Spodoptera littoralis
(cotton leafworm)

The northernly distribution limit of S. littoralis in Europe corresponds to the climatic zone in which winter frosts are infrequent. It occurs throughout Africa and extends eastwards into Turkey and north into eastern Spain, southern France and northern Italy. However, this boundary is probably the extent of migrant activity only because although the pest overwinters in southern Spain, it does not do so in northern Italy or France. In southern Greece, pupae have been observed in the soil after November and the species overwinters in this stage in Crete. Low winter temperatures are therefore an important limiting factor affecting the northerly distribution, especially in a species with no known diapause (Miller, 1976; Sidibe and Lauge, 1977).

EPPO has listed S. littoralis as an A2 quarantine pest (OEPP/EPPO, 1981). CPPC, NAPPO and OIRSA also consider it to be of quarantine significance. It is a potential pest of areas where the average annual minimal temperature is not below -10°C. S. littoralis is already fairly widespread in Mediterranean countries and does not present a phytosanitary risk there. The most significant phytosanitary risk for S. littoralis is the possible introduction into glasshouses in most parts of Europe, where it could damage many ornamental and vegetable crops. Although control with insecticides is possible, there have been many cases of resistance and the lack of available biological control methods means that introduction of S. littoralis into glasshouses could necessitate insecticide treatments that could interfere with existing biological control of other pests (EPPO, 1997).

S. littoralis first appeared in UK glasshouses in considerable numbers in 1963. It was found that the eggs were being introduced on imported cuttings, especially chrysanthemums and carnations.

The host range of S. littoralis covers over 40 families, containing at least 87 species of economic importance (Salama et al., 1970).

In many of the published reports of host plants, it is difficult to distinguish between S. littoralis and S. litura but the tabular data refers entirely to records from the distribution area of the former. Both species are totally polyphagous (Brown and Dewhurst, 1975; Holloway, 1989).

Female moths lay most of their egg masses (20-1000 eggs) on the lower surface of younger leaves or upper parts of the plant (Khalifa et al., 1982). On cotton, the first three larval instars feed mainly on the lower surface of the leaves, whereas later instars feed on both surfaces. The larvae feed mainly in the dark, although this behaviour pattern may be less noticeable in early instars (Hassan et al., 1960). Pinhey (1975) recorded that >50% of the nocturnal larval population consisted of early instar larvae. In summer the majority of fifth- and sixth-instar larvae leave the plants during mid-morning until sunset, returning to climb the plant at night (Baker and Miller, 1974). Third- and fourth-instars rest on the plant and remain stationary unless overcrowded.

On pupation the fully grown larva pushes the loose surface of the soil downwards until it reaches more solid ground 3-5 cm deep. It then creates a clay 'cell' or cocoon in which it usually pupates within 5-6 hours (Pinhey, 1975).

Emergence of adult moths occurs at night and they have a life span of 5-10 days (Shalama and Shoukry, 1972). The reproductive capacity, egg facility and life span of moths are affected by the difference in ages between males and females. The highest ratio of egg fertility was obtained by mating between 4-day-old males with fresh females (Nasr and Nassif, 1978). There is also a correlation between the host plant and the longevity and fecundity of S. littoralis (Dimetry and Nadia, 1972). The majority of adults mate on the first night of emergence, copulation lasting for 20 minutes to 2 hours. Approximately 50% of mated females lay their eggs on the same night of mating, before sunrise (Hassan et al., 1960). Adults fly at night, mostly between 20.00 and midnight (Nasr et al., 1981). Flight activity is governed by atmospheric conditions, increases in relative humidity and decreases in air temperature inducing flight (Hassan et al., 1960). The flight range during a 4-hour-period can be up to 1.5 km (Salama and Shoukry, 1972).

The moths have chemoreceptors on the ventral surface of the tarsi and the distal portion of the proboscis. These are highly sensitive and respond to a certain number of sugars mainly present in nectar. Pheromones (comprising of tetradecadien-1-ol acetates) have been isolated and successfully used in traps (Kehat and Gordon, 1975; Campion, 1977).

The minimum constant temperature for normal development in all stages is 13-14°C. Resistance to cold generally increases through the larval stages and is greatest in the pupal stage (Miller, 1977). At 18°C, egg, larval and pupal stages last 9, 34 and 27 days, respectively. At 36°C, egg, larval and pupal stages last 2, 10 and 8 days, respectively. Data on survival and development at different temperatures are provided by Sidibe and Lauge (1977), Baker and Miller (1974) and Ocete Rubio (1984). Information on development on different host crops is given by Dimetry and Nadia (1972), Hirakly and Bishara (1974), Abdal-Fattah (1977), Zoebelein (1977) and Badr et al. (1983). Studies in Egypt indicate that there are seven overlapping generations of S. littoralis when feeding on cotton, and that there are three peak infestation periods (El-Shafei et al., 1981; Khalifa et al., 1982).

Natural enemies that have been observed in the field (Hegazi et al., 1977). Delvare and Rasplus (1994) described the pteromalid parasitoid, Spodophagus lepidopterae, from S. littoralis from Madagascar. Most work on the natural enemies of S. littoralis has been done in Egypt and Spain.

Generalist predators of S. littoralis include ladybirds, which feed on egg masses and young larvae, Paedeus fuscipes (staphylinid rover beetle), Orius albidipennis, Labidura riparia, Creontiades pallidus, Calosoma chlorostictum and Polistes gallicus.

Pheromeone traps can be used to monitor the incidence of S. littoralis (Rizk et al., 1990). Kehat and Dunkelblum (1993) found that the minor sex pheromone component, (9Z,12Z)-9,12-tetradecadienyl acetate in addition to the major component (9Z,11Z)-9,11-tetradecadienyl acetate was required for in order to attract males.

S. littoralis is often confused with S. litura, and the variability and similarity of the two species makes correct identification difficult and examination of adult genitalia is often the only certain method. For more information on morphological discrimination between the adult, pupal and larval stages of the two species, refer to Schmutterer (1969), Cayrol (1972), Mochida (1973) and Brown and Dewhurst (1975).

Although markings on larvae are variable, a bright-yellow stripe along the length of the dorsal surface is characteristic of S. litura.

On dissection of the genitalia, the ductus and ostium bursae are the same length in female S. littoralis, whereas they are different lengths in S. litura. The shape of the juxta in males in both species is very characteristic, and the ornamentation of the aedeagus vesica is also diagnostic.

An EPPO standard provides guidance for the identification of S. littoralis, S. litura, S. frugiperda and S. eridania (OEPP/EPPO, 2015).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Biological Control

Numerous studies have been carried out on possible biological control of S. littoralis. Parasitoids (braconids, encyrtids, tachinids and ichneumonids) and predators have been extensively documented. A nuclear polyhedrosis virus has been evaluated against S. litura (Elnagar and El-Sheikh, 1990; Jones et al., 1994), whereas fungi and microsporidia have also been recorded as pathogens. Parasitic nematodes such as Neoaplectana carpocapsae have also been evaluated. However, direct use of these biocontrol agents has not been commercailized. Treatment with Bacillus thuringiensis has been used (Navon et al., 1983), but only some strains are effective as S. littoralis is resistant to many strains (Salama et al., 1989).

Chemical Control

The chemical control of S. littoralis has been extensively reported, especially in relation to cotton in Egypt. Numerous organophosphorus, synthetic pyrethroids and other insecticides have been used, with appearance of resistance and cross resistance in many cases (Issa et al., 1984a; 1984b; Abo-El-Ghar et al., 1986). However, compulsory limitation of the application of synthetic pyrethroids to one per year on cotton in Egypt has stopped the appearance of new resistance (Sawicki, 1986).

Chemicals used against species of Spodoptera also include insect growth regulators. There is interest, especially in India, in various antifeedant compounds or extracts, and in natural products, such as azadirachtin and neem extracts.

IPM

Integrated pest management techniques, favouring beneficial arthropods, are applied against S. littoralis on cotton in Egypt. These involve hand collection of egg masses, use of microbial pesticides and insect growth regulators and slow-release pheromone formulations for mating disruption. If these measures are taken, relatively few applications of conventional insecticides are necessary (Campion and Nesbitt, 1982; Hosny et al., 1983; Campion and Hosny, 1987). Damage thresholds have been established by Hosny et al. (1986). Pheromones have also been used for mass trapping using a lure and kill strategy (McVeigh and Bettany, 1987) and for monitoring populations. Souka (1980) experimented with irradiation for sterile-insect release, but this technique has not been widely applied in the field.

Phytosanitary Measures

For planting material, EPPO recommends (OEPP/EPPO, 1990) absence of the pests from the place of production during the last 3 months, or treatment of the consignment. For cut flowers, pre-export inspection is considered sufficient.

Cold storage of chrysanthemum and carnation cuttings for at least 10 days at a temperature not exceeding 1.7°C will kill all stages of S. littoralis, but may damage the plants. Storage at slightly higher temperatures or shorter durations does not eradicate S. littoralis, but differences in response to cold have been observed both between strains and within developmental stages of the pest (Powell and Gostick, 1971; Miller, 1976). Irradiation has been investigated as a treatment for cut flowers (Navon et al., 1988). For cut chrysanthemum flowers, Wang and Lin (1984) suggest enclosing buds in perforated polythene bags to exclude the pest and dipping the cut stems in insecticide solutions.