Stem borers | ASEAN FAW ACTION

Palm Pests and Diseases

Spodoptera littoralis

Stem Borers

Spodoptera littoralis, the Cotton leafworm, is a highly destructive agricultural pest that can cause serious damage to a wide range of economically important crops belonging to more than 40 families. Host plants include wheat, maize, rice, sugarcane, soybeans, fruits, vegetables, ornamentals, orchards, and cotton. It is native to Africa and is widely distributed across the Middle East and throughout Southern Europe. There are reports of this insect being identified in India. It is important to remain vigilant for this serious pest in Southeast Asia.
For example, see India Zoological Annual Report 2022-2023 (2023) 2002-2023 citation

Pig 1. Larva and adult of Spodoptera littoralis
Source: CABI

Life Cycle and Appearance of Spodoptera littoralis

Eggs Spherical, somewhat flattened
Width x height is approximately 0.45 mm x 0.35 mm
Produce 20 to over 350 eggs per batch
Whitish-yellow in colour (changing to black just before hatching)

©MAARTEN VAN MERRIËNBOER/PPS, AALSMEER, NETHERLANDS

Larvae grow to 40-45 mm and are hairless
Variable in colour (blackish-grey to dark green, becoming reddish-brown or whitish-yellow)
The sides of the body have dark and light longitudinal bands
Spots on the first and eighth abdominal segments are larger than the others

Source: CABI

47-Pupa-of-Spodoptera-littoralis-dorsal-

Pupae are green with a reddish colour on the abdomen, turning dark reddish-brown after a few hours
Shape is cylindrical

©NPPO, The Netherlands

Colour: grey-brown
Body Length: 15-20 mm
Wingspan: 30-38 mm
Forewings colour: grey to reddish brown with paler lines along the veins
Hindwings colour: greyish white

©GEORG GOERGEN/IITA INSECT MUSEUM, COTONOU, BENIN

Damage Symptoms

On most crops, damage occurs due to extensive feeding by larvae, which can result in complete stripping of the plants. On cotton, leaves are heavily attacked, and bolls have large holes with yellowish-green to dark-green larval excrement protruding. Maize stems are often mined, and young grains in the ear may be injured.

Pig 2. The damage caused by S. littoralis infestation
Source: Koppert

Spodoptera littoralis vs other Spodoptera species

S. littoralis, S. litura, S. frugiperda, are important Noctuid pest species known for their highly polyphagous nature. Due to their polyphagous nature, they can be found on almost all types of commodities of plants or above-ground plant parts. More than 100 species of host plants are reported, many of which are economically significant (EPPO/CABI, 1997; Robinson et al., 2010).

Spodoptera littoralis

Spodoptera Frugiperda

Spodoptera litura

Distribution

Refer to the EPPO Global Database. (https://gd.eppo.int/)

Identification

It is most reliable to conduct morphological identification at the adult stage. While experts familiar with this genus can sometimes identify species based on the morphology of immature stages, especially larvae, within a specific context, species-level identification is generally only possible at the late larval or adult stage. Due to variability and overlapping characteristics between species, it is not feasible to reliably identify all species of the genus Spodoptera morphologically. Molecular methods are necessary for accurate species-level identification at these stages.

Larvae

The larvae of S. littoralis and S. litura are nearly indistinguishable in appearance throughout most stages of their development. However, they can only be differentiated during the later stages of development.

Pig. 3. The 3rd instar of Spodoptera littoralis, interruption of the spiracular line faint. Photo M. vd Straten © NPPO, The Netherlands

Pig 4. The 3rd instar of Spodoptera litura, interruption of the spiracular line pronounced. Photos M. vd Straten © NPPO, the Netherlands

The larvae of S. litura can usually be distinguished from those of S. littoralis by a morphological feature. In S. litura, the dark lateral spot on the first abdominal segment extends laterally and strongly interrupts the spiracular line. In S. littoralis, the dark spot also extends laterally, but the interruption of the spiracular line is less pronounced.

Pig. 5. The last instar of Spodoptera frugiperda. Photos M. vd Straten© NPPO, the Netherland

The larva of S. frugiperda is very different from the other Spodoptera larvae. The fully grown larva is about 30 (Pogue, 2002) to 40 mm (EPPO, 2004) in length and variable colour, ranging from pinkish through yellowish, olivaceous, brown and dull grey to almost black. The head has a reticulate pattern and is variable in colour, from yellowish to very dark brown; the thoracic shield is the same colour as the head. Further detailed information on the larva of S. frugiperda, including a setal map, can be found at http://idtools.org/id/ leps/lepintercept/frugiperda.html (Gilligan & Passoa, 2014).

Adult

Nevertheless, the wing pattern is a first and handy way to distinguish the possible quarantine species from most non-quarantine Noctuid species, primarily when the specimen's origin is known. The general morphological identification of adults is also used.

S. litura colour and pattern are almost indistinguishable from S. littoralis. They are reniform spotlight brown outlined in white
S. frugiperda has some white and brown transverse line markings, and the ground colour varies from greyish brown to rust brown. This species is smaller than the other species that have the bold forewing markings
Male genitalia: S. litura large opening at the base of valve (similar to S. frugiperda)
The juxta is the most uncomplicated character of the male genitalia to distinguish between S. littoralis and S. litura because it is triangular in the latter (see Fig. 6)
Females of S. littoralis can be distinguished from S. litura by the ductus bursae, which are elongated in S. litura and much shorter in S. littoralis.