HortFACT
Identification
The adult is 44 mm long, and is covered in iridescent scales when it first emerges from the pupal skin. The scales give it a multi-coloured sheen when viewed through a magnifying lens, although to the naked eye the overall colour is fawn. Characteristic features are the white stripes on the thorax (one central and one down each side) and the square 'shoulders' of the elytra, or wing covers. As the weevils get older the scales wear off, revealing the shiny black cuticle.
The eggs are cream and shiny when first laid, but turn black within 2 days. They are spheroidal and about half the diameter of a pinhead.
Newly hatched larvae are about 1 mm long and about 0.2 mm wide. They grow to a length of 7 mm and a width of 2 mm. The body is soft and milk-white, and the projecting head capsule has a mid-tan coloration. The legs are almost invisible.
The pupa is slightly shorter than the fully grown larva and has a yellowish tinge. Sometimes the legs and wings of the developing adult can be seen under the pupal skin. The pupa may wriggle if disturbed.
Host plants
When first found in 1974 the sitona weevil was restricted to burr and spotted medick growing along the coast of Hawkes Bay. It has since been seen feeding on lucerne. It does, however, use plants of many other species as 'staging posts' during migrations or as resting places during winter. Adult weevils will eat small quantities of the leaves of white and red clover and Lotus in laboratory conditions, but only in the absence of lucerne. The larvae consume the roots and nodules of lucerne (and burr medick).
Damage
When there are few weevils in a paddock, the only evidence of their presence is the occasional U-shaped notch in the edge of a leaf, usually about 3 mm along the margin. This is perhaps the 'normal' situation. Moderate infestations can be recognised when leaves have many notches eaten out of them, giving them a scalloped appearance. In heavy infestations the leaves are skeletonised.
Larvae too cause damage, because of their propensity for attacking root nodules. Symptoms of a heavy attack are, therefore, the same as for nitrogen deficiency, i.e., stunted yellow plants. Signs of damage on the leaves are a clue that the soil around the roots should be inspected for larvae and pupae.
Distribution
Sitona weevils are strong fliers, and are able to locate isolated lucerne paddocks with apparent ease. It is unlikely that any lucerne paddock in New Zealand can be considered immune from colonisation by this species.
Life cycle
| Egg |  |
| Grub | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
To understand the life cycle of sitona weevil in the South Island one must remember that the larvae depend on young lucerne roots for their food and shelter. The female weevils lay their eggs at random. When the larvae hatch they burrow into the soil and search for lucerne nodules. The young larva digs into a nodule and eats the central core. When it gets too big for its 'burrow' it digs out and finds more nodules on which to browse. The larva will eat roots if it cannot find nodules. All underground stages are found within 100 mm of the soil surface.
Thus, even though females are able to lay eggs from April to December, the cycle cannot be completed unless nodules and roots are growing. Lucerne plants shed most of their nodules during late autumn, and do not grow new ones until the following spring. Because of this August is the earliest month in which larvae are found in Canterbury. The August larvae turn into pupae and then adults; eggs are laid from October onwards. The progeny of this spring generation start to turn into adults in mid December, but their ovaries and testes do not develop until the end of the following March. Some surviving weevils of the spring generation often continue to lay eggs during summer, but the larvae and adults from these eggs are of little economic importance. Overwintering females start laying eggs in April, but it is doubtful whether any eggs survive. At this time of year the soil cools down, and root nodulation virtually ceases until spring.
Many adults migrate during late March and early April. There is also a smaller migration when the weevils of the overwintered generation emerge around Christmas time. Most hide in leaf litter or in cracks in the soil during summer, and some feed sparingly on lucerne foliage.
Sitona weevils can live for up to a year, and a female can lay perhaps as many as 2000 eggs. Accordingly, large populations can build up quickly when circumstances are favourable to the larvae and adults.
In the North Island there is apparently only one generation each year. Larvae are found in September and October. Only adult weevils can be found in November, but they disperse during summer and reappear in autumn.
J. A. WIGHTMAN