Microtheca ochroloma Stal Coleoptera Chrysomelidae

Natural History

Distribution. This insect is a native of South America, where it is known from Argentina, Brazil,

Chile, and Uruguay. It was first found in the United States at Mobile, Alabama, in 1947. It is now distributed along the Gulf Coast from Florida to Louisiana.

Host Plants. The yellowmargined leaf beetle prefers plants in the family Cruciferae. It has been reported to damage cabbage, Chinese cabbage, col-lards, mustard, radish, turnip, and watercress. Occasionally, it has been found on potato. Ameen and Story (1997b) conducted choice tests with larvae and adults, and reported a preference rating of turnip > mustard > radish > collards > cabbage. It has also been collected from pepperweed, Lepidium virginicum; dock, Rumex sp.; and various clovers and vetch, but it is not clear whether the insects were feeding or just seeking shelter on these plants. Fecundity and longevity on several host plants were reported by Ameen and Story (1997a). They found that turnip was the most favorable host, with an average of 490 eggs produced per female, followed by radish (440), mustard (425), cabbage (271), and collards (199). Adult longevity varied from 68 days when fed turnip and collards to 105 days on radish. Surprisingly, there was not a significant relationship between longevity and fecundity.

Natural Enemies. This insect is inadequately studied, and no natural enemies are known.

Life Cycle and Description. The life cycle is poorly known, and no evidence of diapause has been reported. There appears to be a single generation annually, but the insect can complete its life cycle in less than a month under favorable conditions, so it is possible that more than one generation occur during mild Gulf Coast winters. In Florida, adults remain active throughout the winter months. Damage is noted in the spring and early summer, when both larvae and adults can be found feeding on crucifers. A summer aestivation from mid-June to October has been suggested.

  1. The adults can begin copulation about six days after emergence, followed by oviposition in another 3-6 days. The eggs are bright orange, elongate, and deposited singly or in small groups on the foliage. Duration of the egg stage is 4-5 days.
  2. The larva is yellowish brown and covered with a fine layer of hairs. The head capsule is dark brown or black. Larvae are gregarious during early instars, but eventually become solitary. There are three instars, lasting 3-4, 3-4, and 5-6 days, respectively. (See color figure 118.)
  3. The larva spins a loose net-like pupal case on foliage, and pupates within. Duration of the pupal stage is 5-6 days. Newly formed adults remain within the cocoon for about two days before escaping.
Adult yellowmargined leaf beetle.

Adult. The adult is about 5 mm long, and predominantly dark bronze or black. The peripheral edges of the elytra, however, are marked with a margin of yellowish or brownish, which is the basis for the common name of this insect. The elytra are also each marked with four rows of deep punctures. (See color figure 119.)

Biological information on yellowmargined leaf beetle was given by Chamberlin and Tippins (1948), Woodruff (1974), and Oliver and Chapin (1983).

Damage

The adults and larvae feed on foliage, making small holes and feeding on the leaf margin. Damage to cruciferous crops is reported in the spring. In the United States, it is a pest only in small or home garden plantings. In South America, it has been observed to damage crops grown on a commercial scale, but only before the advent of modern insecticides.

Management

Generally, this is a minor pest that does not require control except on an isolated plant or planting. If the insects are observed to be numerous, foliar insecticides can be applied to provide suppression.

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