Why is it a problem? Feeding by the large soil-dwelling white grub larvae result in large circular holes in potato tubers, greatly 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.
Larvae are large, soft-bodied, 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 shining.
When potato plants are young, larvae feed on the potato root causing plants to dry up. Because roots are destroyed, affected plants are poorly anchored and can be pulled effortlessly from the ground. Once tubers are developed grubs cut holes in the tubers.
Confusion with other pests: White grubs are very characteristic. There could be more than one white grub species involved in Bhutan, but that shouldn’t affect how they are managed. Feeding damage in 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 under-ground, 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 into manure heaps. Use of farmyard manure as fertilizer could therefore be a source of spread into potato fields for eggs and larvae.
When can damage be expected? White grubs appear to be present throughout Bhutan, but generally only affect potato crops at low levels. Severe outbreaks have never been reported.
Hosts: In Bhutan it has only been reported as a pest of potato. However, although the species has not been identified, it is likely to have a wide host range include pasture 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.
Larvae 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 larvae.
Effect of variety
- 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 predators.
- When fertilising fields with farmyard manure ensure that it is well decomposed to prevent movement of beetle eggs and larvae.
Chemical control is difficult and not economically justified in Bhutan. Frequent applications (weekly to monthly) are usually required and only gives limited control.
Version: NPPC 2017. White grub V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Contact: email@example.com
Image acknowledgements: ARDC Wengkhar & https://entomology.ca.uky.edu/ef304