Several species of introduced Pinus are grown as major tree crops in some highland regions of Papua New Guinea. Larger Pinus patula plantations for instance were established in the Eastern Highlands Province since this species is well adapted to the poor soil conditions of that particular location and, to a certain degree, can resist fire. The major pest is the Lymantriidae moth Lymantria ninayi that can severely defoliate Pinus during periodically occurring outbreaks. In the years 1975 to 1978 there was a major Lymantria ninayi outbreak at Lapegu plantation in the Eastern Highlands Province causing the repeated defoliation of Pinus patula over an area of 200 ha and resulting in the death of 50 ha and a loss of several millions of Kina. The calamity was terminated by the impact of a natural pathogen, a nuclear polyhedrosis virus (NPV, Baculoviridae). Lymantria ninayi outbreaks occur at intervals of seven to nine years. After this major outbreak there were several less severe attacks of minor economic importance. The most recent infestation started in 1997. Apart from Lymantria ninayi there are other species of this genus that defoliate Pinus spp., ie. Lymantria rosa and Lymantria novaguinensis.

top row: Lymantria ninayi (female), Lymantria ninayi (male)
bottom row: Lymantria rosa, Lymantria novaguinensis

General Biology

The female Lymantria ninayi lay their eggs in patches on the bark of the host tree, usually under a branch. Thus the eggs are protected from rain. After hatching, the caterpillars feed on the foliage of the host, according to the caterpillar’s size. The younger ones feed on the new needles, the older caterpillars eat the older, mature needles. The larvae have a very ‘uneconomic’ way of utilising their source of food, because they just bite through a bunch of needles, so that the needles fall down and pile up under the tree. The larvae feed during the night. During the day they are hidden at the base of the host’s stem or in the litter under the tree. Lymantria ninayi has five larval instars. The sex of the young caterpillars can be already determined, since the males are smaller and have red dorsal marks. Once grown big enough, the caterpillars either attach themselves to the foliage or hide in the litter beneath the tree and turn into pupae. The pupae are housed in shelters assembled from needles woven together with a silken thread. The pupal shelter was opened up so that the chrysalis becomes visible. The adults are strongly sexually dimorphic. The large females have filiform antennae and are of much brighter wing coloration than the fairly small males with pectinate antennae. The adults are also nocturnal and rest on the bark of the host during day. The life cycle is completed in three to four months. The natural host of Lymantria ninayi is the yar tree Casuarina (Casuarinaceae).

(left) young male Lymantria ninayi caterpillar; (centre) older female Lymantria ninayi caterpillar; (right) opened pupal shelter made from needles that are tied together with a silk thread