The development rates of FAW at different temperatures was studied in the laboratory at the ARC. Laboratory trials were conducted at 3˚C intervals (18, 21, 24, 27, 30, 33 and 36 ± 1 °C). There was no development of FAW at temperatures 0 to 15˚C. However, the FAW developed successfully from egg to adult stage at 18 to 33˚C. However, the eggs did not hatch at 36˚C, although L1 larvae placed at this temperature developed successfully up to adult stage. The extensive data set clearly shows the areas where the winter average temperatures are below 15˚C, the FAW populations cannot persist for long as these temperatures are below the developmental threshold. The FAW cannot tolerate freezing temperatures. Likewise, at average temperatures above 36˚C, the FAW populations will not persist as eggs do not hatch, although larvae can survive for short periods. Therefore, it is considered that the FAW can survive anywhere in South Africa during summer, but it can only survive and develop in the warmer areas during winter. The shortest generation time occurred at 33˚C, when the generation (egg to egg) took only 19 days.
FAW moths are active during the evening and hide during the day. They are sometimes found hiding between maize leaves or in the whorls of the maize plants. Male moths find females by following pheromone plumes released by the females. Mating takes place and eggs are laid in masses on maize and sorghum plants, two or three days later.
FAW eggs are laid in clusters of 50-500 in a single layer, and are mostly attached to foliage. Eggs are dome-shaped and are a grey-white colour. After egg deposition, the female deposits greyish-coloured scales over the egg mass, giving it a hairy or moldy appearance. The presence of egg masses plays an important role in the scouting process for FAW infestations by farmers.
Fall armyworm egg masses. D. Visser ARC-VOP Roodeplaat.
Newly hatched and second instar larvae (L1-L2) are pale green with black heads. At this age, the larvae are difficult to identify morphologically as they closely resemble those of other Noctuidae moth larvae. These small FAW larvae spin silken threads, which can then catch the wind to transport larvae to other feeding sites. This behaviour is called 'ballooning' and can transport the small FAW larvae over considerable distances. During the third and fourth instar (L3-L4), the head turns to an orange-brown colour, the dorsal area changes to a tan colour, whilst the ventral area turns green. These younger instars feed on the outer leaves, first feeding at their hatching site and then spreading out throughout the maize plant where they consume the leaf tissue, except the veins and midrib. Feeding results in characteristic semi-transparent patches, or "windows", on the leaves (Figure 2). However, older larvae are cannibalistic and often tend to eat each other when in contact at high infestation levels, which reduces the numbers of mature larvae per plant. The older L5-L6 instar larvae feed in the whorl in corn plants where they can cause considerable damage.
Fall armyworm damage on maize.
Young larvae – length 6-9 mm
Small FAW larvae (caterpillars) are mostly lighter in colour compared to the older or larger individuals. Green and/or yellow colours are most common, which may possibly camouflage the larvae on the green leaves, but also cause confusion with the identification of other caterpillar pests. The heads of the young larvae may be black or orange, and the spots on the body may not always be equally clear or apparent. Marks on the head may not be easy to observe on small larvae.
The lateral bands are not yet clear on the small larvae compared to the larger larvae. The colour of small individuals may include green, yellow, brown and black, but mostly greenish/yellowish. A pinkish tint is common on the sides of young individuals. The hairs and bumps are more pronounced on small individuals, similar to all stages of the bollworm. Some may walk with an action similar to the semi-looper caterpillars (but not as pronounced).
Young fall armyworm larvae (6-9mm). Left: Topview. Right: Sideview. Photos: D. Visser, ARC-VOP, Roodeplaat.
Mature larvae – length 30-36 mm
Fully grown FAW larvae vary in colour from pale green to almost black, with a reddish-brown head. They closely resemble true African armyworm and the corn earworm larvae in appearance. The difference is that the heads of FAW larvae have a prominent inverted "Y" marking and prominent black tubercles from which hairs arise (pinacula). Such tubercles are also arrayed throughout the body. They also have four dark pinacula (raised spots) arranged in a square on the 8th abdominal segment and in a trapezoid on the 9th. They also have visible white/beige lateral stripes. These visible characteristics were determined to be the best method of identifying FAW larvae in the field.
The lower lateral sides of the FAW are usually lighter in colour, with a brown or black band just above the light band. The general appearance is brown, although blackish or greenish individuals may be encountered. The head may be black, brown or orange. Larger FAW larvae never walk with a looping action.
Mature fall armyworms (30-36mm), Left: Topview. Right: Sideview. Photo: D. Visser, ARC-VOP, Roodeplaat.
PUPAE, SOIL CELLS AND COCOONS
Pupae of the FAW are usually found in the soil and are therefore not frequently encountered. They do, however, sometimes pupate on the plant, e.g. when feeding in the maize ears. The mature larva drops to the ground, burrows shallowly into the soil and makes an earthen cell by constructing a flimsy cocoon into which sand particles are incorporated. Pupae are approximately 15 mm in length and the earthen cells 20 to 25 mm. The pupae have no diapause (no dormant period) and are not able to overwinter in the soil. An adult moth will emerge from the pupae after 7-14 days depending upon soil temperatures.
Fall armyworm pupae, soil cell and cocoon. Photo: D.Visser, ARC-VOP, Roodeplaat.