Grubs are insect larvae. The "source" will depend on your location which you don't indicate and the type of grubs you have. See link below for pics and info.

Planters are normally filled with a soil-less container mix, not soil/dirt. Do that and you won't have any grub problems.

Dave

Here is a link that might be useful: Grub Pests

Maybe I'm misunderstanding the question, but I would think that they came from Junebugs or Japanese Beetles laying their eggs in the soil. This summer, not sometime back before you bought the stuff. It's not really something you can prevent, insects flying in from nearby. There is a pretty major difference in the size of the two types of grub you mentioned, if you are intent on figuring out which they are. Actually, there are many other members of the scarab family that make white, C-shaped grubs, and not all of them eat the same stuff. If it was one of the two you mentioned, they tend to eat the roots of grasses, did your bed get weedy over the summer? Something made them think your bed would be a good place to lay eggs. I know nothing about making plants thrive in containers, so I can't help you more than that. Good luck!

Let me clarify some points:

1. The large container was built and filled with purchased soil in June of this year. It is not an older or established bed.

2. Using a local university website with drawings of hair patterns at distal end of grub and using a magnifying glass, these appear to be japanese beetle or june bug larvae.

3. This new bed was not at all grassy or weedy and located on concrete patio, far away from grass.

In looking at grub lifecycle, it does appear that egg laying takes place in july, which is indeed after the planter was planted. It was just interesting to me to find such a prolific amount in one area only. Looks like i need to treat for beetles and in late spring/ early summer even though I've never seen any in my yard.

June bug is a nickname.
If you have Japanese Beetle, then the grubs are feeding on roots, a big lawn problem.
But you may have the Green Fruit Beetle, this grub eats ONLY compost, Not roots of plants.
I have tons of these grubs every time I mix raw coffee waste in a raised bed. I plant right in the bed & get great crops, with no harm from the grubs.
The GFB grub is bigger then the JB grub, as is the GF beetle is bigger then the J beetle.
I find that for composting the raw coffee waste the Green Fruit Beetle Grub is better then the earthworm. The grub does well in the hot open beds then the earthworm.

I concur. GFB grub does no harm to my garden. However, it's raccoon delicacy so they come and dig up the garden.

To recap again, This was nowhere near grass. Yet I do not think green beetle or scarab based on images. Attached is an image (jap. beetle) which looks like a dead beetle I found amongst the grubs in the soil. I have tried to also attach pix of grub, but don't know if I can do only one at a time.

I am aware that japanese beetles are a huge lawn destroyer. Just curious how these got into my only bed container, when I have plants in ground, and in over 50 pots, etc, and NEVER saw these before.

Thanks Dave for the input on container mix :-)

Picture I found that looks like the grubs.

Are you within 2-3 miles of grass? The beetles fly, lay eggs where they think they can hatch and have a good food supply to turn into new beetles. Again, size of the grub is meaningful. Finding a shell of a dead beetle is not very meaningful when it comes to IDing the grub. That could have fallen into the soil from a spider web or blown in on the wind. IDing the grub would be nice so you know whether it is actually worth worrying over. Some grubs can be damaging, others are benign. But it is easier said than done.

Japanese beetle, green fruit beetle, junebugs, these are ALL scarab beetles, much like tiger, ocelots, and house cats are all cats. There are also many other scarab beetles, such as dung beetles, chafer beetles, and Rhinoceros beetles. These all have grubs that look like that, and some other families of beetles also have grubs that look like that.

If you are sure that your planter was not full of tasty grass roots, then I question the ID of Japanese Beetle, especially without knowing the size of the grub. A detritus-eating grub becomes more likely. However, if you are certain that it is a Japanese Beetle, milky spore is often used on lawns to control them and is considered organic. It does not, I believe, act on other beetle grubs. Personally, my method of dealing with grubs in the garden is to throw them out on the lawn where they can either do their thing or get pick up by foraging birds. I might even gather them in a jar, then place them in a dish for the birds or my chickens. I don't really worry about them, but move them just to be friendly with the birds and to err on the side of caution.

I'm attaching a link to a great website for learning about bugs. The particular page is one of a grub of a randomly selected beetle in the scarab family, just so you can see that they all look alike. Figuring out what attracted them would probably be better to help you ID them than looking at grub pictures. Cheers!

Here is a link that might be useful: BugGuide, Chafer Grub

Thank you sunnibel7! Going to ck out the site now. But I'm wondering if finding over 300 grubs in a 12cubic ft. container stocked with purchased soil, planted with veggies, is normal? Just trying to make sure a problem is not repeated.

I agree that is a lot of grubs, but I have seen things like this. First though, it is Always good to know your location, which should rule out specific insects. I doubt Japanese beetles because it is almost impossible to not know what they are if you have them. I suppose you could be at the edge of a coming infestation. By the way, the grubs are very big coming from a fairly small beetle.

Whether or not they might be Japanese beetles depends upon where you live. For instance, those particular critters haven't yet successfully colonized the states on the western coast.

We can't identify your beetles, or suggest an effective remedy, unless we see pictures of the critters you've found in your garden.

Sorry, but pictures you've found on the internet *aren't* at all helpful.

Thanks for the replies.

Jonfrum- that was my point to this entire post from the get-go. Why over 300 grubs of whatever in my 12cu ft container that was created and planted late last may?

I wanted to make sure the soil wasn't the problem.

To the rest... sorry, I did not photograph the grubs themselves. They are long gone for now.

I am just wanting to prevent a future problem. Yes, grass close by (well within 2 miles). I am in the eastern most point of the SF bay area, lots of heat in summer, but mostly cooler nights.

Planters are redone, and on wheels now to move around... we took the big one and cut it in half, and added sides (and new, safe bottoms of course). I took the advice from here to get container soil, not garden soil. I just put in several arugula and kale. And an artichoke for fun.

Hopefully will grow well!

My granddaughter was doing some gardening and was going to use some planter mix from my bag of Kellogs raised bed planter mix, organic plus. She showed me these larvae like worms. See the photo.
My search online indicated Cockchafers or May beetle larva. We picked enough to fill a quart sized bottle from that one bag. I spread the bags contents on a bare dirt part of the garden so we could pick them out and I then spread diatomaceous earth over it all. I hope that kills them. Is there a natural pest control that I can make using vinegar and ??

Here is a link that might be useful: white grubs - Phyllophaga and other species

To answers your question: The source are other insects, their parents. If you bought your soil it is highly unlikely your soil was infested with insects.

If you dont want any critters in your soil, beneficial or not. There are a couple of things you can do.
1. Put your bed over lawn wire. This also prevents moles and gophers from getting in there.
2. Put a bird bath near your garden and a small bird feeder. But be prepared the birds may choose to eat your vieggies and there are several ways to prevent that.
3. Get diatomaceous earth and follow the directions.

Less than 10% of the bugs you see in your garden are destructive. Identification is crucial. If they just gross you out and prevent you from enjoying your garden, which is the main point to all your work then get rid of them. Just keep in mind no insects at all = more work and less production.

Beneficial nematodes were really helpful in my garden and milky spore is another option for grub control. Asian beetles are a huge problem here, the University is studying it right now but unfortunately, the traps they are working on have an attractant and they're putting them all over the neighborhoods so more are attracted to the area and find they like gardens better than the traps. What they're after is nice fluffy moist garden soil to lay their eggs in.

The grubs themselves aren't usually a problem but when they reach the numbers that we see in this area (1 to 2 DOZEN per shovel full at times,) they ARE a problem because they kill plants by disrupting the roots. I let them stay in my compost where they are great workers but screen them out before adding it to my beds.

It's the Kelloggs dirt. I used the same in my four raised bed planters (the fabric bag type) and now there are hundreds of grubs in each bed. We have 0 beetles in the yard, have never seen a single one in the entire garden or other beds where this brand was not used. My other organic beds with homemade compost are filled with worms, a good healthy soil. I am furious! Who would sell infected dirt as organic to people? Waste of $120 and a perfect growing season.

I purchased the Sunshine 3 cube potting soil and 3 weeks after planting my plants began dying. I decided to change the soil out and found Japanese white grubs. This was in a brand new garden tower vertical planter that is on a platform in a cemented area away from grass. Has anyone found that they can be in the bagged soil? The soil was well compacted and dry so I am wondering if it is even possible for eggs to live in it and hatch when the soil becomes wet.

goodnessbeadsupply I had exactly

the same problem!! Gorgeous raised beds planted 1 year ago. Produced all sorts of veg and herb prolifically thru 2 summer and spring. Cleaned out 2 beds for my Fall planting, spread 1 bag Kellogg 3cf Raised Bed & Potting Mix into 1 bed, spread it out and fertilized. Came out 1 week later to finish: spread the other 3cf bag of Kellogg's into the bare bed, and proceeded to till and plant the other bed... 100's of grubs. 100's!!!!! I had to remove all soil from both beds, and now have to begin again.

I'm so angry at having wasted all of that time... although I guess digging is never wasted time if you love to garden...

June bug is a nickname.
If you have Japanese Beetle, then the grubs are feeding on roots, a big lawn problem.
But you may have the Green Fruit Beetle, this grub eats ONLY compost, Not roots of plants.
I have tons of these grubs every time I mix raw coffee waste in a raised bed. I plant right in the bed & get great crops, with no harm from the grubs.
The GFB grub is bigger then the JB grub, as is the GF beetle is bigger then the J beetle.
I find that for composting the raw coffee waste the Green Fruit Beetle Grub is better then the earthworm. The grub does well in the hot open beds then the earthworm.

he GF beetle is bigger then the J beetle. I find that for composting the raw coffee waste the Green Fruit Beetle Grub is better then the earthworm. The grub does well in the hot open beds then the earthwor

Insects sometimes may attack plants in containers over plants in the ground if the potting mix isn't as rich in minerals and nutrients (e.g. calcium, silica, potassium, and minerals generally). Shaded plants may also get attacked more by certain pests (whether or not they're in the ground). If the pH is off, that could also cause deficiencies. A stronger plant is less of a target.

organic 101

Anyone have good information on what grubs are good for the soil. I live in the desert and told, "we don't have grubs." Not true, but I have to wonder if the grubs I find are ok. I do compost and use on trees. The trees seem to be doing fine but i have no idea how long it will take if these grubs are killing the trees. The grub pictured above with ccl1968 are what I find.

"Phyllophaga, of which there are over 100 different species"

It could be anyone of the 100 species or more than one.

Here is what I would do, also if your state has no regs against importing red worms for composting, then I would get them for my compost.

https://www.ecolandscaping.org/05/landscape-challenges/pest-management/controlling-grubs-milky-spore-disease-or-beneficial-nematodes/#:~:text=The%20nematodes%20will%20give%20more,milky%20spore%20inoculation%