HortFACT
The eggs are usually found singly; they are about 2 mm long and ovoid, swelling to an almost spherical shape before hatching.
The larvae which are similar in general form to grass grub larvae, may be easily distinguished by the following characteristics.
Size: the overall size of the larva is much greater than that of the grass grub, reaching about 2.5 cm when fully grown.
Colour: the head is light brown, and the body greyish or creamy white except for the hind end Which appears black owing to the contents of the gut showing through the body wall.
Spiracles [breathing pores]: these are more prominent in black beetle than in grass grub, and show clearly as orange spots down the sides of the larva.
Anal cleft [opening of the anus]: in black beetle this is distinctly half-moon-shaped, but in grass grub it is prominently Y-shaped.
The pupae are found in earthern cells, and at first are light yellow, becoming reddish brown before the adult emerges.
Host plants
Black beetle is most commonly found attacking pasture grasses, particularly paspalum and ryegrass. It is also often associated with various crops notably maize, sweet corn, potatoes, kumara, and strawberries. A more extensive list of plants attacked may be found on page 396 of "Plant Protection in New Zealand", by ED. Atkinson et al
Damage
The damage done in pasture by black beetle is very like that done by grass grub, m severe infestation the pasture browns off, and can be rolled back like a mat owing to the complete destruction of the root system by the larvae (see photo). In less severe cases the pasture becomes dominated by clover (legumes are not a favoured food) or open and susceptible to weed invasion. Adults chew at the base of plant stems, and seedling maize can be destroyed in this way. The tubers of potatoes and kumaras are bored into by the larvae, and the aerial parts are destroyed below ground level by the adult beetles. Black beetle outbreaks are associated with higher than average spring and summer temperatures.
Distribution
Black beetle is found throughout the northern part of the North Island, occurring as far south as Cape Egmont on the west coast and Mahia Peninsula on the east coast. In the centre of the island it appears to be still spreading south, and has been found around Rotorua. It favours sandy, peaty, or freedraining loam country, and to a markedly lesser extent the heavier clay soils of the hill country. It is a native of southern Africa and also occurs in Australia.
Life cycle
| Egg |  |
| Grub | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
Black beetle has only one generation per year, but it is quite common to find life stages out of phase with the main generation. In some areas up to 20 percent of the population over-winters as third stage larvae or as pupae, and the adults which develop from these stages probably lay their eggs in January and February, so that virtually all life stages may be found in the soil at any time of the year.
The usual cycle is for adult females to lay 7-10 eggs in the soil from late September, most egg laying occurring from late October to late December. Larvae develop through three instars over the summer and pupate in February-March. New adults begin to emerge from late February and feed actively for a few months. When numbers are high, usually following summer drought, massed dispersal flights may occur in autumn. Adults overwinter in the soil, emerging in spring to start feeding again and to lay eggs.
M.J.ESSON