Feeding on dead matter
Saprophagous species – those that feed on dead plants or animals – live in leaf litter, rotting logs and animal carcasses. They include members of the genus Saphobius or dung beetles, which eat decomposing vegetation, and the tiny, beach-dwelling Phycosecis limbata, which scavenges washed-up carrion.
Numerous species are fungus feeders, many of which are found in the forest – like the tiny, spore-feeding mildew beetles, and members of the Ciidae family. Pinhole borers (also known as ambrosia beetles) are weevils that grow fungus in galleries which they form by boring into wood. They inoculate the wood with spores they carry in specialised cavities. Both adults and larvae eat the fungi.
Predatory beetles include most species of ground beetles. Among the most voracious of these are the tiger beetles, such as the boldly-patterned common tiger beetle or pāpapa (Cicindela tuberculata), often seen on clay banks. The adults are fast runners, darting about to catch prey in their massive, sickle-shaped mandibles. The larvae, in contrast, sit anchored to their burrows – heads near the entrance, jaws open – waiting to seize any prey that blunder past.
Other predators are the rove beetles, identified by their shortened elytra (hardened forewings) and flexible abdomens.
A few of New Zealand’s beetles are parasitic, and in New Zealand two families – Bothrideridae and Rhipiphoridae – have species that parasitise wood-boring beetle larvae.
One of New Zealand’s most intensive beetle surveys was made in the Auckland suburb of Lynfield. Fifteen years of collecting from the beaches, back yards and bush yielded 982 species, 753 of which were unique to New Zealand. The study showed that most native beetles kept to remnants of native bush, and very few had adapted to urban areas.
The majority of beetles are herbivores – they only eat plants. They may eat leaves, stems or roots, or they may bore into wood. Dominant among the herbivores are the weevils, longhorns and leaf beetles.
Because they are under constant threat of attack, beetles have many ways to avoid capture. Ground beetles use chemical defences, and rove beetles and darkling beetles secrete foul smells to ward off predators. Some species are camouflaged, such as Pristoderus bakewelli, whose knobbly upper surface blends in with the surrounding bark. Others rely on mimicry. The longhorn beetle Drototelus elegans imitates the bold warning coloration of the parasitic wasp Xanthocryptus novozealandicus.
Beetle behaviour is poorly understood, although it is known that complex relationships exist. Some beetles can be social, such as ground beetles of the genus Megadromus, some of which tend their young. Stag beetles (Holloceratognathus passaliformis) have been found in native ant nests, but the nature of the relationship is not known. In other countries, many beetle species infiltrate ant colonies and prey on the ants.