You'll get up to about a 50% kill at this point (it's late September, so I'm using the October number at this point since Trichlorfon's base kill is only about 20% anyway). See below...

When you were working there, did you see grubs? If not, dig in surrounding areas (the grubs would have moved on when the grass died) to check for them. At this point, in late September, grubs will be small (first instar) if they're Japanese beetle.

If you still see them, you might want to use something like Sevin to kill them in the areas where they're still active, plus a good margin around that.

Unfortunately, Sevin's much wider spectrum, but if there are really enough grubs to actually kill turf, then that level of eradication may very well be required here across a large percentage of lawn.

Thank You morph,

Should I apply Grubex in early May of 2021?

If you dug and saw grubs, then absolutely. You'll get a fair percentage of this year's grubs, but you'll definitely want to make sure that next year's are less of a problem. The GrubEx will be more effective against the grubs when they hatch (and less damaging to everything else as it's a weaker insecticide).

I WAS DIGGING HOLES FOR NEW PLANTS AND I NOTICED WHITE WORMS/LARVAE IN THE DIRT; I'M CONSIDERING "BIO ADVANCED GRUB KILLER PLUS" AND I WOULD LIKE TO KNOW HOW LONG DO I WAIT BEFORE PLANTING THE NEW PLANTS AND WILL THIS TREATMENT HARM THE NEW PLANTS ??? THANK YOU. MRS. TEDDER E-MAIL: lmiraman@gmail.com (small 'L' )

Mrs. Tedder, if you saw a few grubs, then you don't have a problem. There are normally a small countable number of grubs in the soil this time of year. If you found an uncountable mass of grubs, then you have a problem. Fewer than a dozen per square foot is fine.

Generally I am against using an insecticide unless you have a serious issue. My reason is that so much of the biology in the soil is made up of microscopic insects. Killing them all with an insecticide, even temporarily, will injure the general health of your soil.