I big thanks to Powell Gammill from Phoenix, Arizona, for sending these pictures along with his comments on grape leaf skeletonizers.
Attached are two pics I took last year that capture a female laying eggs
(note precise characteristic pattern)
 |
| Grape leaf skeletonizer laying eggs on the grape leaf surface. Picture courtesy Powell Gammill. |
and a pic of newly hatched 1st instar
larvae (and one 2nd stage) in uncharacteristic (non-line) feeding pattern
(which I hope may indicate a viral infection is established around my vines)
and a couple of organized egg clusters. You can reprint them if you wish.
 |
| Grape leaf skeletonizer eggs laid (right bottom) with larvae hatching from eggs at center, bottom. Photo courtesy Powell Gammill. |
In addition to the Btk and spinosad, you can use dormant oil before leafing and
horticultural oil to try and kill the molting larvae under the vines (but this
would kill any emerging parasitic wasps as well). By rotating treatments
you can hopefully prevent resistance. I have found a easy method of
reduction is to look under the leaves and remove any infested leaves before
they hatch or get large enough to move on. They strip a leaf at a time
and are not too hard to spot if they are still young and on a leaf. Also
the adults can’t fly very well and look inelegant in flight.
 |
| Grape leaf skeletonizer adults. Photo courtesy Powell Gammill. |
As you said, if you pick the 3rd and 4th instars off manually do so with gloves
and remove them as if you just drop them unharmed they may either find their
way back to a vine or go to ground and cocoon.
I think these were drying out getting ready to start flying. Note, ours
(Phoenix, AZ) have an orange head. They too are reportedly irritating
(cyanide?) to the skin…I know the larvae have irritating bristles.