Pest insects in tropical pastures/ Spodoptera frugiperda

In the agricultural sector, when establishing a crop, it should be taken into account that there are a number of biotic and abiotic factors that influence crop production, in the particular case of pastures, the production of forage biomass is affected, which is the food par excellence to feed ruminants. Among the biotic factors are pest insects, which appear on forage surfaces depending on the time of year, whether it is rainy or not rainy, since their respective biological cycles require certain agroecological conditions to proliferate quickly.

This publication describes an insect that has spread in different countries, because it is a pest that has a preference for the poaceae family including crops of global importance such as corn (Zea mays), rice ((Oriza sativa), Sorghum (Sorghum bicolor), this arthropod is recognized by FAO as a pest that threatens food security, as it can destroy corn plantations quickly if not controlled in time. Based on these premises and taking into account that it also negatively influences grassland crops, aspects related to ecology, biological cycle, habits, damage and insect control are addressed below.

The Spodoptera frugiperda is known as corn fall armyworm or grass sweeper, both vulgar names are appropriate since the larvae feed on the central leaves and in the growing regions of corn, in the same way grass growers assign the name of sweeper, because they can reach high populations inside the paddock and destroy it or sweep it quickly consuming all its foliage leaving only the central rib of the leaves.

The insect is native to tropical and subtropical regions and according to Angulo (2000), it is a larva of the night butterfly Spodoptera frugiperda, belonging to the genus Spodoptera, species frugiperda , order Lepidoptera and family Noctuidae. They have a chewing oral apparatus causing a chopping effect on the leaves of the plants, in addition it is a polyphagous insect that causes numerous losses in various crops when the rainy period begins, slightly decreasing its attack when the rains are prolonged and intense.

On the other hand, the susceptible grass species are those of the genus Pennisetum, the tanner (Brachiaria arrecta), the Paez (Brachiaria mutica), in addition to the Cynodon spp and Digitaria spp. As for the grasses that show some resistance, there is the guinea Megathyrsus maximus.

The Spodoptera,to reach its adult stage must go through different phases that will be described below:

*Egg stage: the eggs are deposited by the adult insect in the bundle and the underside of the leaves closest to the ground, when the checks are carried out, a large number of eggs covered by a kind of transparent substance can be observed, according to the Farmer's Service Foundation (2005), they indicate that the eggs hatch after approximately 3 to 5 days.

*Larval stage: when the eggs hatch, the larvae are born green and as the days pass they turn black, they have three stripes or elongated yellow lines on the back with black dots and on the head they have stripes in the shape AND inverted. These larvae, according to Chango (2012), go through six or seven instars or stages and during their development variations in size and morphology occur and could go unnoticed until they reach 2 cm when the damage to the crop is severe. This larval phase can last from 14 to 22 days approximately.

*Pupal phase: it is necessary to point out that the pupal phase is a transitional stage where the insect ceases to be a larva and prepares to become an adult, the insect forms as a kind of hard shell where it will remain for a certain time without eating and moving until leaving the pupa become an adult. In the case of the Spodoptera frugiperda the pupa is reddish in color, these are buried in the ground at about 3 or 5 cm forming a kind of pupal chamber, where they remain approximately between 8 to 18 days, depending on the temperature present in the agroecosystem. Some researchers, such as Murúa (2014), reported that in Argentina, in some areas where temperatures range between -2 and -8 degrees Celsius, they observed a high percentage of mortality of the insect in the pupal stage.

*Adult phase: the adult is a moth with a color between gray and brown, with an approximate size of 3 cm, these have nocturnal habits and stay among the leaves of the grasses, weeds or any shady place inside the paddock. The adult female tends to lay the eggs on the leaves and stays alive according to some researchers approximately between 10 to 21 days.

As you can see the duration of the complete biological cycle can range approximately between 30 and 60 days, depending on the agroecological zone where it is located. In Table 1. The different biological stages of the Spodoptera frugiperda in relation to temperature are shown.

As can be seen in Table 1, the temperature has a considerable influence on the biological cycle of the fall armyworm, if the results are analyzed well we can deduce that when the temperatures oscillate around 19°C the larval phase lasts approximately 39 days, which is harmful, since it is in the larval phase that the insect causes damage to the crop. On the other hand, it is possible that in tropical areas the larva causes more damage in rainy periods, since during that time temperatures will decrease considerably.

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Capinera, L. (2001). Manual of Plant pests. San Diego: Academic Press.

Chango, L. (2012). Control of the fall armyworm of corn. Unpublished degree work. Technical University of Ambato, Ecuador.

Fundación Servicio para el Agricultor (2005). Pastures (2nd ed.). Maracaibo: Author.

González, M.; Reyes, N. and Hernández, I. (2015). Biological products for the control of Spodoptera frugiperda (Lepidoptera: Noctuidae). Rev. Colombiana de Entomología, 41(2), 200-204.

Pannuti L.; Baldin, E.; Hunt, T. and Moraes V. (2015). Larval movement in the plant and feeding behavior of the fall armyworm (Lepidoptera: Noctuidae) in the reproductive stages of maize. Revista Entomología, 45, 192-200.