Why is it a problem? Feeding by the white grub larvae results in large circular holes on potato tubers, reducing their market value.
Where and when is it a problem? Although present in all potato-growing areas, it normally does not cause sufficient damage to be economically important. When potato are planted in pasture lands, which are their preferred habitats, white grubs can cause significant harm.
White grubs (Source: Ugyen Dorji, ARDC Bajo)
Larvae are large, soft-bodied, and white with brown heads. Their body is curved into a C shape.
Adults are rarely seen, but are large (12–25 mm), yellow to dark reddish brown to black, oblong, and shiny.
Grubs feeding on tubers (Source: Ugyen Dorji, ARDC Bajo)
White grubs feed on roots, stolons, and tubers. Initially, grubs feed on roots, causing plants to dry up. Once tubers are developed, grubs feed on the tubers, resulting in large holes.
White grub damage on tubers and roots (Source: Ugyen Dorji, ARDC Bajo)
Confusion with other pests: White grubs are very characteristic. Feeding damage on tubers is much larger than that caused by red ants.
Lifecycle: Eggs are laid in grassland and patches of weeds in cultivated fields. Larvae pupate underground, later emerging as beetles. They live in the turf-root zone, where they feed on roots and decaying organic matter. One generation is likely to take at least one year.
Dispersal: Beetles are thought to also lay eggs in manure heaps. Farmyard manure used as fertilizer may thus be a source of eggs and larvae spread into potato fields.
When can damage be expected? White grubs appear to be present throughout Bhutan, but they generally only affect potato crops at low levels. Severe outbreaks have never been reported.
Hosts: In Bhutan, it has only been reported as a potato pest. However, although the species has not been identified, it is likely to have a wide host range, including pastures and weeds. Larvae, probably of the same species, can also be found in manure and compost heaps.
Cultural control is generally sufficient for managing this pest.
Grub can be found when land is being prepared for planting. They are difficult to find once potatoes are growing. If wilting symptoms are noted, then dig around the plant roots for signs of the grub.
Effect of variety
Desiree is susceptible to grub damage in Bhutan.
- Use well decomposed and eggs/larvae free farmyard manure in the field.
- Remove grubs by hand during land preparation or weeding.
- Avoid planting directly into pasture grass land in areas where white grubs are known to be a problem.
- Deep tillage (40 cm deep) can help expose the larvae to adverse environmental conditions and predators.
Chemical control is difficult, and frequent applications (weekly to monthly) are usually required and only give limited control. It is important that insecticides reach the root zones where grubs are feeding. This can be achieved by applying water right after insecticides are applied.
Version: NPPC 2022. White grub V2.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 19 December 2022. Contact: firstname.lastname@example.org
Image acknowledgements: ARDC Wengkhar, https://entomology.ca.uky.edu/ef304 & Ugyen Dorji (ARDC Bajo)