:JSIR DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Lemon tree borer life cycle

CLEARWATER, J. R. 1981: Lemon tree borer, Oemona hirta (Fabricius), life cycle

DSIR Information Series No. 105/33

ISSN 0077-9636

Identification The adults (photo A and front cover drawing) are medium-sized (15-25 mm) slender beetles with very long antennae. The body is sparsely covered with orange hair; there are smaller, dense patches at the back of the head and surrounding the bases of the antennae and a small, round spot where the two elytra (wing covers) and thorax meet. Fine transverse ridges ornament the prothorax (the segment behind the head). Males are smaller than females, but can be reliably distinguished only by examination under a microscope. This beetle flies' well, usually in the early evening (7-9 p.m.) and morning (5-7 a.m.), when mating occurs. It may enter houses, and if caught will stridulate (squeak) faintly by nodding the head and thorax. This movement rubs a knob on the rear of the prothorax over a patch of very fine ridges just forward of the elytral orange spot.

The white eggs (photo B) are large (2.0-2.2 mm), and have a fine surface pattern. They are laid singly in leaf/stem junctions and cracks in the bark. Dead twigs on the outer parts of the tree are preferred. Fresh pruning scars are also used.

The larva (photo C) bores directly from the egg into the wood, where it excavates long tun­nels with - sho.rt side branches through which frass (droppings) is ejected. When fresh the frass is semi-liquid and orange, but it dries to resemble fine sawdust. The larva grows up to 35 mm long, and is pale cream with orange gut contents. The mandibles (biting jaws) are short and black. The true legs are very small, but the· 1arva is able to move very rapidly with the help of dorsal and ventral body ridges.

The pupa (photo E) is 20-25 mm long. Its cell consists of a short length of tunnel blocked with two plugs formed from short strips of wood. The long antennae are apparent at this stage, folded into a hook-shape above the legs. Short, black spines on the abdomen allow the pupa to move around the cell with a screw-like motion.

Host plants All commercial citrus varieties are attacked, though the borer prefers navel oranges to tan­gelos. Grape, apple, almond, gooseberry, tama­rillo, cherry, fig, peach, pomegranate, walnut, plum, blueberry, and macadamia are also attack­ed. Shelter-belt species such as Hakea and poplar support populations of borers that may invade orchards. Mahoe (Melicytus) appears to be the principal native host. Some damage to gorse is the only known benefit from this insect.

Damage Young larvae cause small twigs to die; the resulting cluster of dead leaves is very noticeable in late summer. Major damage can result from the tunnels of larger larvae, with branches break­ing under wind pressure or fruit load. Branches may be girdled and die. Typical damage is shown in photo D.

Distribution The lemon tree borer is native to New Zealand, and is found throughout the country.

Life cycle At least 2 years are required for the development of this insect. The larvae hatching from the new season's batch of eggs, laid from September, achieve moderate growth (up to 15 mm) and bore approximately 150 mm during their first summer. Activity is reduced over winter. October sees a sharp increase in boring activity and much frass is produced when the larva attains its full size. ' Pupation begins in June. Adults remain in the cell for several days until their integument ('skin') hardens. They are not sexually mature until 4 days have passed. The adults may live for 2 months; peak flight activity occurs in October and November.

J. R. CLEARWATER

Department of Scientific and Industrial Research, Wellington, New Zealand, 1981. DSIR Information Series No. 105/33

LEMON TREE BORER (Oemona hirta) LIFE CYCLE

ADULTS LARVAE DAMAGE PUPAE

:hart prepared by B.S.Eykel © Crown copyright

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