Nun moth (Lymantria monacha)

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Kolk A. and Starzyk J. R. 1996.The Atlas of Forest Insect Pests. The Polish Forest Research Institute. Multico Warszawa. 705 pp.

Contents

Occurrence

The nun moth is distributed throughout Europe, however a major damage is caused in Central Europe (Poland, Germany, Czech Republic, Austria, Romania, Byelorussia, etc.). Usually it occurs in lowlands. In highlands it is observed up to 700 m above sea level. Occasionally it may spread up to 1400 m above sea level. In Poland it occurs in Scots pine monocultures growing in poor site conditions.

Host Plants

Outbreaks of the nun moth are often observed in the Scots pine and Norway spruce stands. However, majority of conifers (except the common yew) and broad-leaved tree species (except alders, the horse-chestnut, black locust, ashes, black cherry etc.) may also be its host plants. The nun moth does not attack the black pine, eastern white pine, mountain pine, silver spruce, tremble aspen, plums, etc.

Morphology

Adult females are much larger than males. They are 15-20 mm long with 45-55 mm wingspan. Adult males are 12-15 mm long with 35-45 mm wingspan. Male antennae are comb-shaped, while female antennae are thread-like. The forewings of both sexes are white with wavy dark bands across. The underwings are brownish-gray. However a color of moths vary much, from white to dark forms. Eggs are round-shaped, grayish-brown of about 1 mm in diameter. The newly hatched caterpillar is 3-5 mm long and about 0.5 mm wide. Young caterpillars are dark with tufts of hair of various length growing from numerous teats. Newly hatched caterpillars have also long hair that allow them ballooning with a wind. It disappears after the first molting. Caterpillars molt 5-6 times. The head capsule width of the second instar caterpillar is 0.9-1.2 mm, about 2 mm of the third instar, 3 mm of the fourth instar, 4 mm of the fifth instar and about 5 mm of the sixth instar. Only after the third molting get caterpillars their characteristic color. They have grayish-yellow head with black and brown spots. There are two lines of teats with tufts of hair on the body sides and light spots on the third, seventh and eight segments. Pupae are 15-20 mm long, green at the early beginning and then are getting dark brown or black metallic with tufts of white hair. Male pupae are much smaller than female pupae.

Biology

Swarming starts in July through August, sometime to mid-September. The adults fly at night. Males emerge earlier than females. Males fly quite long distances to find calling virgin females. A sex pheromone may attract males from a distance of about 500 m. During the day the moths usually stay on lower parts of tree trunks. A male can mate with a few females. Mated females stop emitting the pheromone and deposit eggs in bark crevices under scales or lichens. A female lays about 200-250 eggs on the average, 20 up to 100 eggs per egg mass. The nun moth overwinters as the caterpillar in the egg. In late April or early May (at the temperatures of about 10-15 degrees C) young caterpillars hatch and stay in groups for a while. Then they climb up to the crown and feed on young needles and male inflorescence. Caterpillars start feeding on old needles only after the first molting. Old caterpillars consume only the basic parts of needles, while the upper parts fell down. The caterpillar development takes about 40 to 80 days. They pupate on tree trunks or in crowns, occasionally on plants of vegetation cover.

Damage

The nun moth is the most serious defoliating insect pest of the Scots pine, Norway spruce and some other tree species. One caterpillar can damage about 300 Scots pine needles or 1000 Norway spruce needles during its development. The spruce tree defoliated more than 50% usually dies. The Scots pine is more resistant to a single defoliation than other coniferous species. Nun moth outbreaks are usually pandemic.

Preventive measures

Planting multi-species stands, active protection of undergrowth, birds, ants in primary outbreak foci.

Forecasting

Counting of female moths on trees is used to forecast the nun moth density in the next year.

Control measures

Biological (based on bacterium Bacillus thuringiensis) and chemical (chitin synthesis inhibitors or pyrethroids) may be used to control the nun moth.

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