When to Apply Grub Control: Preventive and Curative Dates Where You Live

Apply a preventive grub control while Japanese beetles and related beetles are laying eggs, usually mid-June to mid-July, so it is already in the soil when the eggs hatch. Chlorantraniliprole, the active ingredient in today’s GrubEx, can go down weeks earlier. Use a curative product such as trichlorfon in August or early September, and only when you find enough young grubs. Spring treatment rarely pays.

Egg-laying follows the weather. By Iowa State Extension’s degree-day rule and NOAA’s 1991–2020 climate normals, Japanese beetles normally start flying around Jun 8 in Washington, Jun 26 in Chicago and Jul 7 in Boston. Dates for 29 cities are below, checked against seven extension services.

Preventive or curative: two different calendars

Preventive products last for weeks in the soil and work best on newly hatched grubs. Curative products act fast but briefly, for grubs you have already found.^4,6

Biological products such as nematodes and milky spore give limited, highly variable control, UW–Madison Extension adds, and milky spore has not proved effective in recent university research.⁴

When Japanese beetles fly in your city

Iowa State Extension entomologists say Japanese beetle adults begin emerging at about 1,030 growing degree days (base 50°F) counted from January 1, and keep emerging until about 2,150.² We applied that rule to NOAA’s normals at the weather station nearest each city.¹ The preventive target is day 8 to day 17 after flight starts, the stretch inside all six published preventive windows we could tie to a place.

These are normal-year dates: in 2020 Iowa State reported emergence 7 to 10 days early.³ The rule is for Japanese beetle, which NC State says lives in most of the eastern US and parts of the West.¹⁹

Does the degree-day rule match extension advice?

Every computed start falls inside the published range or within about a week of it; New England runs a little late.^{3,8,6,11,5,9,10,13} The six preventive windows open from 35 days before the computed start (UW–Madison) to 8 days after it (Purdue’s E-271), and all overlap from day 8 to day 17.

Early-to-mid-July is almost always optimum timing for preventive applications.

GrubEx (chlorantraniliprole) goes down earlier

Chlorantraniliprole moves slowly: UMass says it takes 60 to 90 days to fully dissipate in soil and advises mid-April to early June, while UNH says late May to late June.^9,10 That is 4 to 12 weeks before our computed flight start for Boston and 1 to 7 weeks before it for Concord, New Hampshire. Kentucky’s 2012 insecticide table allowed May 15 to late July for the professional version, Acelepryn.⁷

Read the active ingredient, not the brand. Kentucky’s 2012 insecticide table and Iowa State’s 2021 answer list GrubEx as imidacloprid,^7,16 but Scotts’ current US product page and UW–Madison list chlorantraniliprole,^17,4 and the two need different dates.

Curative treatment: late summer, small grubs

Kentucky’s best curative window is early to mid-August, while grubs are small; by late September some move down out of reach.⁶ Penn State gives August through mid-September, and UW–Madison August to October, best early.^8,4 Once grubs are full size, often by mid-September in New England, UMass says curatives seldom cut their numbers by more than half.⁹

Spring generally is not a good time for curative grub control.

Overwintered grubs are large and hard to kill, and a spring curative leaves nothing for summer’s eggs.⁶ UNH advises no curative after May 15, when grubs stop feeding.¹⁰ To repair dead patches, reseed in the late-summer overseeding window.

Southern lawns and other grub species

In Texas the main lawn grubs are May/June beetles and the southern masked chafer; Texas A&M says to treat in June in South Texas and early July in North Texas, around the time the grubs hatch.¹⁴ NC State times curatives for August through October, later in the mountains than in the east.¹² Species differ too:⁴

European chafer runs about two weeks ahead of the other species, UMass notes.⁹ Purdue says it can also damage lawns in spring, and that May/June beetles take two to three years per generation.⁵

Check for grubs before you treat

Only a small share of lawns need treatment in a given year, Kentucky notes.⁶ Cut out a square foot of sod at the edge of a brown patch in July or August and count. Purdue says fewer than 5 grubs rarely need treatment and Japanese beetle usually takes 10 or more; Penn State uses 5 to 10, Illinois about 12.^5,8,11

Kentucky also notes that:

• Heavy watering in June and July attracts egg-laying beetles (see our lawn watering guide).⁶
• Moderate nitrogen from October through December helps turf resist grub injury (when to fertilize).⁶
• Tall fescue tolerates grubs better than Kentucky bluegrass (which grass do you have?).⁶
• Killing grubs will not stop moles, which also eat earthworms.⁶

Skip beetle traps: Purdue says catches don’t predict grubs, and NC State says traps draw beetles in.^5,12 And a crabgrass pre-emergent is a weed product, not a grub preventive.

Water it in, and protect bees

• Water it in. UW–Madison says about ½ inch right after applying, or control is poor; Kentucky suggests ½ to 1 inch.^4,6
• Clear the thatch. Kentucky suggests mowing and raking first, and dethatching if thatch is over ½ inch.⁶
• Keep it off flowers. Purdue advises not treating where dandelions or clover bloom, mowing just before spraying, and leaving a 2–3 foot untreated strip by flower beds with neonicotinoids.¹⁵
• Weigh the product. UMass reports chlorantraniliprole has no activity against bees, while carbaryl is very toxic to them.⁹

neonicotinoid insecticides used for grub control can pose risks to bees and other pollinators

Related guideWhen to Apply Pre-Emergent: Soil Temperature and Dates Where You Live

How we worked out these dates, and their limits

We took NOAA NCEI’s 1991–2020 monthly normal highs and lows at the station nearest each city, interpolated them to days, and summed degree days above 50°F from January 1, capping highs at 86°F (a common method; Iowa State’s notes don’t name one).¹ A simple daily-mean method would shift dates a median 9 days later. A hot spring brings beetles early.³ Details are on our methodology page.

Next step: put your city’s preventive target on the calendar, and check the sod in August before buying a curative. More in our weeds and grubs section.