European settlers brought predatory species such as rats, mice and hedgehogs, which found beetles to be easy prey. As with New Zealand’s native birds, the large, flightless beetles are particularly vulnerable. Some species have vanished altogether, such as the large weevil Tymbopiptus valeas, known only from fossil specimens. Others have become locally extinct, like the Cook Strait click beetle (Amychus granulatus), which is confined to predator-free island refuges, although fossils show that it was once present on the mainland.
Forest clearance by Māori and European settlers has reduced species diversity. Deforestation has decreased the numbers of many species, and there is little doubt that many more species have been lost. Although widespread forest clearance has now stopped, the continued fragmentation of forests may cause the larger, flightless, and less common beetles to become extinct.
Few native beetles have adapted to pastureland and towns – introduced species tend to dominate these environments.
Saving threatened species
Legal protection has been given to 17 of the most endangered beetle species, and the Department of Conservation has identified many more that are threatened. Conservation efforts are hampered by the poor knowledge of beetle life histories and by difficulty identifying many species.
Nevertheless, there is a growing awareness of the importance of conservation. In what was probably a world first, a reserve was set up in 1979 to protect the rare Cromwell chafer beetle (Prodontria lewisi). This species is restricted to the tussock-covered inland dunes of Central Otago, many of which have been lost to pasture, and it is now found only in the 81-hectare reserve.
Some species presumed extinct have been rediscovered, like the Pitt Island longhorn (Xylotoles costatus) and the Canterbury knobbled weevil (Hadramphus tuberculatus).
About 350 species of beetle are introduced. Many arrive almost unnoticed in New Zealand and have little impact. A few beneficial species have been deliberately introduced for the biological control of pests such as the aphid-feeding 11-spotted ladybird (Coccinella undecimpunctata). Others are pests, such as pasture-eating sitona weevils (Sitona discoideus) and house borers (Anobium punctatum), infamous for eating through the wood of houses and furniture.
The whirligig beetle (Gyrinus convexiusculus) is a self-introduced Australian species which lives in ponds and lakes in Waikato and Northland. These beetles use their broad, paddle-like legs to scoot about on the water’s surface in search of prey. Their eyes are divided into upper and lower parts – one for viewing above the water, the other for below.
Native beetle pests are relatively rare. One is the grass grub or tūtaeruru (Costelytra zealandica) – the glossy brown adults are often seen flying around lights on warm summer evenings. Damage caused by the root-feeding larvae, which can number several hundred per square metre, make this one of New Zealand’s most serious pasture pests.
Beetles can play a vital role in helping forest litter to decompose, and as predators of pest species. They can be useful indicators of forest health. Beetles are also used to estimate historical environmental conditions – fossil beetles indicate past types of forest, which are evidence of a changing climate.