Organic Control of Curl Grubs in Lawn

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Curl Grub larvae (credits – CSIRO Australia scienceimage)

If you’ve ever been digging in the garden and discovered plump, pale coloured grubs beneath the soil that curl up into a “C” shape when disturbed, these are Curl Grubs.

Curl grubs are the larvae of various species of cockchafers or scarab beetles (family Scarabaeidae). Depending on where you are in the world, these curl grubs can be the larvae of the Japanese beetles, African black beetles, Christmas beetles, June beetles, Brownish cockchafers, Blackheaded pasture cockchafers, Black soil scarabs and several others. Once these larvae go through their life cycle beneath the soil and emerge as beetles, they’re relatively short lived, living for 1 to 9 weeks. The female beetles can lay 20-40 eggs in the soil, from which new curl grub larvae will emerge.

 

Identifying Curl Grubs

It’s fairly easy to identify curl grubs, they have white, pale cream or greyish-white soft bodies, with 6 legs, and a small hard brown, yellowish brown or reddish brown head. They vary in size and can grow to a size of  4mm to 60 mm long. When resting or disturbed they curl up into a C-shape, hence their name. Their rear end is enlarged and has a grey tinge from the faecal matter in their hind gut which is visible through their bodies.

The natural habitat of scarab beetle larvae is grasslands, where they feed on organic matter and plant roots. In large numbers, curl grubs can be very destructive, damaging lawns, gardens and plants in pots by feeding directly on the roots or cutting roots off as they forage for organic matter.

They are most active during mid spring to mid summer, and usually will only feed in the warmer months, but if winters are mild they may continue feeding throughout the year. Being most active during the hotter weather, they damage plants when they are most likely to be heat stressed, causing them to wilt and not respond to watering, eventually drying out if the root damage is extensive. Most curl grubs are active just below the soil, feeding within 1 to 2 cm of the soil surface.

Curl grub damage in lawns is quite distinct – during late summer and early autumn, irregular brown dying patches of lawn will appear, which can easily be lifted off and peeled  away like carpet from the soil surface, as the roots have been eaten away.

Curl grubs will feed on the roots of a wide range of plants, but not those of the legumes family such as beans and peas.

 

Assessing the Curl Grub Problem

It’s important to point out that having curl grubs in your soil does NOT mean you have a curl grub problem! As a general rule, a healthy lawn can tolerate up to five curl grubs per square foot.

If you want to check if you have a problem with curl grubs, dig up one square foot (30cm x 30cm) of lawn beneath one of the brown patches that is dying back, to a spade depth, and check the soil for curl-grub numbers. It’s best to dig up a few of the dying patches of lawn in different locations to get an idea of the extent of the problem.

  • If you find up to 5 curl grubs in each square foot of lawn, then you don’t need to do anything, a healthy lawn will cope with this number.
  • If you find more than 5 curl grubs in each square foot of lawn, then you need to take action to control the pests.

Another way to asses the numbers of curl grubs present in the soil is to water the lawn well, then cover the brown areas of dying lawn with a layer of newspapers, cardboard, a hessian bag, a piece of old carpet or similar material, then place something on top such as a few bricks to keep it in place and stop it being blown away by the wind. Leave in place overnight, and in the morning lift the layer to check for curl grubs.

 

Organic Curl Grub Control

If curl grub numbers are high and require control, there are various options available which are safe and allowable in organic gardens.

  1. Neem Oil – the natural insecticide neem oil is extracted from the seed of the Neem tree (Azadirachta indica), and contains the active compounds Azadirachtin A & B. It has two modes of action – it disrupts the pest insect life cycle by preventing the insect from moulting and progressing to the next stage in its life cycle, interfering with its growth and development. It’s also an effective anti-feedant which stops pest insects eating, causing them to starve to death.  Insects will stop feeding once they ingest neem oil, but will die off a few days later, depending on the type of insect and how big it is. Neem is safe for beneficial insects as they don’t eat plant leaves which neem is usually sprayed on.For curl grub control, apply neem to lawns, garden beds and potted plants as a soil drench using a watering can. The best time to apply neem for curl grub control (or any other curl grub pesticide, natural of synthetic, safe or unsafe) is from mid spring to mid summer, as this is the time when their eggs are hatching and larvae are near the surface of the soil, where the insecticide can reach them more easily.
  2. Beneficial nematodes – these tiny microscopic worms are a a biological control, they live in the soil and carry bacteria which kill curl grubs and other pests. They actively pursue the pests in the soil, and once inside it, they release the bacteria which kills the pest. These beneficial nematodes are harmless to people, pets, plants or beneficial insects. In Australia, they’re sold as Nemassist® nematodes, and can be applied to the soil with a watering can or pump sprayer. Soil temperature needs to be above 15C at the time of application for the beneficial nematodes to be active, and they can be applied as a preventative measure from spring to winter.
  3. Milky Spore – the soil-dwelling beneficial bacterium Milky Spore (Paenibacillus popilliae) is another biological control which primarily infects Japanese beetle grubs. It’s applied as a powder over the soil and watered in thoroughly. When the grubs feed close to the surface, the ingest the spores, become infected and die. The decomposing  grubs release more of the spores into the soil for the next generation of lawn grubs. The spores will remain in in the soil for 10-20 years, maintaining control of the pests. Milky Spore requires the curl grubs pests to be present in the soil in the first place so the bacteria can multiply and take effect. Since the spores need to multiply to reach sufficient numbers in the soil to work effectively, this process may take around two to four years, so it’s not an immediate control, but it’s effective over a very long period.Another advantage of Milky Spore is that it can be used together with beneficial nematodes which help disperse the bacteria throughout the soil. If you want to use the two together, sprinkle the Milky Spore on the soil and then water in well using a watering can filled water containing the beneficial nematodes.

 

Curl Grub Prevention

One of the best ways to avoid pest problems is to NOT create environments that favour pests. Curl grub beetles prefer to lay their eggs in thin, sparse grass, so it’s best to keep lawns thick and healthy, and cut fairly high, a minimum of 5cm (2”). Water when necessary and fertilise (using natural fertilisers to preserve the natural soil ecology which controls pests, and to avoid pollution of waterways!) to maintain the condition of the lawn, and water deeply to promote deeper, stronger roots.

Curl grub populations are naturally reduced by extreme weather and disease, parasites and predators. Keeping your gardening natural means keeping it safe for living things, which encourages the living organisms which normally control these pests in Nature. Do not use poisonous synthetic chemical pesticides, herbicides and fungicides in the garden!

The most useful natural enemies which control curl grub numbers are parasitic wasps and insect-eating birds. In Australia, Magpies, a member of the Corvidae (crow)family, are a major predators of curl grubs, you’ll often see these birds digging them out of lawns. They can be encouraged to stay if there are clumps of trees for them to nest in. Many flowering native plants and companion plants attract beneficial wasps into the garden, as they provide a nectar source.

Keeping the ecological balance in the garden and maintaining a healthy ecosystem, rather than creating a chemical-warfare trashed ecological wasteland is critical. If you don’t poison or kill beneficial predatory insects, and create a natural setting that give them somewhere to live, they’ll do the work for you. Both the larval stages of click beetles (family Elateridae), and the adult and larval stages of carabid beetles (predatory ground beetles) (family Carabidae) eat curl grubs.

 

Conclusion

Perspective matters, a few curl grubs in the lawn or garden are not a problem, it’s only when their numbers get too high – which is because of an ecological imbalance, often human created, that we need to take action. Natural and organic-certified controls are available to treat curl grub problems, so there’s no need to turn your lawn or garden into a toxic playground. Seeing a few big beetles buzzing around in the garden add to the experience of enjoying Nature. Happy gardening!

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3 Responses to Organic Control of Curl Grubs in Lawn

  1. Bronwyn says:

    Thank you for this fabulous article.

    We’ve had a very dry year so far in Sydney and I believe this favoured the reproduction of curl grubs. I have a blueberry in a large pot and when I realized it was looking unhappy I pulled it up to find at least 40 curl grubs in the pot below the shrunken root ball they had been feeding on. Same problem in a 1m2 raised bed of strawberries all now dead!

    I recently learnt of another method to control curl grubs in pots – place a layer of shade cloth over the soil, under the mulch, to prevent the beetles getting in to the soil.

    Like

  2. tonytomeo says:

    These grubs do not live here, but others do. Some specie (or perhaps only one species) also happen to like the margins of sparse spots in a lawn, but others like thick lawn. It is important to know which species or specie of grubs are the problem. In the past, I ignored a few grubs because I did not care if the lawn had bald spots. However, the skunks dug for them and left messy gouges in the lawn that were not so easy to ignore. Fortunately, the skunks seemed to get all the grubs rather quickly, so I still did nothing . . . but patch the gouges.

    Like

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