HortFACT
Identification
The adult is 8 mm long, straight-sided, and has a longer, narrower snout than Fuller's rose weevil. It is predominantly earth-brown in colour; there is a white median stripe on the thorax and a characteristic white 'chevron' towards the rear of the wing covers. The insect is covered with a water-repellent layer of scales interspersed with stiff, upstanding hairs, except on the head, legs, and underside, which have hairs only. The hindwings, folded beneath their hard covers, are well developed, but the insect is rarely observed to fly. It usually remains hidden during the day, and is most easily found after dusk, when it can be dislodged from plants on which it is feeding.
The spherical eggs are 0.64 mm in diameter and usually laid singly; they are lightly stuck to the stems, low foliage, and particularly the crowns of host plants, or pushed into the soil. They are pearly white at first, but change through yellow and grey to black shortly before hatching. The female lays about 10 a day, and up to 1900 in a season. Hatching occurs after 14-47 days, depending on temperature.
The larvae, or grubs, grow from 1.6 mm to 13.5 mm long in four instars (stages). They feed on the aerial parts of plants, and their colour is derived from the chlorophyll they consume. Thus, those feeding on leaves of cabbage are dull white, whereas those on carrot tops are green. The head is dark and distinctively mottled. The underside of the body, with its soft lobes and absence of hairs, is adapted for adhesion to plant surfaces - larvae are usually found on the lower surface of leaves. Young larvae shelter in and feed on new growth, and often eat out the growing point. Older larvae may burrow into the soil by day and feed at night.
Prepupae (fully fed larvae) make a cell just beneath the soil surface, in which they moult to become pupae. These are about 8 mm long and pale yellow, changing to brown just before the adult weevil emerges. In pupae the long hindwings can be seen protruding from beneath the elytra, and there are no cusps (expendable teeth) attached to the mandibles. In these respects they differ from pupae of whitefringed weevil and Fuller's rose weevil.
Host plants
Most vegetables, and some flowers and fruit trees, have been recorded as hosts; the brassicas and tobacco are particularly favoured. Weeds, especially hog cress (Coronopus didymus) and the chickweeds (species of Stellaria) are also hosts to this species, providing cover as well as sustenance.
Damage
Feeding by adults and larvae on the aerial parts of host plants causes a variety of damage; that illustrated is typical.
Distribution
The vegetable weevil has been present in New Zealand since the 1920s. It has spread into cultivated areas throughout the North Island, and in the South Island as far south as Timaru. It is widely distributed in Australia and the Pacific Islands, and occurs in the southern United States and South Africa. In South America, where it is native to Brazil, Argentina, and Uruguay, it extends as far south as northern Patagonia (latitude 41oS). Both sexes occur there, whereas in New Zealand and elsewhere outside the native range only females are known.
Life cycle
| Egg |  |
| Grub | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
Pre-adult development is restricted to the moister, cooler months. The life cycle is short compared with that of the white-fringed weevil. Development from egg to adult is completed in approximately 3 months, of which 3-6 weeks is devoted to feeding by the larva. The duration of the different stages varies with temperature, and hence with latitude: egg 14-47 days; larva 30-46 days; pupa 10-16 days. The new adults emerge during spring and live about 12 months. They are inactive during hot, dry periods, and do not lay eggs until late summer. There is only one generation each year. Reproduction in New Zealand is parthenogenetic, which means that fertilisation is not required for propagation of the species.
B. M. MAY and A. M. FERGUSON