Biological control is an important and sustainable approach for managing widespread invasive weeds. In biological control, host specific natural enemies (usually insects) are introduced with the goal of establishing a permanent population that will provide sustained, cost-effective control of the plant. The overall goal of this project was to preserve and enhance the effectiveness of biological control for weeds, including on Department of Defense (DoD) installations, through an understanding of the phenological constraints that may arise with a change in climate as a result of the insects’ combined responses to photoperiod and temperature. This work applies to three important weed species that are currently (or soon to be) targets for biological control: purple loosestrife, tamarisk, and Japanese knotweed.

The project team developed an advanced geo-climatic phenology model, which provides an innovative approach to tracking life cycle responses to both temperature and photoperiod. The model can be used to predict phenology, voltinism (number of generations per year), and the degree of asynchrony between the biocontrol agent’s life cycle and the host plant’s growing season under new climate conditions. Experiments carried out in controlled environment chambers were used to quantify and compare the photoperiod response among different geographic populations of all three biocontrol agents and, along with field phenology observations, to obtain parameter estimates for the model. A common garden field experiment set up in Corvallis, Oregon, compared the phenology and voltinism of geographically distant source populations of biocontrol agents under a common climate, while also testing our model’s ability to predict performance of agents moved into in new climates. This experiment also quantified the follow-on consequences of insect photoperiodism and climate change for impacts to the host plant.

Results from the experiments and model indicate that photoperiodism plays a crucial role in determining each biocontrol agent’s phenological response to a change in climate. The project team found that local populations of both Galerucella calmariensis and Diorhabda carinulata have evolved photoperiod responses that improve local fitness by allowing an appropriate number of generations for the season duration. For D. carinulata this adaptation has occurred in conjunction with a rapid range expansion to the south. With G. calmariensis, the local differences in photoperiodism can affect voltinism, and the resulting level of impact to the weed, when moved into new locations. For the knotweed psyllid, A. itadori, the model predicts one to three attempted generations throughout the range of North America where knotweeds are invasive. However, the insect is likely to attempt too many generations for the season duration in northern areas and too few generations in the south. A sweet spot with voltinism matched to the season duration is present at middle latitudes. Geographic patterns of phenological mismatch were similar for all three species and represent a generalizable pattern for other introduced insects sharing a similar multi-voltine life cycle cued by photoperiod.

The project team dedicated considerable effort toward sharing the model and encouraging its application for other systems, including other biocontrol agents, invasive pests, and threatened and endangered species. The model is now publicly accessible online as part of an advanced degree-day phenology modeling system developed and managed by the Oregon Integrated Pest Management Center (formerly Integrated Plant Protection Center) at Oregon State University. Key results that affect management decisions were shared with DoD resource managers in customized reports. As the most comprehensive geo-climatic phenology model that incorporating photoperiodism, this project has contributed greatly to a general theoretical understanding of how photoperiodism and thermal responses interact for introduced organisms and those exposed to change in climate.