Fall Readiness Part II - Japanese Beetle Edition

The weather will be cooling off soon, let’s hope we catch up this fall on much needed precipitation. What about the bugs—specifically the Japanese beetle that rampaged ornamental plants in several Front Range neighborhoods and pained the souls of thoughtful gardeners from July to September?

Online Photo from bioadvanced.com

Thankfully adult Japanese beetles die off each fall after their one-year life cycle from egg, larva, pupa, to adult. Gone, but not forgotten—they leave next year’s army of plant eating progeny all over the neighborhoods they visited. Right now, their offspring (white grubs) are deposited in fields, pastures and lawns in home landscapes, parks, schools, golf courses, even cemeteries. 

Each female Japanese beetle lays between forty to sixty eggs in her short summer life of eight weeks or so. When totaling the hundreds to thousands of Japanese beetles in yards, that's quite a few eggs!

Females prefer laying eggs in lawns or fields near her feeding source. Cool season lawn grasses including Kentucky bluegrass, perennial ryegrass and tall fescue, which are most common in Colorado, are preferred hosts for egg laying. 

Some warm season grasses like Bermuda grass and buffalograss are less favored for egg laying. 

Once eggs become larvae, also called white grubs, they become grass and plant root eating machines for sustenance—well into October.

If white grubs are numerous, lawn damage generally shows up in August to October in the form of patchy dead areas. If severe, the dead grass can easily be rolled back like a carpet to expose eaten turf roots. High numbers can also attract foraging animals like racoons, squirrels or birds that happily dig up lawn areas in search of a white grub meal.   

White grubs in lawns are mostly inactive from late fall until the soil warms up in spring when they resume feeding. They survive all types of winter conditions and shouldn’t be expected to be killed off by cold or snowy weather. White grubs will move deeper when it gets colder or drier, but generally reside four – six inches below the soil. They transform to the pupal stage in spring (usually in May) and emerge as adult beetles from lawns sometime in late June or July to begin another season of plant eating carnage. 

What Now ...

Recommended controls to kill white grubs in lawns are all about timing. Products generally require one summer treatment and are most effective when applied from May through August. Treating lawns isn't one and done forever, lawns needed to be treated every summer.

There are a couple of products to consider applying right now while lawns are still warm, but don't wait much longer. If you miss the windows to apply lawn products, your best bet may be to wait until next summer.

Two effective biological (organic) controls for lawns include beneficial nematodes (microscopic worms) that reproduce inside the Japanese beetle white grub causing them to die within a few days and a bacterium stomach poison called grubGONE! that kills larvae. Choose one or the other.  

Beneficial nematodes need to be mixed and then sprayed on the lawn using a hose end or pump sprayer. Easy to follow mixing and spraying instructions are on the package. Your local independent garden retailer (not box stores or mass merchants) carry beneficial nematodes (refrigerated). They must be the heterorhabditis bacteriophora strain, which will be well labeled on the package. 

GrubGONE! is another organic product that is very effective in killing white grubs in lawns and is very safe to use around people, pets and pollinators. Just add the granular product (based on your lawn square footage) to your fertilizer spreader and water it in. Also purchase grubGONE! at local independent garden centers or online.  

Check out this very informative fact sheet on Japanese beetles in Colorado from Dr. Cranshaw at Colorado State University at http://extension.colostate.edu/docs/pubs/insect/05601.pdf. He includes detailed information about control products, trade names, timing and helpful comments. 

Also check out my Japanese beetle website for additional information, look for my 2020 classes on Japanese beetle management which will be posted later this fall.