Clover root weevil
In 1996, established populations of clover root weevil (Sitona lepidus) were found at two sites in the North Island. Over the next few years this pest spread rapidly, and in 2006 was reported in the South Island. Adult weevils feed on white clover leaves, making notches around the edges. Early-stage larvae do considerable damage by eating the nodules on clover roots where rhizobacteria fix nitrogen. Older larvae feed on roots. The cost of this pest to the pastoral industry is estimated at $300 million per year.
African black beetle
The African black beetle (Heteronychus arator), another major pest of grasses, was first recorded in New Zealand in 1937, and is thought to have arrived from Australia rather than from its native South Africa. The species became established in the northern half of the North Island, then spread down the east coast into Hawke’s Bay and down the west coast as far as Taranaki. Adult beetles are 15 millimetres long and reddish-brown, changing to shiny black as they mature. They feed at the base of grass tillers, causing most damage to seedlings. The larvae do the most harm, by eating the roots of grasses in summer.
The root aphid (Aploneura lentisci) originates in the Mediterranean region and, like the pasture mealybug, sucks grass sap and produces a white wax. It is found throughout New Zealand. This insect has been given little attention until recently, but is now thought to contribute to failing ryegrass in pasture.
Attempts by farmers in the 1970s to use special pastures such as lucerne were hampered by pest problems, despite lucerne being resistant to grass grubs. Larvae of the white-fringed weevil (Graphognathus leucoloma), which was first recorded in 1944, and the sitona weevil (Sitona discoideus), first noted in 1974, feed on lucerne roots. Lucerne productivity and persistence were further affected by invasions of three aphid species, and lucerne is no longer widely farmed.
Parasitic nematodes are not insects, but they also damage the roots of clover and ryegrass. The clover cyst nematode Heterodera trifolii is found in most parts of New Zealand. The larvae penetrate the roots, where the hardening of a cuticle around female nematodes causes cysts to form.
Two species, Paratylenchus and Pratylenchus, feed on the root systems of grasses. Damaged roots lose the ability to absorb nutrients and water, resulting in diminished plant growth. The nematodes are invisible to the naked eye, so they often go undetected.