The eggs of beet armyworms appear woolly and green. They are shaped like a cone with a round bottom. Larvae vary in color from green to yellow, and they become darker and develop stripes along their sides and dark dots on their bodies as they mature. The larvae do not have hairs or spines on their bodies, and they look smooth. The larvae grow from 1 mm to between 25 and 30 mm in length. Later in its development, a larva forms a brown pupa that has dark brown markings and is 15 to 20 mm long. When the moth emerges from the pupa after metamorphosis, it is brown, gray, and white, with brown and gray forewings and white hindwings. Adults have an average wing span of 25 to 32 mm.
Adult males and females look similar. Only the females produce sex pheromones (odors released from their bodies) to attract mates. The sex of a pupa can be determined by examining its anatomy using a magnifying lens. (Capinera, 2006; Chen and Chang, 1989)
Spodoptera exigua, an insect known as the beet armyworm, originally came from southeast Asia. As adult moths, beet armyworms are extremely mobile. Their ability to fly long distances has enabled them to greatly expand their geographic range outside of Asia. In fact, beet armyworms have become an invasive species and crop pest throughout the world. Beet armyworms are found in temperate and subtropical regions around the globe, including China, Japan, the United Kingdom, Scandinavia, the United States, South America, Africa, and New Zealand. They have been studied extensively in the United States and China. Beet armyworms were first recorded in the United States in Oregon in 1876. Since then, they have spread through most of the southern states, reaching as far north as Maryland and northern California. Armyworms can reproduce multiple times in a single warm season, cycling through several generations in one year; thus, they often spread to higher latitudes during the summer into regions where they cannot survive during the winter. Researchers still do not agree on where the insects overwinter every year. Experts disagree about whether some adults migrate to low enough latitudes during the fall to survive the winter. (Feng, et al., 2003; Ruberson, et al., 1994; Zheng, et al., 2011a; Zheng, et al., 2011b)
Beet armyworms are found throughout temperate zones and tropical areas. Their habitat ranges from crop and grass fields to temperate and tropical forests. Adult armyworms lay their eggs toward the bottom of plants, on the top and bottom of leaves, which prevents the eggs from drying out or being eaten by predators. Beet armyworms lay their eggs on many different plant species, including vegetables, grasses, weeds, flowering plants and other crops. When the eggs hatch, the larvae completely consume their host plants, leaving only the roots and leaf veins. The environmental conditions and the host plant species have a large effect on the development of the larvae. A warmer climate and nutritious host plants allow the larvae to grow, develop, and pupate more quickly.
After a larva completes metamorphosis and emerges from the pupal stage as an adult moth, it flies to elevations where winds can carry it long distances for migration. Most moths glide at 200 m above the ground, but some have been known to fly above 500 m. If it is early in the season, adults may stay within the same region and reproduce in the spring or early summer. When the moths do not migrate before they reproduce, armyworm population densities can reach infestation levels, and infestations by this insect can lead to severe crop damage. On the other hand, if it is late in the season, the moths migrate to warmer regions or other countries. In warmer areas, armyworm offspring have a higher chance of survival because more food is available and temperatures are higher. Adults and larvae often are found together in forests and fields. Unlike the larvae, which cannot move very far, adults have a larger habitat because they can fly. (Feng, et al., 2003; Zheng, et al., 2011a)
After 3 or more days as eggs, beet armyworm larvae hatch and then eat their host plant, feeding on the foliage and fruit. The larvae also feed on nearby plants. The length of time in the larval stage varies, because the development time depends on the temperature of the environment. When larvae are exposed to higher temperatures, they develop to the adult stage more quickly. Larval development time also depends on the species of host plant, which ranges from vegetable crops to common weeds and plants. The larvae grow quickly on pigweed and slowly on cabbage. Beet armyworms feed throughout the entire larval stage and grow through 5 instars (stages of larval growth). As they grow through each instar, larvae get bigger, and their coloration gets darker.
When a larva is ready to pupate, it drops to the ground and digs an underground chamber 1 to 2 cm deep and weaves a silk cocoon. The pupa can tolerate temperatures as low as 4 ºC in its underground cocoon during the overwintering (hibernation) stage. The silk of the cocoon insulates the insect against cold weather. The adult moth completes metamorphosis when it emerges after about 6 to 7 days. The sex of this insect can be determined at the pupal stage by using a magnifying lens to examine its anatomy. (Capinera, 2006; Zheng, et al., 2011b)
As soon as it emerges from its pupa, a beet armyworm moth is reproductively mature, and males find females by detecting sex pheromones (odors secreted by the females to attract mates). Adult males can detect female sex pheromones from long distances. They can migrate to breeding areas that sometimes are thousands of kilometers away. Adult males and females have 4 to 10 days to find mates before they die. The average male mates with 5 females, but the number ranges from 1 to 11. Females also mate multiple times, but the exact number is unknown. Older armyworms mate longer and less frequently than younger armyworms. Mating lasts from 30 to 60 minutes the first time, but takes as long as 180 minutes in older moths. As a result, younger moths are more successful at finding mates than older individuals. (Luo, et al., 2003)
When beet armyworms reach adulthood, they reproduce in the spring and summer until they die. Female moths usually start laying eggs 3 to 7 days after emerging from the pupa, and they lay a total of 300 to 600 eggs in their lifetime. The eggs are laid in clusters of 50 to 150, and each cluster is laid in a separate place, either on the same plant or on different plants, to maximize survival. After laying their last egg clusters, adult females live for just a few more days. (Capinera, 2006; Takai and Wakamura, 1995; Zheng, et al., 2011a)
When beet armyworm larvae eat highly nutritious plants, the larvae benefit by growing faster and larger. The identity of the host plant has an important effect on the growth and survival of the larvae. Adult females choose to lay more eggs on high-quality plants. For example, females consistently lay more of their eggs on pigweed than on cotton, bell pepper, sunflower, or cabbage.
At high temperatures, the egg stage lasts about 3 days, the larval stage lasts 18 to 20 days, the pupal stage lasts about 5 days, and adult moths live 4 to 10 days. Thus, the shortest lifespan of beet armyworms is about 27 days. On average, though, most beet armyworms live for about 30 to 40 days. (Capinera, 2006; University of Arkansas Division of Agriculture, 2006)
A beet armyworm larva does not tolerate cold very well, but it can overwinter (hibernate) as a pupa. It digs into the soil to form a chamber that is held together by thin strands of silk. It prefers to form a pupation chamber in the soil right underneath the canopy of its host plants. During the larval stage, beet armyworms are gregarious (they gather in large groups), especially during the first and second instar stages. Large numbers of larvae have been observed feeding together. As armyworm larvae mature, they become less gregarious. (Bradshaw, 2012; Capinera, 2006; Feng, et al., 2003; North Carolina Cooperative Extension Service, 1997; Zheng, et al., 2011b)
Beet armyworm larvae basically stay in one place through the pupal stage, but adults are highly mobile and can travel distances of up to 179 km.
Olfaction (detecting odors) is an important sense in beet armyworms. Adult females choose where to lay their eggs by smelling plants, and the moths use olfaction to find mates. To attract males, females release odors (sex pheromones) from their bodies, and researchers have discovered that the specific compound (Z)-9-tetradecen-1-ol is an important chemical that helps beet armyworms attract their mates.
Larvae also have sensory hairs that can sense the wing vibrations of potential parasites, such as parasitic wasps that burrow into the larvae. When they sense the vibrations, larvae stop moving until the danger is gone, or they may even fall from the plant to avoid being attacked. (Showler, 2001; Wakamura and Takai, 1990; Yong, 2008)
Beet armyworm larvae use chewing mouthparts to feed on leaves and fruit. When feeding on fruit, larvae make shallow, superficial wounds on the surface. The larvae mostly feed on the undersides of leaves, leaving behind small webs of silk. Beet armyworm larvae feed in such a way that leaves become skeletonized--the leaf tissue is all gone, and only the leaf veins are left, still in the shape of the leaf. Larvae also are known to weave the leaves of plants together using their silk, so that they form a temporary cocoon around themselves for protection when feeding.
As a species, beet armyworms can feed on more than 50 plant species around the world. The insect causes damage to crops like corn, alfalfa, peas, tomatoes, potatoes, legumes, soybeans, onions, lettuce, cotton, and tobacco, among others. The larvae also have been known to feed on ornamental plants such as roses, sunflowers, geraniums, and carnations. In addition, larvae have been observed feeding inside the flower buds of some species, which protects them from predators and insecticides. Sometimes beet armyworms even cannibalize each other, especially when food resources are low.
Beet armyworm eggs and larvae are especially vulnerable to predators. They are eaten by a few species in the groups Heteroptera (such as damsel bugs and shield bugs), Coleoptera (beetles), Neuroptera, and Hymenoptera, along with a few unidentified species of Arachnida (spiders). Most of these predator species are generalists that prey on many different organisms.
When threatened by ant predators, armyworm larvae defend themselves by regurgitating (throwing up). Ants are repelled by the regurgitant (the vomit, which is a unique oral secretion), and they stop their attack. When ants come into contact with the regurgitant, they begin to groom themselves intensively.
Larvae also can detect the wing vibrations of wasps and other parasitoids as the attackers approach. When these predators come near, armyworm larvae become motionless on the plant until the threat passes, or they drop from the host plant to avoid an attack. The most common parasitoids that attack armyworm larvae are wasps such as Chelonus insularis, Cotesia marginiventris, and Meteorus autographae and the tachinid fly Lespesia archippivora. (Alborn, et al., 1997; Capinera, 2006; Rostás and Blassmann, 2009; Ruberson, et al., 1994)
Beet armyworms are hosts to numerous wasp and fly parasitoids, such as Chelonus insularis, Cotesia marginiventris, Meteorus autographae, Hyposoter exiguae, and Lespesia archippivora. Parasitic wasps and flies can be attracted by the plants on which armyworm larvae feed. When the saliva of a beet armyworm larva contacts the leaf it chews, the host plant releases volatile compounds (chemicals in the gaseous state). The volatile compounds waft through the air and can be detected by parasitic organisms that use the compounds to find their beet armyworm prey.
Fungal diseases have been observed in beet armyworm populations, and the nuclear polyhedrosis virus is an important cause of mortality in larvae. (Capinera, 2006; Natwick, et al., 2012; Ruberson, et al., 1994; )
Beet armyworms are considered one of the worst agricultural pests in the world. Beet armyworm larvae consume many different types of plants, which can cause disastrous damage to crop production. The armyworm is a well-known pest of vegetables, flowers, and other commercial crops such as cotton and tobacco. Beet armyworm larvae feed in such a way that leaves become skeletonized--the leaf tissue is all gone, and only the leaf veins are left, still in the shape of the leaf. This leaf skeletonization decreases the value of ornamental plants and slows the growth of many crops. The damage leads to decreased yields and high economic losses for the agricultural industry. Larvae consume the seedlings, squares, and bolls of cotton plants, which also reduces yield and negatively impacts the textile industry. The holes that larvae create in tomatoes and lettuce can lead to rotting. Larvae also may leave feces in crops, remain in the crop, or cause significant scarring, all of which negatively affect crop yields. (Capinera, 2006; Natwick, et al., 2012; Zalom, et al., 2011)
Beet armyworms have no known positive economic importance to humans.
Currently, there are no efforts to conserve this species.
Wilber Hua (author), The College of New Jersey, Serge Zemerov (author), The College of New Jersey, Elizabeth Wason (author, editor), Animal Diversity Web Staff, Keith Pecor (editor), The College of New Jersey.
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