HortFACT
Identification
The beetles are 10-12 mm long and noticeably narrower than the beetles of the New Zealand grass grub (see HortFACT 401004). They are dark brown to black, and shiny. There is a 'waist' between the thorax, which is almost spherical, and the 'square-shouldered' elytra (wing-covers).
The eggs are cream-coloured spheres about 2 mm in diameter. Clusters of up to 60 are laid 80-130 mm below the soil surface.
The larvae or grubs are superficially similar to New Zealand grass grubs and small black beetle larvae (see HortFACTS 401004 and 401006). They can be distinguished as follows.
Their head capsule is chocolate brown, compared to the tan coloration of the heads of the other two species.
There are brown plates just behind the head, one on each side of the first thoracic segment.
The body is whiter than in the other two species.
NOTE - Pink or mauve larvae are sometimes found. These are infected with a fungal disease. This disease makes itself more obvious in the later stages of infection when brown 'horns' appear from behind the head of the immobilised larvae and push to the soil surface. They develop fruiting bodies which release spores, some of which will infect more larvae in the following season.
Full-grown (winter) larvae are smaller (up to 15 mm) than New Zealand grass grubs (up to 25 mm) at the same time of year. Black beetle larvae are found in December and January, when the Tasmanian grass grub is normally in the pupal, adult, or egg stage.
The grubs make tunnels in the soil in which they stay during the day. They come up to the soil surface only at night, to feed on such plant material or dung as is available within a few centimetres of the entrance of their tunnel.
The pupae are white at first, but appear to darken as the beetle hardens inside the skin. The larvae form cells about 100 mm below the soil surface before they pupate.
Host plants
The larvae feed on the foliage of grasses, clovers, and lucerne in pastures, home gardens, and recreational grassland and turf. They bite off pieces of leaf, which they drag to their burrows to eat later.
Damage
The damage caused by this species is typically in the form of patches of bare soil which appear from autumn to late winter. These can be small areas only 100 mm in diameter on lawns or bowling greens, or extensive areas of pasture from which all the 'useful' grasses and legumes have been stripped. This leaves the land open to invasion by weeds such as storksbill (in Canterbury), which are not eaten by this pest. Close examination of the soil surface will reveal the tunnel entrances (3-5 mm in diameter) through which the grubs emerge. This damage is easily confused with that caused by the porina caterpillar (see HortFACT 401001). However, the caterpillar always covers both its droppings and the excavated soil around its tunnel entrance with a network of fine webbing.
Heavy rain and hard frosts deter the grubs from venturing to the surface, and damage is severe after prolonged periods of either condition. The damage caused by Tasmanian grass grub can also become noticeable after heavy grazing in autumn or early spring, because the more succulent regrowth is within reach of the larvae. The beetles have not been observed to damage plants.
Distribution
Tasmanian grass grubs live in light soils in the coastal areas of Canterbury (particularly), Marlborough, Auckland, and Northland. Infestations tend not to cover wide areas, but are patchy and severe over relatively small areas of up to 1 hectare. Other regions with light soils and good drainage are clearly at risk, particularly those near the coast
Life cycle
| Egg |  |
| Grub | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
The beetles fly at dusk on still, warm nights between mid January and mid March. Dense swarms aggregating around street or shop lamps in urban areas are common. The beetles presumably mate on the soil surface. The female excavates a tunnel at the end of which she lays a cluster of eggs. The larvae (3 mm long) hatch after about 3 weeks and pass through three instars (growth stages) to become fully grown sometime between April and October, depending on the locality and the quality and quantity of the food supply. They pupate any time from November to January. A 1-year life cycle appears to be the rule.
J. A. WIGHTMAN