HortFACT
Identification
Weevils, sometimes called "snout beetles", have the head extended into a beak, or rostrum, which carries the antennae and mouthparts. The whitefringed weevil belongs to the "short-nosed" group Which is characterised by the presence of a scar on the mandibles [jaws].
The adult is large enough at 10 x 4 mm to be seen easily when moving, but its grey and white striped pattern is obscured among plants on the ground. Its body is covered by water-repellent scales and some fine black hairs.
The eggs are white, soft, and somewhat shapeless. Clusters of 20-60 are pushed between pieces of ground litter and cemented together with bubbly mucus which hardens into a protective film, enabling them to withstand drought. Almost certainly it is in the resistant egg stage that this South American weevil has spread around the world.
The larvae hatch in about 3 weeks under favourable conditions, or 1-2 days after the onset of rain if the eggs were laid in a dry period. They then burrow into the soil, where they feed throughout the winter. The mature larva is about 15 x 5 mm, cream coloured and robust. Like most other weevil larvae it has no legs. The pale head is tucked back into the prothorax, with only the black mandibles protruding. Younger, smaller individuals are difficult to separate from closely related species that may be present at the same time.
When fully grown the larvae cease feeding and move towards the surface ready to pupate. The prepupa is dense white owing to the fat which has accumulated. It prepares a pupation cell by rotating its body while exuding droplets of mucus from the anus to bind together soil particles into a strong, smooth lining. It remains there for several weeks before moulting inside the cell to become a soft, white pupa with the adult features visible.
The newly emerged adult remains in the cell until its cuticle has hardened. It has a pair of expendable cusps [teeth] attached to the mandibles with which to tear a hole in the cell. The cusps drop off soon after emergence, leaving a characteristic scar.
Host plants
Clovers, lucerne, lotus, peas, water melons, tomatoes, potatoes, rye grass, wheat, brassicas, and pine seedlings have been recorded as host plants.
Damage
Since larvae feed on roots, the damage they cause is noticed only when plants begin to show stress be becoming yellow or stunted and wilting; seedlings and drought- or grazing-stressed plants are worst affected. Mature plants of taprooted species such as lucerne and red clover are more tolerant to attack than fibrous-rooted legumes such as white clover and Lotus spp. In potatoes, larvae tunnel inside the tubers in much the same way as slugs. Adult weevils feed on leaves, especially of clover, but this damage is insignificant.
Distribution
Graphognathus leucoloma was first recorded in New Zealand in 1944, and is now widespread in the North Island. In 1972 it was recorded from Canterbury in the South Island. Apart from its area of origin, South America, the species is established in eastern U.S.A., eastern Australia, and the southern tip of South Africa.
Life cycle
| Egg |  |
| Larvae | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
Reproduction is parthenogenetic, which means that fertilisation does not take place and males are not required to propagate the species. Only females are known, and each one can produce over 1,000 eggs.
Most eggs hatch in autumn, and the larval stage can last 8-20 months, which means that larvae of all sizes can be found at most times of the year. Pupation occurs during the warmer months, particularly in December and January, and takes 10-14 days. Adults are present in summer and autumn, though in warmer areas of New Zealand they may be found throughout the winter. On favourable foods such as clovers, lucerne and other legumes, adults live up to 3 months and lay up to several thousand eggs, whereas adults feeding on grasses and cereals live for less than 3 weeks and lay less than 10 eggs. In warmer areas of New Zealand, adults can be found throughout the winter.
D STEVENS