Accidental introductions of pests from overseas have posed some major threats to New Zealand agriculture, especially in recent years. New Zealand has increased its efforts to detect potential pests before they become established.
Argentine stem weevil
The Argentine stem weevil (Listronotus bonariensis) arrived in New Zealand in 1927 and has become a major pest of ryegrass, estimated to cause annual production losses of $250 million. The weevil can be found throughout New Zealand and generally produces two or three generations a year. Adults are 3 millimetres long, light grey to dark brown, with a hard body. They feed on leaves, and lay their eggs in the stems of grass tillers (sideshoots). Larvae kill plants by burrowing into these tillers, eating out the central part.
Black field cricket
The black field cricket (Teleogryllus commodus) is a problem in some North Island areas. These insects shelter in cracks in the ground which have opened up in summer. They eat the foliage of various pasture plants, and grow to about 25 millimetres in length.
Tasmanian grass grub
Aphodius tasmaniae originates in Australia, where it is known as the black-headed cockchafer. In New Zealand it is called the Tasmanian grass grub. These small and shiny black beetles fly between January and March, and are attracted to fresh dung. The larvae dig tunnels in the soil, surfacing to graze on foliage, particularly clover.
Also damaging to clover is the clover flea (Sminthurus viridis) from Australia, which is a springtail that jumps readily when disturbed. Their numbers can rapidly increase in localised areas when the climate is favourable in spring and autumn. They cause serious damage to clover by grazing on soft leaf tissue.