Summary The cabbage stem flea beetle (CSFB), Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae), is a serious pest in winter oilseed rape (WOSR) Brassica napus L. with variation in abundance and damage between years. The adult beetles invade fields at the time of crop emergence and cause a mostly minor damage by feeding on the leaves of young plants. The main damage is caused by the larvae mining the petioles and later stems from the autumn to following spring. Forecasting is widely based on monitoring the activity density of the adult beetles with yellow water traps in the main period of field invasion. Uncertainties are attached to this forecasting as it is based on monitoring of a non-damaging stage. Further, there is not always a direct correlation between trap catches and subsequent larval density since temperature influences the number of eggs laid as well as the number of eggs hatching in the autumn. The focus of this project is the problematic aspect of the existing monitoring of the activity density as the only basis of forecasting and the variation in abundance between years. The main focus has been on the influence of temperature on five parameters of reproduction (the preoviposition and oviposition period, the total and daily egg-laying capacity and female longevity), egg development and larval survival at low temperature. Tests were carried out in the laboratory. Reproduction was studied at five constant temperatures 4, 8, 12, 16 and 20˚. Larval survival was studied at -5 and - 10˚C and the influence of cold acclimation and larval stage tested. As part of the project, an assessment of the initial plant injury was carried out and evaluated as an alternative monitoring method to estimate CSFB abundance in the field. This was tested in a field cage experiment with densities of 1, 2, 4 and 8 pairs of beetles at a plant density of 24. The assessment of the initial plant injury showed a low level of infestation in terms of both plant injury and larval density per plant, and this monitoring not to be reliable indicator of beetle density. There was not a significant correlation between beetle density and the initial plant injury, assessed as number of damaged plants or “feeding holes”. There was a significant but very small effect of beetle density on number of damaged leaves and a significant effect on larval density per plant. Temperature had an effect on the preoviposition period, the total and daily egg-laying capacity and female longevity, development and hatching rate of eggs as well as larval survival at low temperature. The temperature of maximum egg-laying capacity was found to be 16˚C. At 16˚C, the mean preoviposition period was 19 days, the estimated total egg-laying capacity and daily egglaying rate 696 and 5 eggs, respectively and the estimated 50% survival time 186 days. The development time of eggs ranged from 12 days at 20˚C to 163 days at 4˚C and the hatching rate of eggs was roughly 70% at all temperatures except at 4˚C. At 4˚, approximately 50% of the eggs developed into larvae. The results gave a requirement of 185 degrees-days (DD) above a developmental threshold of 5.1˚C to complete egg development. Temperature, time of exposure, cold acclimation and larval stage had an effect on larval survival. The larvae were capable of surviving at -5˚C relatively long, whereas exposure to -10˚C was much more harmful. Cold acclimation increased cold tolerance of the larvae. Estimated time until 50% (LT50) of acclimated and non-acclimated larvae had died at -5˚C was 9.6 and 7.4 days, respectively. Estimated LT50 of acclimated and non-acclimated larvae at -10˚C was 70.5 and 32.6 hours, respectively. Second instar larvae were more cold resistant when exposed to -5˚C, whereas first and second instar larvae were equally resistant when exposed to -10˚C. Weather data from 1990 to 2013 at two locations in Denmark showed that a sufficient number of continuous days with temperatures causing a high larval mortality rarely occur. Based on the existing monitoring with yellow water traps, the time of field invasion can be estimated. With the new results and monitoring of temperature, it is possible to estimate the start and intensity of egg-laying, larval appearance in the field and larval mortality during winter. This can contribute to an improved basis of forecasting and decision support in pest management.