|O. hirta by Des Helmore|
The lemon tree borer (Oemona hirta) is a longhorn beetle endemic to New Zealand. Its larvae are generalist feeders, boring into the wood of a wide variety of trees, native and introduced. When citrus orchards were first established in New Zealand, this beetle started inflicting serious damage, and so gained the name "lemon tree borer".
The lemon tree borer is a large beetle, reaching from 15 to 25 mm (0.6–1.0 in) long. It has a slender body with elongated antennae. Adults are brown in colour, with pale yellow hairs on the elytra (wing cases). Females are larger than males, but have proportionally shorter antennae. A distinctive feature of O. hirta is the transverse wrinkling on the dorsal surface of the pronotum; this is especially distinctive on males. Larvae are a creamy white in colour, with large dark brown mandibles.
Life cycle and mating behaviour
Lemon tree borers have a long life cycle, averaging around two years. Around October, mature adults mate and lay single, large (2–2.2 mm (0.079–0.087 in)), white, waxy cylindrical eggs into the crevices or fresh pruning scars of living trees. Females can lay around 50 eggs during their adulthood. The eggs are mainly laid in twigs, but can also be found on larger branches and sometimes dead wood. The eggs hatch after 9–13 days, and larvae immediately start tunnelling into the wood, going first into sapwood and then heartwood. Larvae occur in low density, with usually only two being present per tree. They eat the wood, creating long tunnels with side galleries and holes for excretion of frass, and bore longitudinally into the stems, going towards the main stem or branch. Occasionally larvae will bore around a branch, causing girdling. The larval stage can last for one to two years, depending on the environment: larvae occurring in milder environments with a shorter winter period will pupate faster. Lemon tree borers can sometimes be found in dead trees, but prefer living trees as they require a certain level of humidity and nutrition to properly pupate to adulthood. The pupal stage lasts between 2–3 weeks, with pupae measuring 20 to 25 mm (0.8–1.0 in). Around mid-June to mid-October the pupae form cells in their host trees, from which they pupate into adults. Newly emerged adults will remain in their pupal cells until their integument has hardened. Once they emerge, they become sexually mature around four days later (although this varies depending on environmental conditions). Adults feed on pollen and nectar of plants. They are mostly nocturnal, foraging and mating at night. During the day, they hide amongst vegetation.
Habitat and distribution
O. hirta is common in New Zealand, present on both islands. It has been collected from sea level up to altitudes of over 1,200 m (3,900 ft), as well as several offshore islands such as Kapiti Island and Mokopuna Island. This beetle is highly polyphagous: the larvae can feed on a large variety of host trees. Host trees include native species such as rangiora (Brachyglottis repanda), tauhinu (Cassinia), and Senecio rotundifolia, as well as introduced European plant species. Pest risk assessments have shown that lemon tree borers, despite their name, can attack over 200 plant host species from 81 families. This allows this beetle to have a large host range and spread widely. Adults are good flyers, with most flying activity occurring in the early morning and evening when most mating occurs. Their flying ability also allows them to colonise favourable habitat, and spread far and wide.
This species is of great economic importance and has become an agricultural pest due to the diet and habit of the larvae, which bore into a wide range of host trees, both native and exotic. Some important crop species are lemon (Citrus spp.), apple (Malus spp.), almond (Amygdalus spp.), chestnut (Castanea sativa and C. crenata), persimmon (Diospyros kaki), cherry (Prunus spp.), walnut (Juglans regia), and grape (Vitis vinifera). As such, quarantine regulations have to be observed when exporting these crops overseas to reduce the risk of O. hirta being accidentally introduced. O. hirta is a threat to the forestry industry, commercial fruit crops, and ornamental garden shrubs. The first symptom of an infestation by larvae is the wilting of foliage and dieback, but this may not always be apparent immediately. Trees will have excretion holes measuring 1–3 mm (0.039–0.118 in), with frass visible on the outside.
This beetle can be accidentally introduced by movement of plants for planting. It was first intercepted in the United Kingdom in 1983 and again in 2010 on Wisteria plants at two different plant nurseries.
Control of Oemona hirta
Lemon tree borers are notoriously hard to control. Physical control can be done by removal of infested wood, but this is very labour intensive. Chemical control of wood borers in general is limited, due to their hidden life stages. In New Zealand, the best way to control the lemon tree borer is through preventive and curative methods.
The parasitic ichneumonid wasp Xanthocryptus novozealandicus is native to New Zealand. The female hunts wood-boring beetles, including lemon tree borer, and lays an egg on the larva by piercing the wood with her ovipositor. X. novozealandicus prefers attacking larvae in their second year of growth.
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