Stefan Buczacki solves your plot problems
Stefan says: I can understand your frustration because you say you’ve been a professional in the horticultural trade all your working life but have never before had such a problem on your own lawns. You say the lawns showed dry and bare patches and when you lifted the turf, you found the chafer grubs and then discovered the entire lawn was infested. You tried applying nematode controls twice in the summer but with little effect.
Chafer grubs are the larvae of insects called chafer beetles. Many people will be familiar with the biggest species, the cockchafer, which often crashes against windows on warm, sultry summer evenings. They’re big as larvae go, up to 4cm (1½in) long, and they feed on practically any underground plant parts, causing significant damage and very often plant death. They occur naturally in grassland and tend to be a problem most often in new gardens when houses have been built on old field sites. I’ve seen many outbreaks in gardens on new housing estates.
The female beetles lay eggs in the soil near to plants during the early summer and larvae hatch a few weeks later. They then feed underground until they’re ready to pupate, which is generally within a year of hatching for the smaller species but may take up to five years for the big cockchafer. Before pupating, the larvae construct little cells in the soil at depths of as much as 60cm (2ft).
You’re correct in that there are no approved chemical controls and a nematode suspension is the only option. You’ve tried it twice without success but my suggestion would be to give it one more go next summer, taking care that the soil is warm and moist. Do also be sure your lawn is well fed and watered to encourage new grass growth. If the area isn’t too large, try hiring a heavy roller in late spring to kill pupae and the emerging adults. If all else fails, you may have to follow your inclination and replace the lawn with hard landscaping.