White Grubs

Adult scarab beetles vary in size and colour by species but are typically stout-bodied and oval. The larvae, white grubs, are creamy white to greyish, C-shaped when at rest, with a brown head and three pairs of legs near the front of the body.

The pattern of hairs and spines on the underside of the last abdominal segment (the raster) is used to distinguish species. Grubs are typically found a few centimetres below the soil surface, near roots.

Adult beetles emerge and lay eggs in soil, often in grassy or turf areas, during warmer months. Eggs hatch into larvae that grow through successive instars, with the largest, most damaging larvae typically present in the final year before pupation.

Grubs move vertically in the soil profile with the seasons, feeding nearer the surface in warm, moist conditions and retreating deeper to overwinter, before eventually pupating and emerging as adult beetles.

Larvae sever roots and tunnel into tubers and underground stems, reducing the plant’s ability to take up water and nutrients. Affected plants may wilt in patches, especially during dry weather, and severely damaged plants can be lifted from the soil with little resistance.

Host crops include maize, potato, and sugarcane, with risk often elevated in fields recently converted from pasture, turf, or long fallow, similar to other soil-dwelling beetle larvae.

Digging and inspecting soil around roots before planting a susceptible crop helps estimate grub density and risk. Monitoring adult beetle flights, for example with light traps, can also help time field inspections and anticipate egg-laying periods.

Because white grubs persist in the soil for extended periods, management relies on assessing risk before planting and reducing conditions that favour large populations.

• Sample soil for grubs before planting susceptible root and tuber crops
• Avoid planting high-risk crops immediately after grassland, turf, or long fallow
• Conserve parasitic wasps, entomopathogenic fungi and nematodes, and insectivorous birds
• Consider tillage timing, which can expose grubs to predators and desiccation