New Zealand grass grub

The New Zealand grass grub (scientific name Costelytra zealandica) is a destructive pest belonging to the Scarabaeidae family, within the order Coleoptera. Originally native to New Zealand, this insect has become one of the most economically significant pests affecting pastures, amenity turf, and various forage crops.

The pest primarily targets a wide range of grasses, including ryegrass, fescue, and bluegrass. During years of heavy infestation, the larvae (grubs) can devastate pasture productivity by consuming the root systems of essential forage species. Beyond grasslands, they may also inflict damage on vegetable crops and cereal seedlings.

The insect follows a univoltine (one-year) lifecycle. Adult beetles emerge in early summer, feeding on the foliage of trees and shrubs before mating. Females deposit eggs in the soil. Upon hatching, the larvae move through three distinct growth stages. The most destructive feeding phase occurs as the larvae mature and aggressively prune roots.

Symptoms of infestation include patches of yellowing or dying grass that can be easily pulled away from the soil, as the root structure has been destroyed. Secondary damage is often caused by birds, such as starlings or gulls, which tear up the turf to feed on the grubs, causing further degradation of the ground cover.

Management and control strategies include:

• Application of biological control agents, particularly entomopathogenic nematodes or fungi.
• Judicious use of soil-applied insecticides when larval counts exceed economic thresholds.
• Cultural management such as maintaining optimal pasture height and promoting vigorous root development.
• Monitoring adult flight activity with light traps to predict high-risk areas for larval outbreaks.