A gardener wrote in recently:

"Hi, I am trying to learn how to Organic Garden.  My 25 x 5 raised garden bed has a major grub worm infestation.  Every foot I dig, there are 10 or more grub worms.  They destroy any seed I plant.  I applied milky spore all over my raised garden bed. What else should I do?  I would love to be able to grow onions and beets this spring.  I will greatly appreciate any and all suggestions or advise for this matter."

This is a great question and I can understand your concern.  We regularly have green beetle grubs in our compost bin over winter, but the problem nearly disappears once spring rolls around due to their life cycle (they fly off, or we hand pick them out).  In an ideal world, we'd all have chickens to let loose on our raised beds between planting fall and spring crops. They'd eliminate the problem for us with no trouble at all. 

I've actually watched the green beetles fly into the compost bin to lay their young.  While these white grubs tend to attack lawns, they have been found in gardens as well.  Here is some helpful information that couldn't be said better than as by Safer Brand, so I'm quoting:

"Natural Predators

What?

Parasitic wasps and parasitic nematodes can be helpful in fighting white grubs.  These insects are great partners to help control white grub   infestations in your organic garden.

 

How?

A parasitic wasp parasitizes a grub and then lays its eggs in the grub. When the eggs hatch, the offspring feed on the grub, killing it if it's not already dead. 

 

Nematodes will feed on the grubs as well. Hb (Heterorhabditis bacteriophora) nematodes are very effective if they are from a reliable source.  (Christy here - you can get these nematodes from Gardens Alive here.)

 

Lure parasitic wasps to your garden by planting nectar or pollen producing plants. (Christy  here - beneficial plants that attract parasitic wasps include letting your cilantro and parsley to go to seed, and planting alyssum, scabiosa, lemon balm, statice, yarrow, and cosmos)

 

Parasitic wasps can be purchased through a commercial insect breeder.  Make sure if you purchase these insects you have an actual garden insect infestation or you may notice the parasitic wasps migrating to your neighbors yard in search of food.

 

When?

Parasitic wasps may take a few years to be effective, when they are in sufficient numbers.  The best time to apply nematodes is early September followed by April.  (Young grubs are more susceptible to the nematodes then rather than after the winter.)

 

Most local greenhouses and garden centers can help you determine the right planting time for nectar and pollen producing plants to ensure that the pollen and nectar will be available when the wasps arrive.

Environmental Controls

What?

Another control idea pertains to water.  Eggs need a moist soil in which to hatch and newly hatched grubs need water as well. By not watering during mid-summer, eggs will not hatch and grubs will shrivel and die.

 

Bag traps for Japanese Beetles are a popular way to control this pest in the yard & garden. (Me again - you can also get bag traps from Gardens Alive here.)  The combination of Insecticidal Soap and  Botanical Pyrethrins will also kill the Japanese Beetles on contact if they are spotted on your plants.  (Christy here - you can purchase a similar Pyrethrin-based control from Gardens Alive here.)

 

How?

Plant those plants that do not attract the insects with a grub stage.  Keep from watering your lawn (and garden soil, if possible) in mid-summer.

Insecticial Soap (Potassium Salts of Fatty Acids) & Pyrethrin - combined are the organic equivalent of a 1-2 knock down punch.  Japanese Beetles, like other hard bodied insects, are tough bugs to kill.  The soap will penetrate their shell enough to weaken and dehydrate the insect and allow the pyrethrin to absorb into the insect and do its job. 

 

Pyrethrin is a powerful nerve agent that will paralyze and kill the insect on contact.  Keep in mind this soap is not like dish detergent; it's a base from a blend of natural plant sources and pyrethrin oils which come from the chrysanthium flower.  An effective and organic combination....... (Christy here again. Keep in mind that these are very strong controls.  Even though they are approved for use in organic gardens, they can still be harmful.  Be sure to read instructions carefully.)

When?

From mid-July through the earlier part of August, if you can keep from watering your lawn, it will help prevent grubs from hatching and surviving."

I hope this helps you in your journey to a grub-free garden. Keep us posted on your progress. 

If anyone else has had success with a particular method, please share it with us here.