If you dig the soil in a garden bed, you will probably have discovered curl grubs. They are up to 4 cm long, have 6 legs, are white to cream in colour and have a brown head and a grey end. They curl up to a C shape.
Curl grubs are the larvae of a range of scarab type beetles like the African- and Japanese beetle or the Christmas beetle. They are not related to wichetty grubs.
In Spring the mature beetles lay their eggs in the soil and with rising temperatures the eggs hatch and new curl grubs are born. This is also the time of the year when more mature curl grubs come closer to the soil surface and start feeding. This is why you can find more of them in the warmer months of the year. Curl grubs prefer dry, hot weather and suffer if a bed is regularly watered.
The larvae feed on the roots of a wide range of plants, they love grass-roots but they keep away from all plants of the legume family.
Don’t be alarmed if you find 4 or 5 curl grubs in a bed, just squash them or collect them and feed them to the birds. Treatment is only required if you find serious infestations with 50 to 100 and more grubs per square meter.
A word of caution when dealing with very big grubs: The grubs are not harmful to humans and don’t carry any poison, but they have strong pincers that can penetrate the skin!
Controlling curl grubs
Due to their life-cycle the best time to deal with curl grubs is mid Spring to mid Summer. Since the parent beetle brings in new eggs each year, curl grubs need to be monitored and need ongoing treatment if they take over.
- Lure them to the surface by watering the bed and then applying a cover (wet cardboard or wet hessian bags, tarpaulin, wet newspaper) over night. Check for grubs in the morning.
- Mix a soapy bucket of water and apply to the soil, check for the grubs surfacing in the next 10 minutes, collect and discard.
- Mix 10g of eucalypt oil and 2.5 g of tea tree oil per litre of water, add a dash of soap and drench the soil using a watering can.
- A commercial product, not harmful to humans, called Eco-Neem can be used to drench the soil.
- A more long-term solution is to infect the grubs with milky spore (Paenibacillus popilliae), a soil bacteria only harmful to those grubs. When the grubs die, more bacterial spores are released into the ground, so the treatment is – to a degree – sustaining itself.