Biology of fall armyworm
Adult FAW is noctuid and feeds for a period which
extends from shortly after dusk to two hours after sunset depending on
temperature and time of the cropping season (Luginbill, 1928). Previous
research findings indicate that FAW is always active in the warm and humid
conditions (Sparks, 1979). The adult starts movement in the evening towards the
host plant for feeding, oviposition and mating (Luginbill, 1928). Virgin female
moths begin signaling male moths for mating by emitting windborne pheromone to
portray their readiness for mating (Luginbill, 1928). The female undergoes pre-oviposition
period ranging between three to four days (Luginbill, 1928). Active oviposition
starts from the initial four to five days of life (Sparks, 1979). Females
oviposit on the underside of the leaves. However, in the event of high FAW
population density, oviposition can be on any plant parts or plant debris
(Luginbill, 1928). Oviposition lasts for a period of three weeks (Luginbill,
1928). The average adult lifespan lasts only ten days with an average range of
7 to 21 days (Sekul and Sparks, 1976).
Eggs
FAW egg is
dome-shaped and the base is flat (Luginbill, 1928). Egg measures 0.4mm in
diameter and 0.3mm in height (Sparks, 1979). Female moths often lay eggs on
light colored surfaces such as fence rails, tree trunks and the underside of
the tree leaves (Luginbill, 1928). Eggs-laying occurs in four to nine days of
female pupation (Luginbill, 1928). The egg mass is covered by greyish scales of
female’s body (Luginbill, 1928). Eggs within the same mass hatch at the same
time (Sparks, 1979) and the eggs contained per egg mass range are between 100
to 200 depending on the moth strains (Luginbill, 1928). The female moth
produces an average of 1500 to 2, 000 eggs in its lifespan (Luginbil, 1928).
Eggs hatch in 2 to 3 days during the warm months of the summer (Luginbill,
1928).
Larval stage
FAW larval stage is the longest in its lifecycle
(Luginbill, 1928). The stage comprises of 6 larval instars (Luginbill, 1928). FAW
larvae are greenish in color with a black head which turns orange in the second
instar (Sparks, 1979). Larvae may be up to 11/2 inches long and
color varies from light-green to virtually black with several stripes along the
body (Luginbill, 1928). The stripes run across the length of the segments (FAO,
2017). The larvae initiate feeding on the fourth day after molting and last for
about 14 days in summer and 22 days in winter implying that larvae stage
development in warm summer is temperature driven (Luginbill, 1928). The net
average development time period for 1st instar is 3.3 days, 2nd
instar is 1.7 days, 3rd instar is 1.5 days, 4th instar is
1.5 days, 5th instar 2.0 days and 6th instar is 3.7 days
at a temperature of 25℃ (Pitre and Hogg, 1983). When larvae hatch from eggs,
they eat the shell; initiate feeding on the host plants and progress to inflict
foliage damage until 6th instars are completed, followed by pupation
(Luginbill, 1928). Larvae require an average of 14,000mm of crabgrass per
caterpillar to complete their development (Luginbill, 1928). Significant damage
is done by larval 6th instars (Luginbill, 1928). In the first 3
stages, larvae are still small and may only utilize less than 2% of the total
leaf consumed implying low damage during these larval stages (Luginbill, 1928).
2nd and 3rd in-stars are discriminated from 5th
and 6th instars by their brownish body color at the dorsal parts while
white lines are formed in the lateral region in 4th to 6th
instars (Luginbill, 1928). The next vital identifying features of big instars
are a set of four spots that form a square on the upper surface of the last
part of the insect’s body and Y-marks on the forehead (Luginbill, 1928).
Pupal stage
The 6 instars drop to the ground and get pupated at
1.3 inches deep in the soil depending on the soil texture, moisture, and
temperature (Vickery, 1929). They spin in a loose
cocoon, oval in shape and 20-30mm in length (Capinera, 2014b), which contains a
typical brown shiny noctuid pupa 18-20 mm in length. Pupation varies from 7 to 37 days depending on soil
opyimal temperature at 25 degree Celsius (Luginbill, 1928), but may take 8 to 9
days during warm summer, however, 20 to 30 days during cooler weather are
required (Pitre and Hogg, 1983). Immature adult, which is reddish brown in
color emerges out from the soil after 10 days in warm summer, and then cling onto
plants. They then stretch out their wings and become adult moths (Sparks, 1979).
Adult stage
Adults
(32-28 mm wing span) can be confused with other Spodoptera spp. However, in S.
frugiperda the veins of the hind-wings are brown and distinct, and in the
male forewings are pale and orbicular stigma has a pronounced pale ‘tail’
distally. In the male genitalia, the valve is almost rectangular and there is no
marginal notch at the position of the tip of the harpe, while the female bursa
lacks a signum (Sparks, 1979). An
adult female longevity is from 10-21 days. Fall armyworm moths are active in
the evenings and hide during the day. Sometimes they hide in sorghum whorls and
between the leaves (FAO, 2017).
2.4.6 Identification of FAW
Larval stage identification in the field requires
expertise and skills as fall armyworm is easily confused with other family
members such as African armyworm (S.
exempta), African bollworm (H.
armigera), cotton leaf worm (S.
littoralis), Spotted stem borer (Chilo
partellus), lesser armyworm (S. exigua), as well as African maize
stalk borer (Busseola fusca) (USAID,
2017). However, there are certain marks developed by taxonomists for
identifying the FAW (Plate 2.2). These include; head with dark net-like pattern
and inverted white ‘Y’ marking, four dark spots at the eighth abdominal segment
and the broad, pale band running along the top body, contrasted by dark stripes
at both sides (USAID, 2017).
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