Weevil (Clover Root Weevil ) Sitona lepidus
Species: S. lepidus
Binomial name: Sitona lepidus
Common names: Clover Root Weevil
Sitona lepidus is a species of weevil found in Europe. It has now spread to North America and New Zealand. Sitona lepidus was found in New Zealand pastures in 1996 in Waikato and Auckland, and by 2004 it had spread throughout the North Island. It was first discovered in the South Island early in 2006 where it is still spreading. It poses a threat to farm productivity since it damages the clover plant. Larvae are most the damaging stage destroying clover roots and root nodules. This reduces the plant's ability to fix nitrogen from the atmosphere and reduces plant growth. A reduction in nitrogen fixed also impacts on the growth of other pasture plants and leads to an increased requirement for nitrogen fertiliser to be applied.
Sitona lepidus adults are a mahogany-brown, 4-6 mm long with a short, blunt nose. Adult are most abundant in autumn and spring. They lay their eggs on the pasture surface and when these hatch the larvae move into the soil seeking food and shelter. These are legless, creamy white grubs, from 1-6 mm long and with a brown head. They may be found by digging into the root zone under white clover plants. Larvae are present throughout the year but are more abundant from late autumn to spring. The larvae are generally much smaller than grass-grub larvae, which can be common in pastures, and they lack the distinctive C-shape that characterise grass grub.
The combination of a favourable environment, lack of competition for an abundant food resource, high reproductive capability of the Sitona lepidus and a lack of natural enemies inevitably meant it was going to become one of New Zealand's most damaging pests of white clover. In the North Island there are two generations a year with adult populations peaking in early and late summer. It is not yet confirmed but in the south temperature may limit this to one. The adults can live for several months with the females laying up to several hundred eggs. Young larvae mainly feed on clover root nodules but as they mature they move onto the roots and stolons. Adults feed on clover leaves producing characteristic notching of leaf margins. Dispersal by flight occurs during the summer and autumn with the proportion of adults having flight or reproductive capability dependent on prevailing climatic and pastures conditions.
Sitona lepidus is a very proficient hitch-hiker in hay and vehicles and it is probably by this method that the spread in the South Island is currently occurring. The use of the parasitic wasp Microctonus aethiopoides as a biological control is under way in Southland, New Zealand.