They're sawfly larvae. They will be specific to their host plant, not general feeders. And yes, you have probably already nearly eliminated them. I like Ken's idea of having some pyrethroid insecticide mixed and ready to go. Pyrethroids are relatively safe for the applicator and the surrounding environment when used in this way. Above all, overall nuclear options are not called for in such a case as this. +oM ps......do not, I repeat, do not get any spray drift from any pyrethroid insecticide in your eyes! It will hurt.

I don't use any oils or pesticides for insect control as I grow organically. google Arbico. they specialize in beneficial insects. I ordered three praying manti egg cases. one for my yard and for my neighbors on both sides. one egg case hatches abou two hundred manti. They devour every insect you hate. and once they live out their life cycle they lay their own egg cases. so now I have an ongoing population of manti. I live an hour north of death Valley so its a desrt hot blazing summers freezing winters if my manti can stay alive year after year your area will support them as well. also consider Ladybugs these are literalyy the toughest bug in the pack they eat every single pest bug you can imagine problem is when they eat everything they move on to find more food. I even take my baby manti indoors to work over my bonsai and houseplants I leave a couple of windows open all the time for my cats to get in and out. so manti have an escape, or entrance if they choose. I grow vegetables, tobacco, fruit, housplants and bonsai. this was a permanent and inexpensive solution to my bug problem. Good Luck!!!!

Last spring , I had a huge crop of it everywhere and I tore it out all spring and hauled it off to the compost heap. When it flowered I pulled the seed heads and bagged them and composted the rest. Irritating plant. Bugs,beetles, flies, bees and butterflies seem to like it. They seem to die in the summer heat and dryness here.

on my sand ... it would make no difference .... you say your drainage is very good .. so i dont see a problem .. in fact.. you may have gotten water down into the whole root mass planted ... which would be perfect .... if you want to know the status of moisture in the soil ... INSERT FINGER ... or dig a 3 to 4 inch hole.. AND FIND OUT ... and then perhaps you can rest easy ... with 5 acres .. and raising lids.. this is one reason i used to carry a timer .... btw .. i would NOT water the top parts of conifers .. so i would also suggest a hose on trickle ... lawn sprinklers are for lawn ... not tree leaves ... i would also suspect it may not need water for a month or 3 .... depending on what you find.. when you dig a couple holes and find out ... keep in mind its fall.. and when temps fall significantly .. especially night temps .. water stays in the soil much longer than in the heat of summer .... ken