Spodoptera exigua (Hübner)
(Sometimes called Laphygma exigua)
Common names: Beet armyworm; lesser cotton leafworm.
Geographical distribution: This species occurs outdoors in the warmer regions of Europe, Asia, Africa and the Americas, and in greenhouses in colder climates. CIE Map #302, 1972.
Morphology: The moth is 12-14 mm in length, forewings yellow-grey, with two rust-colored markings and white margins with black dots. Hindwings white to pale grey, with darker margins. The young larvae are pale green, later becoming greenish-brown, darker in crowded populations, with a dark lateral stripe, but no hairs or spines. Final larval length about 30 mm.
Host plants: Highly polyphagous.
Life cycle: A female lays up to about 1000 eggs, in batches of 50 to several hundreds in several groups. The egg cluster is covered and protected with greyish hairs. The majority of clusters are sited on the lower surfaces of leaves, the eggs completing their development within 2 days in warm weather. Initially the larvae are gregarious, remaining together even during day. Older larvae are solitarious and disperse over the whole plant. They feed at night, hiding during day in the ground or in shaded and damp parts of the plant. The mature larva moves into the ground to pupate within a smooth cell. The threshold of development is at 10°C and about 450 day degrees are required to complete a generation. Moths usually live for 10 days, and in the Middle East they raise 7-9 annual generations.
Economic importance: The larvae initially feed on the lower leaves, later dispersing over the whole plant, feeding nocturnally on entire leaf tissues except the main veins. They tend to burrow holes through thick areas of plants, moving from plant to plant. Occasionally the larvae migrate in large numbers (hence “army worm”) and can destroy entire crops within a few days. They damage beets, clover, cotton, soybean and other crops, especially when irrigated. In the Mediterranean region of Turkey the pest attacks maize sown in July. They eat grapes as well as tomato and eggplants fruits, attack and injure ornamentals, industrial, field and medicinal crops.
Monitoring: Populations can be monitored with sex pheromone traps. Frequent visual sampling for damage and larvae during summer is nedcessary because the moths often invade from surrounding crops or weeds.
Horticultural Methods: Removal of weeds reduces alternative hosts. Appropriate planting dates may reduce pest activities. For example, most damage can be avoided if maize is sown in southern Turkey during mid-June, before the pest’s population peak.
Mating disruption: Saturation of the atmosphere with the pheromone around susceptible crops may greatly reduce mating and pest fecundity.
Chemical control: Insecticide Resistance to many insecticides is a major problem in the management of this insect. Pyrethroids, Insect growth regulators, preparations of Bacillus thuringiensis Berliner (Bt) and of NPV have been reported to control S. exigua in the Mediterranean region. Other options include some oils, insecticidal soaps and neem.
Biological control: Many natural enemies attack this pest. In Turkey they include several hymenopterous and tachinid parasitoids. Of the former, Hyposoter didymator was the most prevalent, attacking 40.5% of the larvae. In Israel Meteorus versicolor was predominant in corn fields and Chelonus inanitus in alfalfa plots. Various predators, such as Anthocoridae and ants attack the larvae, as do some Entomopathogenic fungi. Entomopathogenic nematodes can infect both larvae and adults of the beet armyworm.
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Eigenbrode, S.D. and Trumble, J.T. 1993. Antibiosis to beet armyworm (Spodoptera exigua) in Lycopersicon accessions. HortScience 28: 932-934.
Gelernter, W.D. and Federici, B.A. 1986. Isolation, identification, and determination of virulence of a nuclear polyhedrosis virus from the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae). Environmental Entomology 15: 240-245.
Prabhaker, N., Coudriet, D.L., Kishaba, A.N. and Meyerdirk, D.E. 1986. Laboratory evaluation of neem-seed extract against larvae of the cabbage looper and beet armyworm (Lepidoptera: Noctuidae). Journal of Economic Entomology 79: 39-41.
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Sertkaya, E., Bayram, A. and Kornosor, S. 2004. Egg and larval parasitoids of the beet armyworm Spodoptera exigua on maize in Turkey. Phytoparasitica 32: 305-312.