It is often noticeable to see damage to crops and pastures having been eaten or infected by insect pests. Damage can occur below ground too, with some insects completing their lifecycle there.

It is beneficial to identify the pest and understand its lifecycle, as it can be futile to control those that live underground with a chemical spray. This is because the spray does not travel deep enough into the soil for effective control. Identifying the part of the lifecycle above the ground provides an opportunity to control the pest then.

The most common subterranean pest found in pastures is the native grass grub (Costelytra zealandica), while in some areas its cousin the Tasmanian grass grub (Acrossidius tasmaniae) is found. The grubs are similar to each other and do significant damage below ground.

Grass grub larvae are soil-dwelling, growing up to 15 mm long. They are grey/white coloured with tan heads. The larvae are typically found lying in a C shape in the root zone of the plants and feed on the roots of many pasture species and other plants. Infestations appear above ground as patches yellower in colour when compared to surrounding pasture, and the plants can be easily pulled from the ground. The patches feel soft underfoot and become easily pugged.

Porina caterpillars (Wiseana cervinata) are found throughout New Zealand, though damaging populations are uncommon in the northern half of the North Island.

The moths fly in spring and summer and live for only a few days. During these flights, female moths may lay up to 3,000 eggs on the surface of pasture or grassy areas. The eggs hatch in 10 to 21 days and the young caterpillars construct silk-lined casings on the soil surface. As the caterpillars grow, they construct burrows in the soil which eventually reach a depth of about 25 cm. They emerge from their burrows at night to feed on grass and clover leaves at the base of plants, dragging these back to their burrows to eat.

The caterpillars appear dirty white in colour with a grey back and a dark brown head. Damage to crops is most noticeable from April until September and mainly occurs one to four years after cultivation, though most often in the second or third winter. It only takes 50 to 75 porina per m² to destroy some plants, open pasture and allow weed species to establish.¹ Major damage results from a population of 100 or more porina per m².¹

Clover root weevil (Sitona Lepidus) is a major pest of clovers, especially white clover. An adult feeding is detected by a distinctive half-moon shaped notching on the leaf margins. The larvae migrate down to the roots, feeding on the roots and root nodules, reducing the clover's ability to fix nitrogen.

With underground pests affecting the performance of plants and crops, contact your local PGG Wrightson Technical Field Representative to recommend a crop protection programme.

¹ Barratt, B.I.P., van Toor, R.F., Ferguson, C.M., & Stewart, K.M. (1990). Grass grub and porina in Otago and Southland: a guide to management and control. Ministry of Agriculture and Fisheries 1-104.