mentioned predators to the pest containment, as well as that of other bark beetles. This study allowed us to assess the most critical periods for these two predators. In fact, prey-predator ratios were even lower in March-May (about 2:1), and in June-August (about 5:1). On the other hand, the September-December period was far better for both predators, with an overall prey/predator ratio of 19:1. This is also the key period to capture I. sexdentatus, since during this study the highest catches of this pest were recorded always in October and November. Thus, based on our observations, NTC should be taken into account when planning mass trapping, especially when NTC may negatively affect the very outcome of the control measure itself. Choosing a period less critical for predators, for example, could, to some extent, compensate the loss of pest catches obtained otherwise in more unfavorable periods. Furthermore, more selective traps (custom traps are currently under study) could increase general prey/predator ratio, allowing the forest manager to adjust more freely the mass trapping period.