Plutella xylostella (L.)
Taxonomic placing: Insecta, Holometabola, Lepidoptera, Plutellidae.
Common name: Diamondback moth.
Geographical distribution: Plutella xylostella occurs wherever crucifers are cultivated, but does not overwinter in temperate zones. Adults are dispersed over long distances at a rate of up to 1000 km per day, and have thus become widely distributed. Although the pest is usually believed to be of Mediterranean origin, a South African provenance has recently been suggested, based on the range of its parasitoids there and on the diversity of cruciferous plants in the Cape region. CIE map # 32, 1967.
Host plants: The pest feeds on those members the family Brassicaceae that contain mustard oils and their glucosides. Crops attacked by the larvae include cabbage (Brassica oleracea var. apitata), cauliflower (B. oleracea var. botrytis), broccoli (B. oleracea var. italica), turnip (B. rapa pekinesis), brussels sprouts (B. oleracea var. gemmifera), kohlrabi (B. oleracea var. gongylodes), radish (Raphanus sativus) and others. The larvae also feed on wild crucifers.
Morphology: The adult moth is grey with three pale triangular markings on the inner edge of each forewing. When the wings are folded, these form a diamond pattern, hence the pest’s common name. The adult wingspan is about 15 mm. The neonate larvae are grey with yellow head capsules, turning light green with dark brown heads at a later stage. They bear five pairs of prolegs, whose posterior pair protrudes from the tip of the abdomen, forming a distinctive V-shaped structure. The larvae are about 10 mm in their last (fourth) instar. The yellow-brown pupae develop in a loosely-woven white silk cocoon either attached to the host leaves or lying in the surrounding leaf litter.
Economic importance: Feeding by the larvae on the growing tips of crucifers causes stunting and distortions that make the heads unmarketable. Feeding damage to leaves is characterized by the removal of all tissue except for veins and the upper epidermis, leaving distinctive transparent patches or “windows”. These later form holes in the leaves as the dead tissue tears. The most significant damage is to young plants; feeding on the outer leaves of more mature cabbage causes little economic loss. The diamondback moth is among the most injurious insect pests of cabbage and other crucifer crops throughout the world. The destruction of indigenous natural enemies by the widespread use of broad-spectrum insecticides and the year-around cultivation of crucifererous crops are considered to be the main reasons for its status of a major pest in most parts of the world. The annual cost of managing P. xylostella worldwide is estimated to be about US$1 billion; in the Middle East, however, it is a less serious pest than in other countries with comparable climates. This may be due to the presence of efficient guilds of natural enemies, and to the fact that crucifers are grown only in winter in the Middle East.
Life cycle: Adults of the diamond moth begin their daily activity at dusk. Mating occurs at dusk on the day of emergence and females begin ovipositing soon after mating. Oviposition continues for up to four days, during which each female deposits up to 250 small oval yellow eggs, singly or in small patches, on the leaf surfaces of host plants. The eggs hatch into small, grey larvae, about 2 mm in length, which immediately commence feeding. The larvae generally remain on the under surface of leaves throughout their feeding period. When disturbed, they rapidly drop off the leaf and remain suspended at the end of a silken thread, behaviour that may offer protection from natural enemies. At the end of the fourth instar the larva spins a delicate open-network cocoon in which it pupates. The cocoons are attached at each end to the substrate, and may be found on the leaf surface of the host plant or among debris on the ground beneath the plant. Adult moths emerge in the afternoon and may live for 2 or 3 weeks in summer or winter, respectively. The duration of the life cycle varies with temperature. In general, development from egg to adult takes 10-14 days in summer, 4-5 weeks in spring and autumn, and up to two months in winter. Low temperatures and heavy rainfall account for high mortality in winter. The moth produces 10 annual generations in Israel.
Chemical control: The diamondback moth has developed resistance to nearly all synthetic insecticides used against it in the field, including Bacillus thuringiensis (Bt) formulations. Because of this, pesticide recommendations constantly change as new compounds come into the market and others become useless.
Biological control: In its pre-imaginal stages the pest is attacked by over 90 species of mostly hymenopterous endoparasitoids, especially members of the genera Apanteles, Cotesia and Diadegma semiclausum. A few dipterous endoparasitoids also attack the pest. Adult moths are eaten by generalist predators, such as birds and spiders. Effective pupal parasitoids are known from the genus Diadromus. Little is known about natural enemies of the diamondback moth in the Middle East. No attempts at the biological control of the diamondback moth have been undertaken in the Middle East to date, probably due to its minor pest status.
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Rami Kfir, ARC-Plant Protection Research Institute, Private bag X134, Pretoria 0001, South Africa.
Moshe Coll and Sharon Tam, Department of Entomology Faculty of Agricultural, Food and Environmental Quality Sciences The Hebrew University of Jerusalem; P.O. Box 12, Rehovot 76100, Israel Phone: 972-8-948-9153; FAX: 972-8-946-6768. e-mail: firstname.lastname@example.org