Curl Grub

Curl Grub And Bonsai

What are curl grub and why are they dangerous to bonsai?


Introduction To Curl Grub

Curl Grub is a common name for what is actually larvae of common beetle species which can include African black beetle, Christmas beetle and scarab beetle. Curl grub are often a creamy white colour and live in the soil of lawns, garden beds and potted plants ( including your bonsai ). If left untreated they can cause extensive damage.

Detailed Overview Of Curl Grub

When treating a pest it is best to understand the pest you are treating first to have better success, lets take a deeper look into curl grub.

Life cycle

Egg : The beetle will begin its life as an egg which are generally laid in the warmer months which is when the beetles reproductive behaviour is at its peak due to the warmer weather. The beetles will lay their eggs in the soil close to the roots of plants. The duration of this stage of development is dependant on the species of beetle and the conditions which include temperature and moisture levels within the soil.
Larva ( The Curl Grub Stage ) : Once the eggs hatch that’s when we get the destructive curl grub we are all too familiar with in bonsai. The curl grub will be a creamy white colour and usually stays in a C shape which is where it gets the common name curl grub. During this stage they tend to feed on organic matter ( your soil ) and also the root system of the plant. This is the stage where you will start to see signs of the curl grub which we will discuss later in this article. Much like the egg duration this can be dependant on the species of beetle and also the availability of food.
Pupa : After the curl grub is nice and fed up on your roots and soil it will enter the Pupa stage where it will begin metamorphosis inside a protective chamber called a pupal which is constructed in the soil.
Adult Beetle : Once the metamorphosis is complete the adult beetles will emerge from the soil where they can now feed on the foliage of your trees and plants and also reproduce which will begin the whole cycle again which could be a bigger problem for your collection as numbers will have now grown.

Signs Of Curl Grub Infestation

Declining or reduced Vigour in the growing season : This might seem obvious and is something that can be linked with about 100 other issue but it is still a sign there are curl grub active within your trees. As they begin to feed on the roots the tree will begin to suffer and show those signs on the top of the tree, if your trees are beginning to show signs of ill health and you know you have been fertilising, watering and pruning properly then it might be time to inspect the roots.
Soil Movement / Birds Showing Interest In Your Soil : A sign that you have curl grub is actually being able to see the soil surface moving, now you have to be paying close attention to your trees to actually notice this but you can visibly see movement in the top layers of soil when curl grub are active, Birds how ever have a much more keen eye for this and if they are paying your soil a lot of attention chances are they have spotted that movement and will be picking at the soil to try and harvest the grub.
Soft Puffy Soil Surface : When curl grub are close to the surface often your soil will take on a very soft and puffy appearance as the curl grub some what mulch the soil ( especially pine bark ). Its hard to describe in words but when you see it you will know exactly what I mean when I say soft and puffy. If you do have pine bark in the soil mix you will notice the pine bark has become very finely mulched which is generally what gives the soil that puffy appearance.
Plastic pots will have soft sides : Curl grub are more common in the development stages of your trees then the refinement stage due to them not liking in-organic substrates as it tears their soft flesh and they cant feed on it. If you are using an organic mix in refinement then there will be no escaping them. For your trees in development how ever that are still in plastic growing pots if you give the sides a squeeze the pots that have active curl grub will generally be able to be compressed on the sides. This is due to the grubs once again mulching the soil and making it very soft and there wont be any roots in that area. A normal pot with no grubs will be hard on all sides and you wont be able to push the sides in at all.

Treating and Preventing Curl Grub

Organic Methods
In-Organic Substrates ( Akadama, Pumice, Kiryu, Zeolite, Perlite ) : Like mentioned earlier that once moving into refinement we generally move into our in-organic substrates which have many benefits for our trees which we wont cover here, but one of the benefits is that curl grub wont survive in these substrates as it cuts their body’s and they cant feed on it. Now in saying that the mix has to be completely In-Organic, I have seen cases where the tree has been moved into its first stage of refinement which generally means we leave a core of organic soil in the middle of the tree to help it transition over. On our next repot we will then remove the organic soil from the core and replace it with in-organic substrate to complete the transition. I have personally seen curl grub in this core which can be a major issue especially if they have eaten all the roots in the core. So keep in mind your tree wont be fully protected from curl grub until you have completed the transition from organic to in-organic.
Beneficial Nematodes : This route may not be as effective but has been show to work, If you are looking for a none chemical way to deal with curl grub this may be your only option besides getting in and picking them out by hand. Nematodes are a microscopic roundworm that can naturally occur in soil but you can also inoculate your soil with them as well. You can purchase nematodes such as Heterorhabditis bacteriophora & Steinernema carpocapsae to introduce to your soil to fight curl grub. To fight the curl grub the nematodes will penetrate the curl grubs soft body and release symbiotic bacteria which will multiply inside the grub leading the the grubs death in a few days. The best time to apply nematodes for this will be in spring and summer, in spring the eggs will hatch into larvae and in summer they will be most actively feeding.
Chemical Treatment
Systemic Imidacloprid Treatment : The most effective way to control curl grub is to root drench with imidacloprid such as the Congaurd we sell. Its important to note that if the grub is too far into its larvae stage it might be too late to treat with imidacloprid and you will need something a little different which we will talk about next. The best time to apply the root drench is about a month before your winter ends, this will give the tree time to load up on the imidacloprid and for it to be very active in the soil which will mean the newly hatched larvae will die ending the cycle. It is recommended that you repeat this process every year as a precaution as once the grubs move further into larvae they become harder to eradicate. One of the major upsides of Imidacloprid though is that it also protects the tree against other pests we commonly see on bonsai including aphids and scale which are also most active in early spring. Please do keep in mind though that if you have bee’s actively pollinating in your area try to avoid flowing species as imidacloprid can kill the bee’s as well. If you do need to treat flowering species places them in a bug net so they still get the natural environment but the bee’s wont be able to get to them.
Carbaryl : If the grubs have gotten to a larger size closer to end of spring early summer you may have to use something with the active ingredient carbaryl. This should kill the grubs on contact if the imidacloprid has not worked.


To wrap this up and to summarise what we have just gone over Curl grub are the larvae of certain types of beetle. The eggs will be laid and hatch in early spring and will most actively feed during summer. The best plan of attack for your trees is actually preventative measures such as in-organic substrates in refinement and imidacloprid treatments for anything with organic matter in the soil in the last month of winter.
I hope this has helped and goog luck with your curl grub control!

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Author : Joshua Hooson

Joshua Hooson is an author and enthusiast of the art of bonsai. He has built his knowledge and understanding of bonsai through a combination of self-experience, lessons learned through hands-on practice, and extensive research. His articles reflect his passion for the subject and offer insights gained through his own personal journey in the world of bonsai. All the information provided in his works is a result of his own experiences and the knowledge he has gained through his studies. He is dedicated to sharing his love of bonsai and helping others grow in their understanding and appreciation of this ancient and beautiful art form.

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