Parts of my lawn is torn up from animals searching for grubs. I sprayed nemotodes last October, but clearly grubs are still present. Do I re-spray now, in spring, or wait until the summer? Do I re-seed now, as my lawn is unsightly? Not sure what to do first.
How unfortunate that you have grubs in your lawn. However, it is a common issue at this time of year. There are two periods when grubs can be targeted by nematodes, in the spring when soil temperatures are above 10°C (often early May to early June) and again in the fall before soil gets too cold (mid-late September to mid October). Your best option is applying during both the spring and the fall to get maximum coverage and protection!
Nematodes search through the soil, locating a host and then enter through natural body openings. A natural bacterium is released that is fatal only to the insect, which dies within 24 to 48 hours. The nematodes reproduce and continue to eliminate grubs as long as soil moisture conditions allow (6 to 8 weeks).
Application should be done when soil temperatures are above 10°C, preferably above 15°C. The lawn must be kept moist for two weeks after application. The solution must be able to penetrate to the root zone area, thus a properly dethatched lawn is advisable. Nematodes will infect grubs within 2 days of application and kill them within 14 days. They will not kill the entire grub population in your lawn, but will reduce it drastically.
Since the grubs are at a stage when the use of beneficial nematodes is effective (early May ), you could overseed at the same time. Both the treatment and grass seed require watering. If the nematodes are effective, the grubs should be dead before the new grass roots form.
Below you will find 5 links from some of our earlier posts with helpful information that should help you understand how the problem occurs, how to deal with it using natural, organic methods and some tips as to how to prevent it from recurring.
https://www.torontomastergardeners.ca/index.php/askagardener/grubs-in-lawn/
https://www.torontomastergardeners.ca/index.php/askagardener/lawn-problems/
The application of nematodes along with having a thick healthy lawn will deter grubs.