[ RNASeq transcripts ]
The fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae) is a polyphagous species, attacking about 100 plant species, such as maize, wheat, rice, soybean, cotton, tomatoes, among others. This great diversity of host plants, associated with its rapid development and high potential to cause economic losses, makes S. frugiperda one of the most important pests in agriculture. In addition, it is considered the most important pest of maize culture in Brazil, due to high economic losses and rapid development, thus enabling its rapid dissemination and multiplication. Populations of S. frugiperda are distributed across the American continent, especially in the regions with the highest temperature, and in different countries in Africa. The presence of this insect species was also registered in Asia, India, and its occurrence in China was recently formalized. In Brazil, S. frugiperda can be found in all maize producing regions.
This insect has a nocturnal habit, lays about 100 eggs on the top of the leaf of the plants, which may show a light green color at the beginning and turn orange at the end of the phase. The egg phase lasts 3 to 5 days at a temperature of 25ºC. Newly hatched caterpillars feed on the eggshell itself, then remain at rest for 2 to 10 hours and then scrape the leaves, a very characteristic symptom of the pest attack on the crop. Caterpillars prefer to feed on new leaves and are usually found one per plant due to their cannibal habit. Over time and their development, these caterpillars increase in size and begin to perforate the maize leaves and move towards the region of the cartridge of the plants, being their preferred place for development and shelter. Initially, the caterpillars are light, turning from dark brown to greenish to almost black, and have three pairs of legs on the chest and five pairs of false legs on the abdomen, reaching 50 mm in length. Caterpillars can have up to seven instars and in the last ecdysis they leave the cartridge, penetrate the soil at 0.5 cm deep, where they become pulps, which are about 15 mm long, reddish or brown in color. The pulp phase lasts about 10 to 12 days in the hottest times of the year. Adults are moths and after mating, usually at night, females lay on maize leaves. In addition, adults measure about 35 mm in wingspan, and the color of the front wings is dark brown, while the color of the rear wings is gray-white. The adult's longevity is about 12 days and the complete cycle of the pest is just over 30 days. According to the literature, generally the dispersion capacity of adults is within a 200m radius, however, other studies have shown the flight potential of around 800m.
Seedlings are fed upon within the whorl. Larger caterpillars can cut the base of the plant. Mature plants suffer attack on reproductive structures. On tomato plants, buds and growing points may be eaten and fruits pierced. Maize leaves are eaten and the whorl (funnel) may be a mass of holes, ragged edges and caterpillar frass. Young larvae skeletonize the leaf lamina in a typical 'window-pane' damage. 'Window-paning' is the most common damage symptom at early whorl; however, this is sometimes indistinguishable from damage that is due to other stem borers. Usually many young caterpillar will be present on the same plant, but normally one or two older caterpillar may be found on a single plant, as others will migrate and feed on neighbouring plants. Later caterpillar instars make larger holes, causing ragged whorl leaves, and produce sawdust-like caterpillar droppings, while fresh feeding produces big lumps. Badly infested fields may look as if they have been hit by a severe hailstorm. Fall armyworm can also destroy silks and developing tassels, thereby limiting fertilization of the ear. Maize plants may have the cobs attacked by caterpillar boring through the kernels. Damage to cobs may lead to fungal infection and aflatoxins, and loss of grain quality. At high densities, large caterpillars may act as armyworms and disperse in swarms, but they often remain in the locality on wild grasses, if available.
For the correct integrated management of the fall armyworm caterpillar, it is essential that the farmer starts with the adequate monitoring of the crop, in order to determine the population level of the pest and the correct moment for adopting the available control measures. Among the different strategies for the control of S. frugiperda in maize, the possibility of adopting chemical control methods stands out; the use of genetically modified plants (expressing Bt proteins) and biological control, the latter with the adoption of different natural enemies, such as the egg parasitoid Trichogramma and Baculovirus spodoptera, biopesticide for use in the control of pest caterpillars. The association of these two natural enemies in pest control has shown a significant increase in demand by producers, helping a lot in the integrated management of the pest in the maize crop. Spodoptera spp. resistance breeding programmes have developed field crop varieties with improved resistance, one example being maize. One resistance mechanism that appears to be operating in maize is increased leaf toughness vis-à-vis a thicker epidermis. On the other hand, transgenic maize containing genes encoding delta-endotoxins from Bacillus thuringiensis kurstaki have been commercialized in the USA and Brazil. Vegetative insecticidal proteins (VIP) have been isolated from Bacillus thuringiensis (Bt) during the vegetative phase of growth which show a wide spectrum of activities against lepidopteran pests, especially Spodoptera spp. These insect species appear to be controlled by these toxins, but the development of resistance is a concern. Field-evolved resistance to the Bt maize expressing the Cry1Ab protein is reducing it efficacy in Brazil. Studies reported that most Bt maize hybrids lost their ability to control fall armyworm within 3 years of introduction in Brazil. In some areas resistance to insecticides may be widespread and control can be difficult. Recommended insecticides for Spodoptera spp. include esfenvalerate, carbaryl, chlorpyrifos, malathion, permethrin, and lamba-cyhalothrin. Recent studies showed that five insecticide compounds used against fall armyworm (cypermethrin, deltamethrin, lambda-cyhalothrin, permethrin, and chorpyrifos) remained in the soil. The sex pheromone for S. frugiperda contains (Z)-9-Tetradecenyl acetate (Z-9-14:OAca) which is common to Trichoplusia ni, Spodoptera exigua and Agrotis ipsilon exigua. Mating disruption may be possible given the successes observed for S. exigua in which (9Z,12E)-9,12-tetradecadienyl acetate released at high concentrations, caused mating disruption in tomato, lucerne and cotton fields.
With the possibility of S. frugiperda becoming resistant to insecticides, several research groups have been developing new strategies that aim to control populations of this insect-pest, mainly through Biotechnology. In order to develop new ecofriendly technologies applied to control of S. frugiperda, the Plant-Pest Molecular Interaction Laboratory (LIMPP), together with the Bioinformatics Laboratory, both located at Embrapa Genetic Resources & Biotechnology (Brasilia-Brazil) and supported by PlantStress Biotech National Institute of Science & Technology (INCT), focused on observing the differences in fall armyworm development when fed on artificial diet and two host crops: cotton (Gossypium hirsutum) and maize (Zea mays). Initially, it was observed the development of caterpillar on each host plant. Here is presented the RNA-Seq (Illumina Platform) data of S. frugiperda caterpillars fed with artificial diet, cotton and maize.