Thank you for the question. Timing is critical to control grub infestations. According to the University of Minnesota Extension, in general, the best time to apply the various insecticides for grubs is from mid-July until early September. The mature grubs start to migrate deep into the soil when the soil temperatures drop to 60 degrees and less. Grubex has very long persistence but moves relatively slowly into soil. According to University of Colorado Extension, the product is Best applied in May/June; some control possible with applications made in April or early August. Fairly slow (weeks) in providing control after application. The slow action is why it's too late to apply it now. The grubs will be long gone by the time the Grubex moves deep enough into the soil. You may find the entire publication useful: http://extension.colostate.edu/topic-areas/insects/japanese-beetle-5-601/
It is thought that efforts in controlling Japanese beetle grubs may not provide desired beetle control next year. This is because the beetles are strong flyers, arriving from miles around to eat, mate, lay eggs, and start the cycle over again.
If you decide to use Grubex next year, remember to read the product package and follow directions exactly. The label is the law.
Thank you for contacting Extension.