Grub Control Timing in Northern Colorado: When to Treat, What to Use, and What to Skip

Short answer: preventive is mid-June to early August, curative is late July through mid-September

Grubs (the larvae of June and Japanese beetles) feed on cool-season bluegrass roots in two distinct windows. The preventive products (imidacloprid, thiamethoxam, clothianidin) need to be down before eggs hatch and grubs start chewing — roughly mid-June through early August in Northern Colorado. Curative products (carbaryl, trichlorfon) actually kill active grubs and work best when the larvae are small and near the surface, roughly late July through mid-September.

Missed both windows? You can still recover the lawn — grubs rarely kill bluegrass outright unless the population is heavy and the lawn is already stressed.

1) Know the NoCo calendar before you buy anything

Northern Colorado sits in a transition zone for grub pressure. Japanese beetles are established along the Front Range; masked chafer (a related June beetle) is the more common local species. Both lay eggs in mid-summer, the larvae hatch and feed through late summer, then burrow down for winter.

• Egg-laying peak: early to mid-July (adult beetles flying into the lawn at dusk).
• Hatch + small grub feeding: late July through August.
• Heavy feeding + visible damage: late August into September.
• Burrow down for winter: October. Curatives stop working once grubs are deep.

2) Preventive vs. curative — pick the right one for the timing

These two product categories do different jobs. Using the wrong one at the wrong time is the most common mistake we see.

Preventive products (the "put it down now" approach)

• Active ingredients: imidacloprid, thiamethoxam, clothianidin — often sold as a single season-long application.
• Window: mid-June through early August. Earlier is better than later in NoCo because egg hatch hits fast once beetles are out.
• What they do: Sit in the root zone and kill grubs as they hatch. They do nothing to a grub already feeding in late August — by then it's too late.
• Watering in: Apply to a dry lawn, then water with about half an inch of irrigation within 24 hours. Skip the watering and the product stays on the blade and breaks down in the sun.

Curative products (the "I waited too long" approach)

• Active ingredients: carbaryl, trichlorfon.
• Window: late July through mid-September, while grubs are still small and near the surface.
• What they do: Actually kill active larvae on contact. Short residual — usually 1-2 weeks of activity.
• Trade-off: They work, but you only get one shot. If grubs burrow down before you apply, you'll have to wait for next year.

3) How to tell if it's grubs — not just heat or watering

Plenty of August brown spots get blamed on grubs when the real problem is a sprinkler head out of alignment. Here's a quick field test that takes two minutes and tells you which one it is.

1. Pick a spot where brown meets green. Use a shovel or spade.
2. Cut a one-square-foot flap of sod about 2-4 inches deep.
3. Fold the sod back and count the C-shaped white grubs in the root zone.
4. Less than 5/sq ft: light pressure, often recoverable on its own with water and fertilizer.
5. 5-10/sq ft: real problem, plan to treat.
6. More than 10/sq ft: heavy pressure, treat and plan to repair sections in fall.

4) If you missed both windows — recover the lawn instead

Once grubs are deep (October onward), no product will help. Switch to recovery mode:

• Lightly rake and overseed damaged patches in early fall — see our overseeding guide for the right NoCo window.
• Water consistently into fall to push root regrowth.
• Mark the calendar for a preventive application next June or early July. One year of damage usually means it'll happen again.

5) What to skip

• Spring grub products. Putting down preventive in April or May in Colorado is too early — the active ingredient breaks down before eggs hatch.
• "Grub and feed" combos as a default. Cheap fertilizer + weak grub product often does neither job well. If you're going to treat, treat the grub problem on its own.
• Treating the whole neighborhood. Grub pressure is patchy. Spot-treat damaged areas plus a 5-10 ft buffer, not the whole lawn.

6) When to call a pro

If you've never checked for grubs and your lawn is suddenly half-dead in late August, you're guessing. The square-foot test, the right product choice for the timing, and a fall overseeding plan are three things that go wrong fast when you skip them.

• You want the right product applied in the right window, watered in correctly.
• Your lawn has had grub or skunk damage in past years and you'd like a preventive plan set up.
• Damage is already done and you need a fall overseed + recovery plan.

Serving Northern Colorado: Johnstown, Loveland, Berthoud, Greeley, Fort Collins, Windsor, Milliken, Timnath, Severance, Mead, Longmont, Evans, Firestone, Frederick, Eaton, Ault.