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Vine weevil goes through a life cycle, where generally there are grubs present in the spring and late summer. Although the grubs can be found in warmer conditions for longer periods. I would expect grubs to be present from late August for the second generation.
To be safe I would apply Vine weevil killer nematodes in September, this will also ensure that the soil or compost is warm enough for the nematodes to be active. A successful application will kill about 70 percent. It is not possible to kill all of them.
The Vine weevil either goes through the winter as a grub deep in the soil or as an adult sometimes. There are no controls available for the adults. Soil temperatures are warm enough for nematode treatments again from about April to May.
The grubs will form new adults in June to July. I would focus on treating pots and containers, where the nematodes are very effective and any beds where you think there are high numbers of grubs. I would not treat the whole garden but focus on those areas and treat in late summer and the spring. I hope this helps.