Spodoptera littoralis

Spodoptera littoralis is often confused with Spodoptera litura due to similar larvae and adult physical appearances. A bright yellow stripe along the dorsal side of the larvae is characteristic of S. litura. However, due to the variability in markings, the only certain way of distinguishing between the two species is by comparing the genitalia.[4] Specifically, ductus and ostium bursae are known to be the same length in S. littoralis females but different in S. litura females.[7] Similarly, it was reported that Prodenia ornithogalli is an American representative of S. littoralis as forms of the two species are extremely close and related. The main difference between these species is that P. ornithogalli is much darker in color and has sharper markings.[8]

There are many reported plant species and families that S. littoralis larvae feeds on. Specifically, the range of host plants on which S. littoralis feeds covers over 40 different plant families and at least 87 different plant species. S. littoralis larvae prefer to feed on young leaves, young shoots, stalks, bolls, buds and fruits. Some of these host plants are also known to affect larval growth. For example, larvae that fed on castor oil leaves had shortened larval and pupal duration, but those that fed on sweet potato leaves had prolonged larval and pupal duration.[11]

S. littoralis is reported to be polyphagous. Few of the economically important crops that are damaged by S. littoralis include Ipomoea batatas (sweet potato), Solanum lycopersicum (tomato), Zea mays (corn), Triticum aestivum (wheat), etc. Some of the other plants/crops that are also damaged by this species include Capsicum (peppers), Chloris gayana (rhodes grass), Hibiscus mutabilis (cottonrose), etc.[4]

Oviposition refers to a process in which animals lay eggs. There are three different oviposition sites of S. littoralis females. Of the three, young leaves are particularly preferred by the females as they serve as food sources for the hatching larvae. Eggs are also often deposited on fully expanded leaves or mature leaves but larvae hatching from these leaves move to feed on young leaves. Larvae that fed on young leaves or fully expanded leaves were found to have significantly faster growth rate compared to others that fed on mature leaves.[12]

Female moths lay a batch of eggs that ranges from 20–1,000 eggs on the lower side of a host plant's leaves. After the eggs are laid, female coats the eggs with scales from its abdomen. These spherical eggs show white to yellowish color and are 0.6 mm in diameter. These white-yellow color of the eggs turn black just before hatching due to the presence of black larval head near the transparent shell.[1][4]

Spodoptera littoralis larvae

Larvae body length ranges from 40–45 mm. The larvae is usually hairless and varies in color. It has been noted that new born larvae has grayish or dark-green body color whereas the developed larvae has brownish or yellowish body color. Dark and light longitudinal bands along with two dark semi-circular spots on the back are noted characteristics of the larvae. The newly hatched larvae up to third-instar mainly feed on the lower side of the leaves but the later instar larvae feed on both sides. S. littoralis larvae in general are known to feed in the dark. It also has been observed that larvae up to their third and fourth-instar usually remain in their host plant but fifth and sixth-instar larvae leave the host plant during the day and climb back up at night.[1][4]

The pupa is around 20 mm long and cylindrical in shape. Initially, the pupa is green colored with reddish abdomen but it turns reddish-brown within few hours. Pupation usually occurs 3–5 cm deep in the solid ground under soil. The duration of pupation is around 5–6 hours.[1][4]

Adult moth has a greyish-brown body that is around 15–20 mm long. The wingspan of the adult moth ranges from 30–38 mm. The forewing of the adult moth has brownish color with paler veins whereas the hindwing is more lightly colored (greyish-white). There are also oblique white bands on the forewing.[4] The adult moth appears at night and live about 5–10 days.[13]

Copulation refers to sexual behaviors observed in animals. Most adult moths mate on the day that they emerge from pupation. Duration of copulation ranges from 20 minutes to two hours. Studies revealed that age difference between the male and female moth is crucial in determining reproductive capacity, adult lifespan and egg quality.[4] The most effective age difference that increased egg fertility was four days difference between the male and female. When a four-day old male mated with a fresh born female, egg fertility was observed to be high. However, females laid most eggs when it mated with a male who was one day older.[15]

Pheromones refer to chemical factors secreted by an organism that effect the social behavior of other organisms that receive them. The pheromone secreted by females for mate calling is (Z,E)-9,11-tetradecadienyl acetate. As most matings occur at night, females producing largest amount of pheromone was observed 2–3 hours into scotophase, a dark phase of light cycle. The amount of pheromone in these females ranged from 6–8 ng. Lowest amount of pheromone was excreted few hours before the dark phase ended.[16] Other than its function to attract mates, female-produced pheromones may also synchronize male circadian rhythm.[17]

Courting is a behavior that displays the attempt of one sex of the species to attract the other sex for copulation. Antennae serves an important function in adult moth courtship. A male answering a female mate-call fully extends its antennae while flying over the female. A sign of rejection in females is noted by rapid wing flicks, whereas lifted wings, curved abdomen and withdrawal of pheromone in females are signs of female's acceptance for copulation. Once male and female are attached during copulation, male move its head downwards. Males without antennae did not answer female mate-call, which resulted in lack of mating. Females without antennae seldom mated but all matings were reported to be abnormal.[18]

Flight in insects refer to a process in which an organism moves from one place to another with the usage of its wings. S. littoralis can fly up to distances of 1.5 km over a four-hour period. Since the species is nocturnal, its peak flight activity is usually between the hours of 20:00 and midnight.[4] The flight activity for the moth can extend up to 10 days while an adult. Research has also shown that wind direction seems to have an effect on the moth's flight activity.[13]

Antenna are paired appendages attached to an arthropod's head that usually has a sensory function. Antenna in S. littoralis has an important function in facilitating mating. Experiments showed that antennal lobe of adult antenna are responsible for detecting different types of sensory inputs that are necessary for the adults to respond to external stimuli. Specifically, antennal lobe are responsible for identifying olfactory and mechanosensory stimulus.[19]

Diapause is defined as a delay in development caused by adverse external environmental factors. Mature larvae dig down the soil where it forms a cocoon to go under pupation. Larvae in its pupal stage can go through diapause but it is vulnerable to continuous frost and cold weather. However, it is generally known that S. littoralis larvae do not go through diapause.[20][21]

Because of S. littoralis' polyphagous behavior, the species has been damaging to many economically crucial crops such as cotton, tomato, maize and various other vegetables. Most of the damages are a direct result of extensive larvae feeding. Often, larvae will either create large holes in the leaves, only leaving the big veins of the leaves, or bear into the bud for consumption.[22] Most of the plants or vegetables that are attacked by S. littoralis are unsuitable for consumption as the damages are too severe. Of different types of host plants, cotton has been one of the main targets of S. littoralis. The species feed on cotton leaves, flower buds, fruiting points, and bolls, leaving the plant unsuitable for any further usage. Such damage of cotton plant is most prevalent in North Africa, especially in Egypt.[4] As a result, EPPO has assigned S. littoralis as A2 quarantine pest.[2] In the United States, S. littoralis has been listed as an exotic organism with high invasive risk.[3] Since 2004, there have been 65 reported incidents of S. littoralis that were present at various U.S. ports of entry.[23]

Due to S. littoralis' devastating effect on crops, numerous attempts have been made to counteract the species' dispersion and activities. These preventative methods are mainly divided into biological and chemical methods. Biological methods include using parasitoids or predators to control moth reproduction and dispersion.[4] Usage of bacteria such as Bacillus thuringiensis in combination with bacterial endochitinase has also been utilized recently to control many bacteria-resistant strains of S. littoralis larvae. Studies revealed that the synergistic toxic effect of Bacillus thuringiensis and bacterial endochitinase successfully resulted in reduced larval weight.[24] However, direct usage of these biological methods are yet not legal.[4] Similarly, chemical insecticides including organophosphorus, synthetic pyrethroids have been widely used to control S. littoralis.[4] Recent studies also revealed that insect growth regulators such as azadirachtin from neem tree are effective in controlling S. littoralis larvae as azadirachtin-treated larvae had overall higher mortality rate as well as reduced growth rate due to increased deformity and growth disruption.[25]