Spodoptera exigua, commonly known as “rosquilla verde” in Spain or “gusano soldado” in Latin America, and “beet armyworm” in the USA and other English speaking countries; is a Lepidoptera which belongs to the super family of the Noctuidae. This is a migratory plague of a polyphagous nature which causes it to affect horticultural produce such as peppers, tomatoes, aubergines, cucumbers, beans, water melons and other plants.
The female worm is extremely fertile throughout seasons of warm temperatures and as a result is able to lay up to 1,700 eggs; the level of fertility decreasing in the cold season to around 500 eggs.
The eggs are laid mainly on the underneath part of the leaves. Once emergence occurs, the larvae immediately begin to eat the newly formed leaves and flowers, moving together in first phase clusters. However, throughout their development, they separate themselves and continue to devour both leaves and fruit.
At the end of the larvae cycle, the larvae drop down or allow themselves to fall onto the ground in order to pupate; burying themselves in the shallow ground and forming a silky cocoon. The length of the chrysalis stage depends on the temperature, and from this point onwards the new adults emerge and a new cycle or generation begins.
The length of the complete cycle will vary according to atmospheric conditions. However, the average cycle will be between thirty to forty days.
The damage produced due to feeding can be recognised as holes chewed in leaves, flowers and also fruit.
Damage to leaves can be observed as perforations, which are almost circular in shape. These are found in the blade or edge of the leaf and tend to be located in the area of the leaf veins. The small caterpillars only eat the parenchyma of the leaf, leaving behind the epidermis. However, the larger caterpillars cause a decrease in the size of the surface area of the leaf, which in turn will affect the vegetative growth of the plant.
Perforations are produced in the peppers from the point at which the caterpillars make their entrance. They feed inside the pepper which causes rotting, and with regard to water melons the caterpillars cause superficial gnawing to the peel of the fruit.
As a preventative measure it is important to take care in the first phenological stages of plant growth, especially by carefully eliminating the remains of prior crops and weeds. It is also important to place nets over the strips of ground used in order to prevent the possible entrance of butterflies. The use of light traps and pheromones can also help to control the adults.
With respect to chemical control, the use of applications which reach the underneath parts of the leaves is highly recommended. These should be made with active materials such as indoxacarb, lufenuron and spinosad.
Biological control can be undertaken with applications of Bacillus thuringiensis and with the use of nuclear polyhedrosis virus for the control of Spodoptera exigua during the egg laying stage or the first larvae stage. This treatment should be repeated every seven days taking into account any natural enemies and the effects caused by general predators.