Cabbage webworm

Taxobox
name = Cabbage webworm


regnum = Animalia
phylum = Arthropoda
classis = Insecta
ordo = Lepidoptera
familia = Pyralidae
genus = "Hellula"
species = "H. undalis"
binomial = "Hellula undalis"
binomial_authority = FabriciusThe cabbage webworm ("Hellula undalis") is a widely distributed webworm native to southern Europe or Asia that is destructive to cabbages and other vegetables throughout the Middle East, Asia and the Pacific, in eastern and southern Africa, and in the Gulf states of the United States. It is a major pest in most tropical and some temperate countries where brassicas are grown, ranging in Asia as far north as Japan. The insect was first identified in Italy.

Principal plant hosts are broccoli, cabbage and cauliflower. It also attacks Chinese cabbage, kai-lan (Chinese broccoli), Chinese mustard, daikon, eggplant, flowering white cabbage, mustard cabbage and radish.

Description

The eggs are oval and creamy white when freshly laid on the surface of leaves or on younger parts of the plant. At temperatures between 25-29° C, a single female lays as many as 150 eggs, which hatch in four or five days.

The pretty caterpillars are creamy white or dusty yellow, with broad light pinkish brown or purplish stripes, and have a black head. Mature caterpillars bear faint stripes and measure 15 mm when fully grown.

The pupae are shiny pale brown with a dark dorsal stripe. Pupation occurs in leaf tissue, in tunnels made by the feeding larvae inside the stem, and sometimes on shed leaves or other debris on the ground. The pupal period lasts seven days.

Adult moths are rather nondescript and gray, with small brown spots or specks.

Development of pest stages is greatly influenced by temperature. The upper threshold of development is about 98º F and the lower threshold is 68º F. The complete life cycle of the cabbage webworm varies from 17 to 52 days, with several generations occurring each year.

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Major damage occurs on young plants but the voracious caterpillars also feed on older plants. Damage is most severe between transplanting and the heading stage of cabbage, although the larvae are present in the field throughout the crop. On cabbage seedlings, extensive damage can occur when the caterpillars feed on the growing points (apical meristem) and the developing leaves, causing severe defoliation. Serious injury also occurs when they tunnel into the main stem. Damage at this stage of plant growth results in stunting, and sometimes the death of young plants. Injury to the shoot from a single larva can produce deformed plants and the formation of multiple small growing points or heads, which render the vegetable unmarketable. On older plants the larvae feed on leaves and by tunneling into leaf petioles.

Because damage tends to be hidden from sight, symptoms of imported cabbage webworm infestations are often not detected until plants appear stunted or deformed. However, when infested seedlings are examined closely, silken webbing and frass are easily spotted on the surface of the inner leaves and stalks, along with obvious feeding holes in surrounding parts of the plant. Sometimes the larvae tie leaves together with silk strands and feed within. At other times the larvae feed under a protective silk web containing an exit hole. On loose-heading cabbage plants, the larvae are often found without webs, feeding on young leaves. Webbing and fecal frass are characteristic of webworm damage, although the untrained observer could easily mistake silk webbing as the product of diamondback moth larvae or the imported cabbage worm. Positive identification can be made only if the insect is found.

Despite the importance of the webworm, information on its natural enemies is generally limited. In a study conducted in Malaysia, [ [http://www.springerlink.com/content/nq31610g2p66j57l Natural Enemies for the Cabbage Webworm, "Hellula undalis" (Fabr.) (Lepidoptera: Pyralidae) in Malaysia.] A. Sivapragasam and T. H. Chua, Population Ecology, Volume 39, Number 1 / June, 1997, Pages 3-10. Text on SpringerLink - Journal Article.] a major predator of the webworm was identified as a fire ant, "Solenopsis geminata", which foraged on the prepupae and pupae. These ants were found to be important in determining the density of webworms on cabbage and their within-generation survival. The caterpillars are also targeted by species of wasps known as ichneumon flies, though these parasitoids were not an important mortality factor for the webworms because they were usually present at the end of the crop season and their numbers were generally low. Pre-emptive spraying of the crop with chemical insecticides, applied once or twice weekly, is a common control method.

External links

* [http://www.extento.hawaii.edu/kbase/crop/Type/hellula.htm Hellula undalis.] Knowledge Master, EXTension ENTOmology, University of Hawaii.

* [http://www.infonet-biovision.org/default/ct/106/pests www.infonet-biovision.org – Cabbage webworm.]

References