- Soybean

Soybean Soybean

Beet Armyworm

Insect

Spodoptera exigua


In a Nutshell

  • Larvae usually are dull green with many fine, wavy, light-colored stripes down the back. They have a dark spot on the side of the thorax above the second true leg. Larvae feed on both foliage and fruit and skeletonize foliage.

Symptoms

Initially young larvae feed in clusters on the underside of old leaves in the lower canopy. Larger larvae become more solitary and disperse over the whole crop leaving irregular holes on leaves. Mature larvae can defoliate small plants entirely or skeletonize leaves, that is, eat up all tissue except the veins. If they run short of leaves, the caterpillars may also attack pods, but stems are not on their diet. Generally, they feed at night, hiding during daytime in the ground or on shaded and damp parts of the plant. Young seedlings can be killed by their feeding activity of Spodoptera exigua, but older plants will possibly recover if the infestation is not too severe.

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Hosts

Trigger

The damage is caused by the larvae of the beet armyworm, Spodoptera exigua. This insect occurs in warmer regions of Asia, Africa, the Americas and Europe as well as in greenhouses in cooler climates. They infest various crops including cotton, beet and maize. Adult moths are of gray-brownish in color. Forewings are mottled brown and gray, with an irregular pattern and a light colored bean-shaped spot in their middle. Hindwings are gray or white in color, and with a dark line near the margin. Females lay eggs in clusters on the lower surface of leaves, covered by white or grayish hairs. The young larvae are greenish-brown and have dark longitudinal stripes on the back. Mature larva are green, with a marked yellow stripe on each flank and a wide yellowish-green band on their back.

Organic Control

One golden rule to reduce populations of S. exigua is to promote natural enemies. Flower bugs (Anthocoridae), (fire)ants, parasitic wasps (Hyposoter didymator), flies and spiders attack eggs or larvae. Entomopathogenic fungi, Bacillus thurigiensis, NPV and nematodes will infect larvae and adults. Botanical insecticides based on fresh Neem, lemongrass and ginger are also effective. Likewise, both eggs and young larvae can be controlled with the foliar application of 5% cottonseed oil. Pheromon traps can also be used to disrupt mating and inhibit or eliminate reproduction (up to 97 % efficiency).

Chemical Control

Always consider an integrated approach with preventive measures together with biological treatments if available. Insecticides are not recommended as they can kill beneficial insects and natural enemies of S. exigua, which in turn could actually lead to outbreaks of this pest. Moreover, this insect has shown a high capacity to develop resistance to many chemicals.

Preventive Measures

  • Plant resistant species with harder leaf tissues.
  • Adjust the sowing time to avoid peak populations.
  • Check your plants regularly for signs of the pest, especially in the late evening or early morning.
  • Use pheromone traps to monitor infestations.
  • Remove weeds and litter in and around the fields as they offer shelter and sites for breeding.
  • Dig a deep trench and fill it with water to drown the caterpillars migrating from adjacent fields.
  • Make a reasonable use of broad-spectrum insecticides, as these can kill natural enemies.
  • Plow and harrow your field thoroughly to expose larvae and pupae to predators.

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