Changing climate conditions and new forest threats such as invasive alien species present new challenges for forest practitioners and demand effective responses that may exceed the traditional information and data needs, served by the available forest monitoring resources and regimes. Motivated by examples in other domains, it has thus been proposed to make use of informal online information sources, so-called social media such as blogs, the micro-blogging service Twitter or the social network Facebook, to supplement traditional environmental information sources. Here we present an example of such an information source using 5074 English and German Twitter messages posted between May 2012 and December 2015 and directly referencing the Oak processionary moth. We focus on different analytical approaches which can facilitate an efficient automatic analysis of large social media data sets in order to extract trends on the public discourse on this exemplary forest threat. Specifically, we present temporal and geographical messaging patterns, a content analysis of the messages including shared web links as well as a network analysis of the users contributing the information. We find that messaging patterns generally show a good alignment with the lifecycle of the sample species, but that distinct differences in the content emphasis and messaging patterns for the Twitter messages attributed to the UK and Germany can be observed. We conclude that, while the chosen example is unlikely to contribute novel primary observations of Oak processionary infestations, the presented methods are an efficient approach to monitor public discussions on this sample forest threat and can directly inform public outreach and engagement strategies, which could in turn support the gathering of primary observations and is furthermore applicable to a broad range of other forest threats such as invasive alien species. Our results suggest that forest practitioners should thus consider social online media as a supplemental information source worthy of further exploration.