The Western Grapeleaf Skeletonizer is a moth not a beetle. As such spreading BT at the base will do nothing. The female moth lands on the leaves and deposits her eggs in tight recognizable clusters underneath the leaf. In Phoenix, AZ they start to appear in May. I assume later in your area.

Control is via hand picking (I find this to be quite easy to do). Go out and inspect. Find a leaf with eggs, or newly hatched larvae (caterpillars) on the move remove the leaf to a bucket for disposal. Unless things get out of hand before you notice them, hand picking is VERY effective.

Use gloved hands if caterpillars are present to pick infested leaves as they have fine spines that will cause serious skin irritation.

BTK will also work quite well sprayed directly onto the caterpillars per label directions. Just one squirt. They eat the leaves that has the toxin and quickly stop eating and become fatally paralyzed within hours.

Spinosad also works applied per label directions to quickly paralyze these caterpillars within hours of being applied and they dry up and die.

Few predators: There are some viruses that will kill these pests and parasitic wasps. When I notice a army of these things either paralyzed (that I didn't do = parasitized by something) or where they have partly liquified (virus infection) I pick the leaf and drop it carefully nearby to the ground hoping to amplify the beneficial. But otherwise their spines offer pretty good protection; birds won't touch them, nor other predators.

Here is a link that might be useful: Skeletonizers

This post was edited by Fascist_Nation on Fri, Mar 14, 14 at 19:54

Here are characteristic egg clusters and 1st instar larvae (note the one big fat one, 2nd instar?). Interestingly these are not organized into their typical line of eating mouths. Either they just hatched (note the empty versus full eggs) or they are infected with something.

The western grape skeletonizer is certainly a moth, but the original poster implies adult beetles were seen, no mention of grubs or larvae, perhaps Japanese beetles are at work here.

Would be an excellent idea for OP to post an image of the critters, whatever they may be.
And another of the damage.
Please!

If BT or spinosad is used, does it pose a risk for eating the grapes that either are forming or will form later?.

Neither BT or spinosad are considered effective against adult Japanese beetles.

For other uses, the labels will specify days to harvest after application.