This caterpillar is a Western Grape Leaf Skeletonizer, Harrisina brillians.

From what I can tell

Sounds deadly. It is skeletonizing my grape leaves but so cute. What color butterfly will it be?

Blackish blueish

Not as pretty as i thought it would be. I sprayed the grape vine with BT. There are many of these on it.

Very cool. I learned something new from this thread, so thanks for posting the questions and answers. I don't grow grapes, but if I do, I'll be able identify these skeletonizers now, thanks. I agree, LOL, very menacing name, hah! Happy gardening!

Don't touch them barehanded. They have numerous spines that impart a trace amount of cyanide that will irritated your skin (hence their non-camouflaged, 'better not eat me' markings). As a result they have few natural enemies.

The adult forms around here look like the image provided without the distinct orange mark (which is a different species "H. americana" found in much of the US). Both species adult forms are poor fliers who flit around. That also limits how far they spread. I often find them resting or mating on non-grape leaves.

The photo you took is of the 4th or 5th instar larvae which means they have been munching for some time and I suspect this may be the second wave (there usually are five waves in the season in Maricopa Co.). Once the grapes have been harvested you can let them consume and poop out the leaves if you want---there are some claims infested ESTABLISHED vines produce better for this reason, and they don't seem to bother the grapes even when the leaves run out contrary to some published claims. It is your choice.

I would vigorously protect first year grape vines however as they need ALL the leaves they have to establish their deep and extensive root system---I actually remove just a part of egg infested leaf if it is not covering the main vein with scissors to preserve the remainder of the leaf.

I choose to hand pick the leaves to control them (easy to spot after the first generation which are laid close to the ground and often in the interior. 2nd gen onward are usually exposed and obvious. But once the army is on the march BTK and spinosad are really your next resort. Either paralyses feeding within hours and leaves them to dessicate on the vine. Any I find obviously paralyzed without my intervention or lysed I leave alone or carefully drop the leaf if already picked down into the understory as this is evidence of a parasitic beneficial or granulosing virus infestation respectively that I want to encourage.

This post was edited by Fascist_Nation on Sun, May 18, 14 at 14:42