Spodoptera litura
Spodoptera litura, otherwise known as the tobacco cutworm or cotton leafworm, is a nocturnal moth in the family Noctuidae. S. litura is a serious polyphagous pest in Asia, Oceania, and the Indian subcontinent that was first described by Johan Christian Fabricius in 1775. Its common names reference two of the most frequent host plants of the moth. In total, 87 species of host plants that are infested by S. litura are of economic importance. The species parasitize the plants as larvae through vigorous eating patterns, oftentimes leaving the leaves completely destroyed. The moth destroys economically important agricultural crops and decreases yield in some plants completely. Their potential impact on the many different cultivated crops, and subsequently the local agricultural economy, has led to serious efforts to control the pests.
S. litura is often confused with its close relative, Spodoptera littoralis. These two species are hard to discriminate between because the larvae and adult forms are identical. Inspecting the genitalia is the most certain way to tell the two species apart.
Description
Sex differences
Morphology
There are slight but obvious differences in morphology between males and females of S. litura that allow for the easy differentiation of the two sexes. Male forewing length is while female forewing length is slightly larger and measures. The orbicular spot on the forewing is also more pronounced in the males.Differences in food regulation
Regulation of macro nutrient input differs between males and females. Experimental results show that when S. litura are presented with two nutritionally complementary diet options, one rich in protein and a second rich in carbohydrates, females tend to consume more protein than males while no differences in carbohydrates exist. Body utilization of the macro nutrients differed as well. Females were very efficient at converting the protein consumed into body growth and mass, reflecting the bodily requirements to produce eggs. Males, on the other hand, were more efficient at depositing lipid from ingested carbohydrates. This fits in well with the migration patterns associated with mating. Males usually go out to find females during mating season, so the lipid deposits are thought to be energy reserves that will help the males in preparation for the migration.Similar species
Spodoptera litura and Spodoptera littoralis are very closely related species. Discriminating between the two species can be difficult because the larvae and adult forms look identical. In fact, these two species are so similar that previous records that have claimed the presence of S. litura in areas such as Russia, Germany, and the UK may actually have been referring to S. littoralis. Since both species are polyphagous, taking note of the host plant is not helpful in correct identification. The only way to properly differentiate between the two is by inspecting their genitalia. In S. littoralis, the ductus and ostium bursae are the same lengths while in S. litura, they are of different lengths. In males, the juxta have characteristic shapes for each species.Range
S. litura is the most common in South Asia. However, its natural range extends from the Oriental and Australasian areas to parts of the Palearctic region as well. The countries with the most widespread population of S. litura include but are not limited to China, Indonesia, India, Japan, and Malaysia. The range of S. litura has also extended into non-indigenous regions through international trade. Moths in their egg, larvae, or pupae stages can be present in the soil, flower, or vegetation that are being transported across various regions. Pupae especially can be moved long distances, provided that they are not crushed, because of the relatively long pupation period.Habitat
S. litura is a general herbivore and takes residence on various plants. The lower and upper limits of habitable temperatures are, respectively. Therefore, it is well suited for tropical and temperate climate regions. As caterpillars, S. litura can only move short distances. However, adult moths can fly up to a distance of for a total duration of 4 hours. This helps disperse the moths into new habitats and onto different host plants as food sources are depleted.Life cycle
Although the length of a life cycle varies slightly throughout the different regions, a typical S. litura will complete 12 generations every year. Each generation lasts about a month, but temperature causes slight variations: life cycles in the winter tend to be slightly more than one month, and life cycles in the summer tend to be less than a full month.Egg
Eggs are spherical and slightly flattened. Each individual egg is around 0.6 mm in diameter with an orange-brown or pink color. These eggs are laid on the surface of leaves in big batches, with each cluster usually containing several hundred eggs. Females have a typical fecundity of 2000 to 2600 eggs. However, experiments have shown that high temperatures and low humidity are inversely related to fecundity. When laid, the egg batches are covered with hair scales provided by the female, which gives off a golden brown color. Egg masses are in total diameter, and eggs will hatch 2–3 days after being laid.Larva
Larvae body length ranges from to. The larva is variable in color based on age. Younger larvae tend to be a lighter green while older ones develop to a dark green or brown color. A bright yellow stripe along the dorsal surface is a characteristic feature of the larvae. The larvae also have no hair. Newly hatched larvae can be found by looking for scratch marks on leaf surfaces. Since S. litura is nocturnal, the larvae feed at night. During the day, they can usually be found in the soil around the plant. There are six instar stages, and by the last stage, the final instar can weigh up to 800 mg.Pupa
Pupation lasts around 7 to 10 days and takes place on the soil near the base of the plant. The pupa is typically long, and its color is red-brown. A characteristic feature is the presence of two small spines at the tip of the abdomen that are about long each.Adult
Adult moths are on average long and have a total wingspan of. The body is a gray-brown color. The forewings are patterned with dark gray, red, and brown colors. The hindwings are grayish-white with a gray outline. The mean female longevity is 8.3 days while for males it is 10.4 days.Mating
There is no mating activity on the first night that the moth emerges. The second night, however, accounts for about 70% of the matings. This night marks the maximum activity. Females mate an average of 3.1 times while the males have a mating average of 10.3. During copulation, males transfer a mean of 1,052,640 sperm per mating. Eggs during mating are laid in a cluster covered with hair from the female's abdomen. This acts as a protective layer from parasites predating on eggs. Since S. litura is a nocturnal moth, all reproductive activities occur during the scotophase. These reproductive activities include calling, courtship, mating, and oviposition. Several studies have pointed out that the female lifespan decreases after mating. The reasons for this are still not fully known. Several possible explanations include physical injuries from the male genitalia or the male accessory gland secretions that force females to commit more resources to reproduction instead of on herself.Male accessory glands
s are a reproductive evolutionary strategy adopted by males to gain higher fertilization. MAGs contain many different kinds of molecules including carbohydrates, lipids, and proteins. When MAGs are transferred from the male to the female during copulation, it exerts a wide range of effects on female post-mating behavior. One of these effects include suppressing female receptivity to future matings by reducing their sexual receptivity or sexual attractiveness. Experiments have shown that females exposed to MAGs do not engage in mating call behavior the night they are exposed to the secretion. A successful mating that resulted in fertilized eggs led to an even longer break from sexual receptivity.Mating also has an effect on stimulating egg production and ovulation. This phenomenon may also be a result of the mechanical stimulation of male genitalia during copulation. However, studies have shown that MAG secretions are necessary for the maximum stimulation of the eggs. As a result, female longevity is negatively correlated with the number of eggs laid because a large portion of resources end up being used for the development of eggs instead of on herself.
Pheromones
In sexually reproductive animals, recognition and attraction of potential mates can occur in the form of pheromones. In moth species, pheromones are produced by the females by pheromone glands and are released to attract males of their own species. Accurate recognition of compatible mates is essential for reproductive success because failure to do so will come with steep costs: wasted time and energy, higher risk of predation, and reduction of viable offspring. Therefore, there is a strong selection for correct mate recognition signals that maximize reproductive fitness. Both S. litura and S. littoralis share the same 11 components that make up their pheromones, with -9,11-tetradecadienyl acetate acting as the major component.There is an inverse relationship between pheromone concentration within the bodies of females and the calling behavior of a female. This is because pheromones are released during female calling. It has been previously stated that the male accessory gland suppresses female calling and subsequently, re-mating. With calling suppressed, pheromone concentration builds up in the body of mated females. Therefore, when pheromone glands are analyzed, mated females will have a higher titre than virgin females. It is important to note that this result is different from previous studies on other insect species.