During the life cycle of some of these insects they come quite close to the surface at some stage; and at other times they migrate deeper down. They feed on organic matter which includes roots. What method you use for control depends upon how deep they are. If they are close to the surface either chemical or biological can be effective. If they are deeper, only certain (and usually expensive) chemicals will be effective. I have no experience in the use of biological agents for grub control.

How many grubs per square foot did you have? 5 or less will not be a problem, while 10 or more could be. Grubs, the larva of the scarab beetle, a large family of beetles that includes the rose chafer, June Beetles, Cicadas.
It would be highly unlikely that the grubs caused your birch to die, a large number of things around here cause some cultivars of birches to die and this is the native habitat of them. The Paperwhites around here grow and die sometimes lasting many years sometimes hardly growing before dieing. They get infested with the Bronze Birch Borer and a couple of other insects that kills them, but birches oeverall are not very long lived trees and a soil that is too dry, or even too wet, one year is enough to kill them.

As kimmsr mentioned, there are many kinds of scarab larvae and they don't all eat the same thing. Most are soil dwelling and most feed on plant roots. I have seen a large species we have here feeding on young aspen roots and killing nursery trees. Also have seen it in Christmas tree plantations feeding on conifers. I would be surprised if they could kill a mature tree.

Without knowing which species you have and what the host range might include, I would be wary of planting a young tree in a spot with a large number of them without treating them first.

The beneficial nematodes sound like a good option since you already have a hole dug down to where they are. Spring is a good time to put them out as grubs are active now.

Thanks kids or the input. The tree that died was planted in 02 and has always been a runt, it never flourished as the other two have. These are River Birch, aka European Birch aka Heat Birch. We were digging down of course and with every shovel full of dirt, there were probably 2 - 3 grubs in a hole dug for a 5 gal tree. We probably pitched 10 - 12 grubs. They were deeper in the soil most likely because we had a very cold winter, unusual for this area for the last several years. We see the June bugs, iridescent green guys but I can't say we see tons of them and frankly, I can't say that we see lots of beetles even. At the moment we are seeing these big moths with the thick 2 - 3inch long bodies and double wings w/a spread of 4 inches.

The other two birch are having trouble also, they are not as lush, and have branches with no leafing. The leaves that remain are cupped under just a bit. I do not see any obvious pest except the ants that love to live under the trunks of these trees. These birch are supposed to be the most trouble free including attack by the bronze birch borer. I have examined the trunks well and I can't see evidence of borer damage. I could have overwatered, though I was attempting to put down the proper amount of water once a week. We had that horrendous heat spell last summer that killed so many plants and then the surprise freezes that wiped out even more plants.

I have continued to spray the area with Outside Pride Humic - Mychozzoral solution hoping this would strengthen the trees. Any suggestions regarding this?

The grubs I can take care of, I just didn't realize that they would be found in soil w/o lawn. And I do quite a bit of digging around and this is the first time I have come across them in a planter bed. I just dug up strawberry plants in the same area about 7 ft away and didn't come across one grub.

I swear this area is so strange to garden in. Yes this area has been populated for only 18 yrs, it was open Mojave Desert prior. It seems as if there is a constant battle with the soil and bugs within and out. Perhaps this is the result of taking desert land and attempting to civilize it with plants that can live here but are not native. I'm about ready to let this $22,000.00 yard return to its original state. I am open to any and all ideas, advice, etc.

Botany Bob, are you by chance around Couer d' Lene? My mother grew up on a homestead in what was called Blue Creek very near CDL. Many stories of the wolves that would scratch at the door, also the night my grandmother beat a fox to death in the hen house, Tubs Hill where crosses were burned against the Catholics and my mom's school called Wolf Lodge. They were real pioneers, 7 children, mother and father away doing lumberjacking, immigrants from Finland around 1896.

I just did a search on grubs and found this thread. It sounds like you have exactly the same problem I do. We built raised planters last year and filled them with a well-rototilled combination of the sandy native soil (which had been under concrete), lots of compost and some manure. I had filled it with a bunch of salvias, some bearded iris, and some other plants to see what would grow there (the sun situation is patchy). I also grew green beans in one patch. This year I decided to replant, and when I dug deep to amend with more compost (it had settled), I found dozens of these grubs. It was 2-3 grubs per shovelfull. I probably tossed a hundred. They were exactly as you describe. I have never had a grub problem before. Normally I don't use insecticides, but I'm wondering if I should here. Can you tell me more about the beneficial nematodes, where I can get them and how they work? Thank you.

Jillalameda:

I just came across your post. Beneficial nematodes can be ordered through the mail from garden supply stores or if you are in a well populated area, a big nursery might also carry them. They are considered expensive but the good thing is that they continue to multiply in the soil. For the whole story on them, do a search on this site for them. There is a lot of discussion about them in the Organic Gardening and Organic Lawn forums also. If you have the Sunset Garden book, you should find info explaining them.