Impacts of climate change on the crop invasion of oilseed rape by the rape stem weevil, Ceutorhynchus napi, in North-Western Germany

1. Introduction

The rape stem weevil, Ceutorhynchus napi Gyll, (Coleoptera: Curculionidae), is a severe pest in oilseed rape (Brassica napus L.) in Europe and can cause yield losses up to 50% (Williams 2010). Based on a refined model by Debouzie and Wimmer (1992), we analyzed interactions between meteorological variables and pest activity. This new impact model was combined with a multi model ensemble of regional climate change projections (Junk et al. 2012). This ensemble of different Global (GCM) and a Regional Climate Model (RCM) covers the period from 1961 to 2100. All projections were based on the A1B SRES emission scenario (Nakićenović and Swart 2000). The range of projected changes in future air temperature and precipitation were comparable with the recently published results presented in the ‘Fifth Assessment Report: Climate Change 2013` by the IPCC. To eliminate systematic biases a non-linear bias correction scheme was applied to the air temperature and precipitation time series. The onset of the crop invasion by the rape stem weevil was projected to occur between 7 days (near future, 2021-2050) and 17 days (far future, 2069-2098) earlier in comparison to the reference period (1971-1990).

2. Materials and methods

The model by Debouzie and Wimmer (1992) predicts the crop invasion, when daily maximum air temperature exceeds 9°C on three consecutive days up to DOY 74. In addition, on these days no precipitation should occur. After DOY 74 only one day is taken into account with the same threshold values. Based on long-term observational data the threshold values of this model were optimized in an iterative process minimizing the root mean squared error (RMSE). The adopted values were 12.0°C, 12.5°C, and 13.0°C for the consecutive days (up to DOY 74). The daily totals of precipitation on these days must be less than 2 mm on each day. From DOY 75 onwards the thresholds values refer to only one day and were 9°C for the daily maximum air temperature and less than 1 mm precipitation. The in situ-observations at Goettingen, Germany (Lat. 51°56', Long. 9°94', 333 m AMSL) consist of field surveys (1989-2010). Daily data of maximum air temperature as well as precipitation were retrieved from the data archive of the German Meteorological Service (1971-2010) and used to calibrate the forecast model of Debouzie and Wimmer (1992) and for the bias correction of the different RCMs. We used 15 GCM RCM combinations of the EU ENSEMBLES project, covering the period from 1961 until 2100 (van der Linden and Mitchell 2009) with a spatial resolution of 25 km. For the air temperature and precipitation time-series a non-linear bias correction (quantile mapping) was applied that corrects the mean and also the width and the shape of the distributions (Te Linde et al. 2008).

3. Results & Discussion

A comparison of long-term averages of the measured (9.0°C) and modelled (8.9°C) mean air temperature of the reference timespan (1971-2000) differed only by 0.1 K. The 30-year long-term averages of the multi-model annual mean air temperatures increased from 8.9°C in the reference period to 10.1°C in the near (2021-2050) and 11.7°C in the far future (2069-2098). The long-term average annual precipitation sum - derived from the multi model ensemble mean - increased from 658 mm in the reference period to 706 mm in the near and 719 mm in the far future. The model by Debouzie and Wimmer (1992) adopted to the local climate conditions is able to reproduce the observed dates of crop invasion with a RMSE value of 10.9. In most of the years the differences are - with less than 3 days - very small. The combination of the multi model regional climate change projections and the immigrations model for C. napi were analyzed for 10-year time slices in the reference period as well as for the near and far future. In the control time-span, first appearance was on average at DOY 77 ± 2.8 (18 March). For the two future time-spans, shifts towards earlier dates are expected: DOY 70 ± 5.7 (11 March) for the near and DOY 60 ± 12.3 (1 March) for the far future. Although higher air temperatures increase the probability towards earlier migrations dates the changes in the precipitation mask this effect considerably. Even under significantly higher air temperatures in the far future the slightly higher precipitation amounts in March could hamper the crop invasion by C. napi.

4. Conclusion

Our study demonstrated that simple models using locally-adjusted weather-based thresholds have the potential to offer sufficiently accurate forecasting of first immigration flights by C. napi. This is the basis for an appropriate timing of insecticide application for controlling this pest.

Acknowledgements

We gratefully acknowledge the financial support of the Ministère de l'Enseignement supérieur et de la Recherche (MESR) of the Grand-duchy of Luxembourg in the framework of the REMOD program.

References

Debouzie D, Wimmer F (1992) Models for winter rape crop invasion by the stem weevil Ceuthorrhynchus napi Gyll. (Col., Curculionidae). J Appl Entomol 114:298-304.

Junk J, Eickermann M, Görgen K, Beyer M, Hoffmann L (2012) Ensemble-based analysis of regional climate change effects on the cabbage stem weevil (Ceutorhynchus pallidactylus (Mrsh.)) in winter oilseed rape (Brassica napus L.). J Agr Sci 150:191-202.

Nakićenović N, Swart R (2000) Special report on emissions scenarios: a special report of Working Group III of the Intergovernmental Panel on Climate Change. Cambridge University Press.

Te Linde AH, Aerts JCJH, Hurkmans RTWL, Eberle M (2008) Comparing model performance of two rainfall-runoff models in the Rhine basin using different atmospheric forcing data sets. Hydrol Earth Syst Sci 12:943–957.

van der Linden P, Mitchell JFB (2009) ENSEMBLES: Climate Change and itxs impacts: Summary of Research and Results from the ENSEMBLES Project. Met Office Hadley Centre, Exeter, UK.

Williams IH (2010) The major insect pests of oilseed rape in Europe and their management: an overview. In: Williams IH (ed) Biocontrol-Based Integrated Management of Oilseed rape Pests. Springer, Dordrecht, The Netherlands, pp 1-43.