Etiella zinckenella Treitschke Lepidoptera Pyralidae

Natural History

Distribution. Limabean pod borer is found throughout the world, but is most common in the tropics and subtropics. It occurs in the warmer regions of the temperate zone, but is absent from cold climates such as northern Europe. In North America, limabean pod borer is widespread in the western United States, from Washington to southern California and east to Colorado and Texas. Limabean pod borer is known from certain eastern states, such as Florida, North and South Carolina, and Maryland, but is not considered to be a serious pest in the East. It is a problem, however, in the Caribbean, including Puerto Rico. In Canada, though limabean pod borer has been detected, it is not considered to be a field pest. It was first observed in the United States in California, in 1885, but its origin is unknown.

Host Plants. This insect limits its attack to plants in the family Leguminosae. Vegetables damaged by lima bean pod borer include cowpea, faba bean, snap bean, lima bean, pea, and pigeon pea. Other legumes such as lupine, Lupinus spp.; rattlebox, Crotalaria sagit-talis and C. incana; locoweed, Astragalus antiselli; and milkvetch, Astragalus trichopodus, also serve as hosts. In California, severity of limabean pod borer damage is inversely related to the distance between crops and wild hosts, particularly lupine (Stone, 1965). In Puerto Rico, Crotalaria spp. are important alternate hosts (Segarra-Carmona and Barbosa, 1988).

Natural Enemies. Limabean pod borer is not heavily parasitized in North America. Several native egg and larval parasitoids (Hymenoptera: various families) have been detected in California and Washington, but none are effective. Numerous parasitoid species are known from Europe (Parker, 1951; Stone, 1965), but attempts to introduce parasitoids into California and Puerto Rico during the 1930s were unsuccessful (Clausen, 1978).

Life Cycle and Description. The number of generations varies according to the weather. The complete life cycle requires at least 60 days, and often considerably longer during cool weather. In California, one generation usually occurs during March-June on lupines or other wild host plants, and then another 2-4 generations occur during June-December on crop plants or perennial lupines. Starting in about July, some of the mature larvae diapause in their cocoons. The proportion of larvae entering diapause in July is small, only about 8%, but the proportion increases to 53% in August, 89% in September, and 100% in October and later. Pupation of overwintering larvae occurs from January to March. Emergence of adults from overwintering larvae begins in March, but it is protracted.

  1. The egg is oval and measures about 0.60.7mm long and 0.3-0.45mm wide. The egg is white when first deposited, but turns pink and then gray as the embryo develops. Duration of the egg stage is about 15.4 days, but varies from 5 to 33 days depending on temperature. The eggs are deposited singly or in small groups of up to 12. They are deposited on the flower, stems, or pod petiole. Estimates of egg production vary widely. Laboratory-reared moths often produce only 50-90 eggs, whereas field-collected moths may produce about 140-260 eggs. The latter values are probably much better estimates of fecundity.
  2. The larva is white or cream at hatching, and measures only 1 mm long. Young larvae immediately bore into a bean pod and develop internally. The entrance hole into the pod usually heals, leaving no indication that a larva was feeding within. There are five instars, with mean head capsule widths of about 0.15, 0.35, 0.65,1.05, and 1.60 mm, respectively. Duration of the instars is about three, three, three, three, and four days, respectively. The mature larva measures

Limabean pod borer larva.

Limabean pod borer larva.

12-17 mm long and is pinkish or tan. The head and pronotum are yellow with black markings. The larva has five pairs of prolegs in addition to three pairs of thoracic legs. Duration of the larval stage is usually about 35 days, but it may vary from 13 to 65 days. At maturity, the larva eats through the wall of the pod, exits, and drops to the soil. (See color figure 79.)

  1. The mature larva burrows into the soil to a depth of 1-5 cm and spins a cocoon. The soil particles adhere to the cocoon, so if dug from the soil the cocoon is an elongate cylinder of soil measuring 15-20 mm long and 6-8 mm wide. The duration of the pre-pupal period is variable, but during the summer months it is typically 8-24 days. During the winter, of course, it is greatly prolonged, because this is the overwintering stage. The pupa, which eventually develops in the cocoon, is 8-10 mm long and 2.5-3 mm wide. Usually, it is amber or light brown. Duration of the pupal period is about 36 days (range 16-101 days).
  2. The adult is a small brownish-gray moth, with a wingspan of 24-27 mm. The most distinctive features are the forward-protruding mouthparts, a characteristic feature of pyralid moths, and a broad white band along the leading edge of the forewings. There is also a transverse yellowish band slightly anterior of the mid-point of the wing. Copulation commences about 24 h after emergence. The pre-oviposi-tion period is usually 4-6 days, and adult longevity is about 10-12 days.

The biology of limabean pod borer was described by Hyslop (1912b), Abdul-Nasr and Awadalla (1957), and Stone (1965). Rearing procedures were outlined by Hattori and Sato (1983).

Damage

The larvae feed on buds and blossoms, and burrow into the pods of legumes to feed on the developing seeds. They typically feed on only a portion of a seed and then move on to attack adjacent seeds. Silk and fecal material accumulate in the pods. In the case of

Etiella Zinckenella
Adult limabean pod borer.

small pods, larval feeding usually causes the pod to drop from the plant, but large pods remain attached. Larvae sometimes leave one pod and move to another to continue feeding, especially following pod drop. Market standards often cause loss in excess of the direct feeding injury by larvae, because even low levels of damaged pods or beans are considered to be undesirable. Formerly, this insect was considered to be an important pest in the western United States, but with the introduction of modern insecticides it has assumed minor status. In tropical climates, however, it remains a serious pest of beans and soybeans.

Management

  1. Light traps can be used to sample populations of moths. Also, a sex pheromone has been identified and used successfully to trap moths in the field (Toth et al, 1989).
  2. Residual insecticides provide good control of borers. As the larvae feed internally, it is essential that the insecticides be on the vegetation at the time moths are ovipositing and eggs are hatching. Protracted emergence of adults and multiple overlapping generations often necessitate numerous applications of insecticides. Bacillus thuringiensis is not usually recommended for this insect.

Cultural Practices. Because larvae overwinter in the soil beneath legumes, tillage can reduce emergence in the spring. Autumn plowing to a depth of at least 20 cm is recommended. Early season planting is also helpful because the crop can reach maturity before pod borers attain high densities.

Host-Plant Resistance. Host-plant resistance is a viable option for some crops. Several pigeon pea and soybean cultivars have been shown to exhibit considerable resistance to attack (Cruz, 1975; Armstrong, 1991; Talekar and Lin, 1994).

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