Grubs in Container of Garden Soil Killing Plants

Grubs found in our compost piles

Q. I have discovered over 200 huge grubs in a 15 x 24 inch container that is about 2 ft deep. I had filled this container with a bag of garden soil from a garden center and planted strawberry plants. Of course the plants all died, so I decided to plant some seeds and discovered all these horrible grubs!!!

I have been told that a product called Grub-Away is safe to use and that nematodes are even safer. But my main concern is all the earthworms that are in my gardens. I put in the worms last year and am seeing tons of babies. I really do not want to harm them. So far, I have been digging up sections (all raised beds) and destroying the grubs.

If the grubs were in the bags of soil I bought, do I need to treat future purchases?

A. Most of the grub control information is focused on lawns. However there are other grub problems such as those you find in the compost and even bagged compost. In most of the United States the problems in compost are considered minor compared to lawns.

White grubs feeding on the roots of a lawn grass, killing it

Here in our desert Southwest where we have fewer lawns the importance of grub control in compost piles and even bagged compost is usually just or even more important than lawns. The nonchemical control of the white grubs in lawns will also work for the most part in compost or bagged compost. Technically, to be legal the recommendation or control should include the site of application. I have to give you that warning to be in compliance with the law.

Adults of one of the compost grubs, masked chafer

For instance if the product is being sold for pest control in lawns, then it should also include your application on the label. If you choose to use it for this purpose you are doing it at your own risk of success or failure. Having given you the Party line, those pest-control products that are biological or natural should be perfectly safe to use.

There are some very expensive bagged composts that are actually very good but those producers believe in NOT sterilizing the compost. And this is a great idea in theory. The reasoning is that they want all of that biological activity added to a garden soil. You go through the expense of quality composting that produces all of this wonderful biological activity for the soil and then you kill it.

Another adult form of compost grub, green june beetle

This biological activity contributes to the breakdown of organic material in the compost and releasing plant nutrients and a wonderful chemical activity that can take years to develop into normal Desert soil. Many organic growers would be dumbfounded in the logic of killing biological activity in compost. This biological activity is precisely one of the reasons why you compost.

On the other hand, not sterilizing a bag of compost will introduce all of this biological activity to whatever you add this compost to. So if you buy a bag of compost that has not been sterilized and you use it with your houseplants there is going to be a very good chance your houseplants will be infested with dusky winged fungus gnats for example. I get many questions submitted to me on how to control this pest in houseplants. This is seldom a problem outside in the garden in our climate.

Another adult of a grub, ten-lined june beetle

So you have to be careful which type of compost you use for which application. In my opinion I would never use an unsterilized compost for houseplants for precisely this reason. Inside the house or greenhouse all of your products should be sterilized if you don’t want these problems. These are closed environmental systems and must be managed as such. You must scrutinize everything you bring into a closed environmental system because there are few checks and balances to help keep introduced pests under control.

I like these newer bagged composts very much and there is a developing market for them primarily due to the development of local markets for food. They are expensive, running $20 per bag or more. One product line that is popular right now is Fox Farm products. If they can keep their quality control as the demand for products like this climb, it will be a good product but check the bag and see if it was sterilized or not.

Worm for composting

If not and you want it sterilized then put the bags in full sun during the summer months for four days and flip the bags over every day. This should heat sterilize the contents provided the temperatures reach 160F for at least 30 minutes. That should be no problem in our environment but if you want to make sure then buy an inexpensive compost thermometer online (I have not seen any available in Las Vegas) and stick it in the compost bag, burying the probe to the hilt. In mid-day you want the temperature to hit at least 160F.

Secondly, you should inspect any compost and get rid of any grubs. These are decomposers and are working at helping to decompose plant materials but are first level decomposers and will attack the roots of healthy plants as well. So screen your bagged compost before using it if you want to maintain its biological activity.

There are natural pest control products for grubs such as beneficial nematodes and bacteria like Milky Spore but they will focus on some grubs and may not others. They are somewhat selective and may be a good alternative treatment that will leave earthworms alone.

Remember that earthworms can move pretty fast. If they don’t like an environment they usually flee. Grubs have a harder time doing this. But the earthworms must be able to flee somewhere and if they are in a bag that is getting progressively hotter they will not escape.

I hope this helps.