Plutella xylostella

Scientific Name Plutella xylostella L.
Common Names English: Diamond back moth; German: Kohl-Schabe, Kohlmotte; French: Teigne des crucifères; Dutch: Koolmotje; Spanish: Palomilla dorso de diamante
Description The adults are grayish-brown, with a body length of about 7-9 mm (0.27-0.35 inch) and a wingspan of about 12-15 mm (0.47-0.58 inch). At rest, the folded wings show a marking shaped like three diamonds, which is formed by the visible parts of a broad band on the forewings.
The larva is pale green or gray and up to 15 mm (0.58 inch) long. This caterpillar is agile and it will twist about and drop to the ground at the slightest contact.

Biology

Damage

P. xylostella attacks exclusively cruciferous plants as they contain mustard oils, which are essential feeding stimulants for this insect. The first instar larvae bore into the leaves and mine inside the mesophyll. Older caterpillars feed on the undersides of leaves, often leaving the upper epidermis intact, which leads to a windowpane-like effect. The last instar larvae are especially voracious. Leaves can be completely skeletonized, everything except the veins being consumed.
When young cabbage plants are attacked, injury to their growing points may cause their heads to deform or to fail to develop entirely.
Older heads, even if only superficially damaged, will often be unmarketable due to numerous holes in the leaves and soiling with frass. Pupae or larvae present inside the florets lead to rejection of the produce by consumers.
In rape or mustard, the caterpillars feed on leaves, buds, flowers and pods; heavy infestation will lead to stunting and in severe cases plants may die off.
Problems with the diamond back moth are much greater during hot and dry weather. At the other extreme, heavy rain may sometimes drown more than half of the young larvae.

Lifecycle

Over a period of up to ten days, each female deposits on average 150 eggs (occasionally twice as many), in small groups of up to eight in depressions or close to the veins on the lower leaf surfaces.
Like all activities of the adult moths, egg-laying occurs at night.
The uncommonly broad ecological range of P. xylostella is reflected in its widely-varying rates of development. Two to eight days after oviposition, the larvae hatch. Their four instars last altogether from 6 to 30 days, then the caterpillar builds a loosely-woven silk cocoon. After a prepupal period of 2-15 days, pupation occurs. Three to fifteen days later, the adult emerges. Accordingly, the number of generations per year varies from two during the warm season in cool, temperate regions to continuous breeding, with up to ~30 overlapping generations, in the tropics.
Plutella xylostella overwinters as pupae attached to plant parts. Whether they can overwinter in cool climates is not quite clear. It is certain, however, that the insects immigrate anew each year into countries with hard winters, often in very great numbers, from warmer regions. While they are bad active flyers, they can be carried by the wind several thousand kilometres in a few days.
When the adults are disturbed on a windless day, they only flutter around jerkily for a moment, staying close to the plant cover. Alarmed larvae typically wriggle backwards rather quickly and then drop from the plant, descending by spinning a silken thread, which they will use again to climb back upwards after some seconds.

Occurrence

Additional Crop Information

Virtually all cruciferous crops.

Agricultural Importance

The diamond back moth is found wherever crucifers are grown, and is particularly damaging in the tropics and subtropics.
P. xylostella is probably the most serious pest of cruciferous crops worldwide, due among other things to its high reproductive potential and voracity. Fields with cabbage crops can be totally destroyed.

Control

Integrated Crop Management

If possible, plant and rotate crops simultaneously in a larger area in order to arrange for crucifer-free periods, and thus to disrupt the otherwise continuous population build-up.
Be sure to destroy harvest residues. Check seedlings before transplanting.
In some countries, intercropping, undersowing and the use of trap crops have shown promising results.

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