Grass grub

Adults fly late spring (Oct-Dec); grubs damage roots through autumn and winter

Grass grub is the larva of an endemic New Zealand beetle (Costelytra giveni) and one of the most common soil pests in Kiwi gardens and lawns. The fat white C-shaped grubs chew through roots, while the small brown adult beetles fly at dusk in late spring and nibble leaves.

How to identify

• Fat, white, C-shaped grubs with a brown head and three pairs of legs, found in the top 10cm of soil
• Patches of lawn that yellow, wilt and lift away easily because the roots have been eaten
• Wilting or collapsing seedlings, strawberries and vegetables despite watering
• Clouds of small brown beetles (about 1cm) flying at dusk on warm evenings from October to December
• Birds, especially blackbirds and magpies, digging up lawn and beds to feed on the grubs

How to prevent

• Keep lawns and beds healthy and well watered so plants outgrow minor root damage
• Encourage birds, which are the main natural control, by keeping the garden bird-friendly
• Turn over soil in vegetable beds over winter to expose grubs to birds and frost
• Avoid leaving large areas of bare grass that adult beetles prefer to lay into

How to control organically

• Dig over affected beds and hand-pick the grubs, or let chickens work the soil
• Water beneficial soil nematodes or apply a Beauveria-based biological control to heavy infestations
• Firm the soil around seedling roots, as grass grub prefers loose soil
• For lawns, treat in autumn when grubs are small and feeding near the surface