HortFACT
Identification
The larvae are C-shaped when relaxed, creamy white in colour, and have a light tan head and a horseshoe-shaped cluster of anal bristles. They moult (cast their skins) three times. Newly hatched larvae are about 5 mm long and weigh only 2-3 mg. When fully grown, third instar larvae are 20-25 mm long and weigh 150-250 mg. In the third instar the grub's black gut can be seen through the translucent body wall. The white material is stored fat. Grubs tinged green, purple, and yellow are sometimes encountered- these are diseased.
Grass grubs can be confused with the grubs of related species, but do not have the dark brown head of the Tasmanian grass grub, and are in the egg and young larval stages when black beetle larvae are considerably larger.
The pupae are shorter and thicker than the last instar larvae. They are also creamy white, but have no visible gut. The developing wings and legs can be seen beneath the pupal skin.
The beetle or adult stage is a rich brown colour and has a hard, shining covering. The paired elytra [wing cases] on its back protect the dedicate wings which unfold to allow it to fly. The beetle is 8-13 mm long.
The eggs are laid in clusters of 5-30, 10-25 cm deep in the soil. They are white and oval, about 2 mm long and 1 mm wide when laid. After a couple of days they swell by absorbing water from the soil until they have the more rounded shape of those in the photograph.
Distribution
The species occurs throughout New Zealand, but is most likely to be found south of Hamilton at altitudes up to 1200 m. It is not found in any other country.
Host plants and damage
Grass grubs attack the roots of most pasture plants, but their numbers are highest under susceptible species such as white clover and ryegrass and very low under the resistant lucerne and Lotus major. Tall fescue supports relatively high populations of grass grub but with little effect on plant production. In infested pasture, grass grub numbers can be so high (up to 1500 per square metre) that pasture production is severely reduced and the sward may die. However, grass grubs are by no means limited to the pastoral scene. The larvae cause damage in horticultural situations (e.g., to strawberries and carrots) and in forest nurseries (e.g., to pine seedlings). The beetles eat leaves. They have little impact on pasture but are known to strip the foliage from grape vines and roses, and from apple, cherry, plum, peach, and apricot trees.
Life cycle
| Egg |  |
| Grub | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
Most grass grubs hatch in December and January and pupate 9-10 months later. They are found up to 150 mm below the soil surface. The first larval stage lasts about 3 weeks and the second about 6 weeks. The third instar is present until the following September or October, but completes its growth and stops feeding about July, depending on the conditions. The pupal stage lasts 3-4 weeks.
The adult beetles live for 2-4 weeks. Most emerge from the ground in October and November, although they can be found as late as February. Female beetles are usually mated as soon as they appear on the surface of the pasture and lay most of their eggs close to the point of emergence. This means that infestations tend to remain localised. However, after laying most of her eggs the female may fly off to infest new sites.
If the soil is cold (e.g., at high altitude) or dry (e.g., in Canterbury) the larval stages of some or all members of a population can last over 2 years, but they still pupate and emerge as adults at the same time of year as I year old insects.
P.G. FENEMORE revised by J.A. WIGHTMAN