I have just found this organic solution - much safer and kinder to the environment, birds, etc than chemicals. After heavy rain or having thoroughly watered any yellow patches in the lawn (or areas around the plants), cover the earth with black plastic sacks and leave overnight. The leatherjackets will come to the surface and can be collected in the morning. Encourage natural predators, such as spiders and garden birds, especially starlings. Sounds a bit fiddly but worthwhile in the long run. Good luck !