In general, insect pests are less of a problem to the forestry industry than they are for agriculture and horticulture.
Conifers such as Pinus radiata can be attacked by a native looper caterpillar, Pseudocoremia suavisi, which also feeds on native trees. Epidemics occurred in pine forests in Canterbury in the 1950s and 1960s, and in Douglas fir in the 1970s. Caterpillar populations have been reduced by a naturally occurring virus and improvements in silviculture.
Moths and butterflies
Tomato fruitworms and leafrollers occasionally defoliate pine trees. They are most problematic in tree nurseries, where weeds provide an extra food source.
Adult beetles such as the mānuka beetle (Pyronota spp.), grass grub (Costelytra zealandica) and Odontria species feed on a range of trees including pine during their flight season. The root-feeding larval stage of these insects can be pests of nursery trees, as are larvae of the black vine weevil (Otiorhyncus sulcatus).
The wood-boring wasp (Sirex noctilo), introduced from Europe, was once considered to be a major pest in pine plantations. By removing weak trees susceptible to attack and introducing parasitic wasps for biological control, the problems caused by this wasp have been eliminated.
Monterey pine aphid
In 1998 the Monterey pine aphid (Essigella californica) became established in New Zealand, and by 2000 it was widespread. Its main host in its native California is Pinus radiata, and it has damaged drought-stressed plants in Australia. In 2008, after 10 years presence in New Zealand, this insect has had no visible effect on tree health.
Wood- and bark-boring insects can be a problem in logs after felling. Adults of Hylastes ater (black pine beetle) and Hylurgus ligniperda (golden-haired bark beetle) tunnel and lay their eggs in fallen logs, and the larvae feed on phloem (plant tissue that transports food around the plant). When adult H. ater emerge from the logs, they eat the roots and collars of seedling pines and kill them. This beetle is also a carrier of sapstain fungi, which causes wood discolouration and is a significant economic problem in New Zealand.
Eucalyptus trees are mainly colonised by Australian insects, with the number of species found having increased over the last century. In 2008, 26 specialist eucalyptus feeders had been reported, although relatively few had become pests.
Eucalyptus tortoise beetle
The eucalyptus tortoise beetle (Paropsis charybdis) was first recorded in New Zealand in 1916. The beetles and their larvae eat eucalyptus leaves and can severely defoliate new growth. Some eucalypt species are less susceptible to attack by this insect, and biological control has had some impact on populations.
Gum tree weevil
Other eucalypt defoliators are the gum-tree weevil (Gonipterus scutellatus) and the larvae of the gum emperor moth (Antheraea eucalypti). Sap-sucking pests such as a psyllid, Ctenarytaina eucalypti, and the scale insect Eriococcus coriaceus can stunt and distort tree growth.
Two native species – the caterpillars of Anetus virescens and the pinhole borer Platypus apicalis – drill into living trees, and can damage the timber.
Parasites and predators have been introduced to control these insects. Another pest, the leaf blister sawfly (Phylacteophaga roggatti), was first recorded in New Zealand in 1985. Four years later the mass rearing and release of the parasite Bracon phylacteophagus reduced numbers of the leaf blister sawfly.