Just in case you didn't already know, the beetles emerging in their hundreds from your lawn are Garden Chafers (Phyllopertha horticola). The larvae of these feed on the roots of grasses and various cereals, while adults feed on leaves of various shrubs and trees. You can treat the larvae with nematodes https://www.harrodhorticultural.com/nemasys-chafer-grub-killer-pid7815.html ) which is very effective if you follow the instructions. Alternatively / also you can trap the adults in a pheromone trap https://www.harrodhorticultural.com/garden-chafer-trap-pid8235.html#product-tabs ) , however i think this second option maybe too late now as the majority of adults would have emerged, mated and laid eggs by now.
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