Phyllophyga sp.
Crop: Potato
Why is it a problem? Feeding by the white grub larvae results in large circular holes in 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 potatoes are planted in pasture lands, which are their preferred habitats, white grubs can cause significant harm.
IDENTIFICATION
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.
SYMPTOMS
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.
BIOLOGY
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.
MANAGEMENT PRACTICES
In Bhutan, the use of forest soil/compost seems to be the primary source of white grub infestations. It is highly recommended to screen and treat to remove larvae and eggs.
On-farm monitoring
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.
Non-chemical management
Heat treatment of compost/forest pile/FYM in greenhouse
- Choose a sunny period when the greenhouse receives ample sunlight
- Spread the compost pile evenly on a flat surface that will ensure even heating.
- Regularly check the compost file for signs of white grub mortality. White grubs may become lethargic or die due to the heat. Collect and dispose of the dead larvae
- The duration of exposure will depend on the intensity of the sunlight and the severity of the infestation. Several hours to a few days of exposure may be necessary.
Insecticide treatment of compost/forest pile/FYM
- Evenly apply the insecticide solution (Cypermethrin 10% EC, 1 ml @ 1 liter of water) to the pile using a sprayer or watering can. Ensure that the insecticide reaches all parts of the compost pile by thoroughly mixing the pile.
Chemical management
Chemical control is difficult. Soil drenching with insecticides (Cypermethrin10% EC- 1 ml @1 liter water) is effective for killing white grubs. Apply an insecticide solution directly to the soil in the root zone of plants where white grubs are active. The grubs come into contact with the insecticide as they feed on plant roots or move through the soil, resulting in their mortality.
Version: NPPC 2022. White grub V2.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 19 December 2022. Late date modified: 29 Feb 2024. Contact: nppcsemtokha@gmail.com
Image acknowledgments: ARDC Wengkhar, https://entomology.ca.uky.edu/ef304 & Ugyen Dorji (ARDC Bajo)
