A Guide To Killing Lawn Grubs

Lawn grubs are the larvae of beetles and an unwelcome garden pest.  They live underneath lawns and can quickly cause severe damage to an otherwise healthy grass yard. Lawn grub larvae eat the roots of the grass, killing the lawn in the process. They also attract predators which will rummage through the grass to feast upon the lawn grubs. The result can be a patchy brown lawn at best, and at worst, a muddy, scruffy mess. 

Lawn grubs do not like sunlight, and so usually stay in the soil, below the surface of the lawn. As they are not obviously visible, unfortunately it’s often the damage which is noticed before their presence is known.  They are especially active during the summer when the weather is warmer, and particularly so after a wet downpour.  

Thankfully, a lawn grub problem is easy to treat, and the first line of defence is vigilance. Let’s look at the different types of lawn grub, symptoms of a lawn grub infestation, and how to treat the problem. 

Types of Lawn Grub

There are many types of lawn grub which are common in Australia: 

• Lawn armyworm (Spodoptera mauritia, Spodoptera frugiperda and Pseudaletia unipuncta). Predominantly dark brown/green in colour, with some patterning, and up to 5cm in length.  It has a very short lifecycle which means that several generations of lawn armyworm may emerge in a year. A single moth can lay over 1,000 lawn grub eggs in 10 days, which means potentially a thousand hatched larvae looking for fresh grass to feed on! 
• Sod webworm (Crambus). Light brown or dark grey with black stripes, this lawn grub reaches up to 3cm in length. 
• Billbug (Sphenophorus brunnipennis). Small at up to 1cm in length, a white or cream coloured body and orange head and no legs. This one is particularly common in kikuyu grass. 

A large family of very familiar grubs are those of the Scarab beetle.  These are not true lawn grubs, but it useful to know about them, as they are often confused. They are distinguished from lawn grub as they have dense bodies, six legs, and have white or cream coloured bodies.  They are shaped like a ‘C’, hence the nicknamed ‘curl beetle’.  Curl beetles include the following species: 

• African black beetle 
• Argentinian scarab 
• Cockchafer  
• Pruinose scarab 

All types of lawn grubs go through a similar lifecycle. The larvae pupate in the soil then emerge from pupation as a beetle or moth. After flying then mating, they return to the soil to lay eggs, which then hatch into a new batch of larvae. The lifecycle repeats until an outside intervention disrupts their reproduction. It’s the larval stage when most damage is done, usually from September through to March, so this is when you need to be most alert to the symptoms of lawn grubs.  

Recognising the Signs of Lawn Grubs on Lawns

Be vigilant for early lawn grub damage, as they can multiply very quickly and are voracious eaters. Key symptoms to look out for are: 

• Patches of brown turf. This is caused by larvae eating the roots of the grass, which then dies off. 
• A spongey lawn. Where grubs have disturbed the soil and grass roots, the surface may feel loose to walk on. It may even be possible to easily lift sections of lawn, as the roots are no longer attached to the soil. 
• ‘Frass’, which is green excrement left by the larvae on the soil surface.  
• Holes in blades of grass. Particularly where lawn armyworm is present, as it likes to munch on the leaves as much as the roots.   

Other activity to look out for include: 

• A sudden increase in birds visiting the lawn. They will eat the grubs, but damage the grass as they retrieve the grubs from the soil surface. 
• Hovering moths. Increased moth activity around the lawn can indicate that they are preparing to lay eggs in the grass.   

If you suspect lawn grubs are present, an easy way to check is to water the lawn heavily and cover with a piece of cardboard over night. Lift the cardboard the next morning, and look for larvae, which will have come to the surface, if there are any. 

Killing Lawn Grubs

All problematic lawn grubs are treated in the same way. The intention is to disrupt the short lawn grub lifecycle.  

Once you are sure lawn grubs are present, apply a lawn grub killer treatment immediately, preferably in the evening. This will most likely be in November or December when large numbers of lawn grubs may become apparent. Expect to apply treatment several times until the problem is under control.  

An alternative way for killing unwanted lawn grubs is to apply Nematodes, specifically Heterorhabitis bacteriophora. This way of tackling a lawn grub problem must be timed correctly, as it acts best on newly hatched young grubs. 

Do check that grubs are causing the problem before treating them, as doing so unnecessarily can harm beneficial insects. 

What is the Best Lawn Grub Killer?

We think that the best lawn grub killer is Scotts Lawn Builder™ Lawn Grub + Insect Slow Release Fertiliser, applied using Scotts® EasyGreen Broadcast Fertiliser Spreader for an even distribution of treatment. 

Are Lawn Grub Killers Safe for Pets?

Scotts Lawn Builder™ Lawn Grub + Insect Slow Release Fertiliser is of minimal risk to pets when the instructions are followed. Apply at the recommended rate and frequency. Keep the product locked away as ingestion of large quantities can be harmful to dogs.   

How to Prevent Lawn Grubs

A well maintained lawn will prevent many problems. Keep the lawn strong and healthy by regularly feeding. mowing and removing thatch, so that there are less appealing places for beetles and moths to lay their eggs.