Rapid climate change threatens to interfere with previously successful biological control by changing the phenology of established biocontrol agents and their targets. Phenological mismatches between trophic levels have been documented in several plant-herbivore systems, but are relatively little studied in managed systems, where reduced synchrony between weeds and biocontrol agents may cause substantial economic harm. Rhinocyllus conicus, the thistle head weevil, has been widely effective as a biocontrol agent for musk thistle, Carduus nutans, but its continued effectiveness may be impacted by climate change. The success of this weevil strongly depends on the synchronization of weevil emergence and thistle development. R. conicus exerts the strongest control over C. nutans in years when peak oviposition coincides with peak availability of thistle buds. We investigated the potential for mismatch in this system using an urban-rural temperature gradient as a proxy for future warming. Urban heat islands can provide a valuable means for investigating the effects of temperature differences without co-occurring variation in photoperiod. We established observation sites spanning the urban-rural gradient in and around Harrisburg, PA, and tracked musk thistle phenology and weevil oviposition along this gradient with twice-weekly observations of ten thistles per site through two growing seasons.
Results/Conclusions
We found significant temperature differences between sites, with sites differing in average temperature by up to 3.12°C over the course of our observations. Sites categorized as urban were typically warmer than suburban and rural sites, but there was considerable variation within each group. Thistle phenology also varied substantially between sites with the average date of first bud appearance differing by up to 8.3 days between sites. The timing of weevil activity varied as well, with peak weevil abundance occurring up to 15 days apart at different sites. Preliminary analysis suggests temperature is an important predictor of the timing of phenological events, but that other site characteristics, including soil moisture and soil nutrient content, are important as well. Understanding the impact of temperature on relative biological control agent phenology, and predicting any corresponding decreases in biological control agent effectiveness, is crucial for managers as they decide whether they will need to supplement biological control with other management practices, such as mowing, as average temperatures increase. This information thus highlights when additional management is necessary and illuminates how complementary interventions can best be deployed to limit negative impacts on biological control.