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.

A grub is simply an insect in it's larval form. Not all grubs are fond of lawns, but the grubs we don't like are fond of eating roots. Are you seeing any plant damage? Lots of times a population of grubs isn't something to be concerned about as not every population is going to harm plants or turn into something bad. However, if damage occurs or you just want to proactively get rid of them the Sevin should work. Read the instructions though as the effectiveness of most grub control products is dependant upon timing of the application. A lot of the products have to be applied before the adult lays the eggs. A BT strain which paratisizes the grubs is a good choice as well, but often the soil has to be warm (70F) at application time. So, you will have to read up on your options and choose accordingly. Your county extension service probably can suggest what will work well for you as many products that have been shown very effective in one area did poorly in a another. Soil chemistry varies widely from one place to the next which is a likely reason for the differing performance so a call to the extension office or a chat with a master gardener in your area is probably your best source of info for this.

You'll get up to about a 50% kill at this point (it's late September, so I'm using the October number at this point since Trichlorfon's base kill is only about 20% anyway). See below... When you were working there, did you see grubs? If not, dig in surrounding areas (the grubs would have moved on when the grass died) to check for them. At this point, in late September, grubs will be small (first instar) if they're Japanese beetle. If you still see them, you might want to use something like Sevin to kill them in the areas where they're still active, plus a good margin around that. Unfortunately, Sevin's much wider spectrum, but if there are really enough grubs to actually kill turf, then that level of eradication may very well be required here across a large percentage of lawn.

one per hole.. or lets say one square foot.. is probably not considered a significant population ... second.. they are more of a lawn problem .. did you remove lawn for your new patch ... ONLY WHEN I WAS A LAWN WARRIOR.. DID I BATTLE THEM... oops ... i never found them to be an issue in the general garden.. yucky as they may be ,.. one per hole.. or lets say one square foot.. is probaly not considered a significant population ... and keep in mind.. they turn into something else soon enough .. and you need to figure out if that end thingee is the real problem ... i suppose the major pest related to such is this: https://duckduckgo.com/?t=ffnt&q=japanee+beetle+grubs&iax=images&ia=images and if you have a lot of those.. then you should probably be treating your lawn .. rather than your berry patch ... and i have no idea how to ID them ... any more specifically ... my bottom line.. pick your battles.. but there is no reason to kill everything.. just because they look disgusting.. lol ... it can become very costly .. and cause you to use too much stuff ma nature might not need thrown on her ... lets see if i am in the majority or minority on such ... ken