Many thanks for your recent enquiry regarding the direct, and associated, problems you are experiencing with chafer grubs. We would not recommend applying the Chafer Grub Killer until August, as the timing of this application coincides with the active stage of young larvae, which are easily killed by the nematodes. This also causes major disruption to the life-cycle of the chafer grub as fewer larvae are around to overwinter and develop into adults in the following years.
Unfortunately the application window is rather small, stretching from August to the end of September – after which the grubs head down to deeper soil and comparative safety.
There are however some cultural methods which may well help you in the meantime; encouraging rooks and starlings into the area will help, as these birds feed hungrily on grubs, and thoroughly soaking the grass before covering with sacking, tarpaulin or plastic sheeting overnight will bring the grubs to the surface – they can then be collected and disposed of.
Practicing good husbandry in the smaller garden will pay dividends; thorough cultivation and good weed control of flower borders and vegetable plots should unearth grubs which can be removed by hand or left for predators.