Yellow starthistle (YST; Centaurea solstitialis L.) is one of the most impactful weeds of rangelands, grasslands, and wildlands in the western U.S. On Department of Defense (DoD) lands, it is a major impediment for military training and ecosystems. Four biological control insects have been introduced with the goal of controlling YST; however, there is a critical need to add additional agents that complement these seed and flower-feeding insects because the weed is not under control over most of its range, and it is expected to further spread with climate change. The overall goal of this project is to enhance the effectiveness of YST management using a newly approved biological control agent, the rosette weevil, Ceratapion basicorne (CEBA) and to understand how developmental constraints may influence weevil’s performance in new and changing climates. In contrast to current biocontrol, CEBA feeds on multiple plant tissues, which can kill whole plants and reduce the growth and reproduction of survivors. The project team will 1) establish field populations of CEBA on or near DoD lands in California, Oregon, and Washington; 2) conduct experiments and field studies to understand developmental cues and constraints of CEBA in relation to host phenology and its environment; 3) develop geo-climatic models to investigate the potential for phenological synchrony and range overlap between CEBA and YST under present-day and future climates; and 4) share basic and applied knowledge gained from the research with land managers, ecologists, biological control practitioners, and the general public.

A series of laboratory experiments to determine CEBA’s developmental requirements including the role of temperature and photoperiod in terminating and initiating diapause will help optimize rearing methods and build populations for use in releases. The project team will work with DoD natural resource managers and state agency biocontrol coordinators to select release and research sites. Following releases, populations will be monitored for establishment, growth, and phenology. The project team will conduct greenhouse experiments to assess how the timing of CEBA’s life cycle relative to YST and different moisture and photoperiod regimes affect insect performance as a biological control agent. Outdoor garden experiments at three different latitudes will help to evaluate the extent of phenological synchrony of YST and CEBA under distinct climate and photoperiod conditions. Additionally, YST phenology will be monitored using time lapse cameras at release sites and up to six other infestations. Geo-climatic models to predict phenology and potential range (climate suitability) will be parameterized primarily using developmental data collected for this study as well as occurrence records from the native (CEBA) or invaded (YST) range. A geo-climatic modeling tool previously developed by the team will be updated and operationalized at USPest.org, allowing end users to easily access the tool and its regularly updated model forecasts. The project team will share practical YST and related weed management knowledge and insights to DoD land managers and other stakeholders through interactive training workshops, field events, and education materials.

The project will provide end users with a dynamic modeling tool and forecasts to help them identify locations where CEBA will likely have success establishing and expanding under present-day and future climates. More effective biological control of YST will reduce the need for chemical and other costly control measures. Established CEBA populations on or near DoD lands will also provide a source for natural and human aided dispersal to surrounding weed infestations. The findings will help inform the extent to which introduced agent populations experience phenological mismatches upon introduction to new environments. Future applications on DoD lands may include biocontrol agents for other high priority weed systems, including those currently under review for use against garlic mustard, tree of heaven, Russian olive, phragmites, and knapweeds.  Adaptation of the phenology modeling tool for threatened and endangered butterflies and their hosts may also have relevance for DOD conservation efforts.