Abstract:
Fall Armyworm (FAW), Spodoptera frugiperda, is an invasive insect pest that causes
severe damage and yield loss to maize. Synthetic insecticides applied for FAW control are
environmentally unsustainable and ineffective due to development of resistance.
Information on life-cycle and seasonal occurrence of FAW is necessary for its effective
management. Reports on appropriate management strategies for FAW are limited.
Therefore, FAW biology and ecology on maize in the South-West, Nigeria (SWN) were
investigated.
A four-stage sampling procedure was used. In the first stage, three major Maize-Growing Agroecologies (MGA): Humid-Forest (HF), Derived-Savanna (DS) and Southern-Guinea Savanna (SGS) were purposively sampled in the SWN. In the second stage, one
Agricultural Development Programme Zone (ADPZ) was purposively selected in each
MGA. In the third stage, 50% of Agricultural Development Programme Blocks (ADPB)
per ADPZ was randomly selected. In the fourth stage, Maize Farmers– MF (n = 212)
were randomly sampled proportionate to size in each ADPB. Data were collected on MF
knowledge of FAW attack period, damage severity, larva description, and control
practices using structured questionnaire. Twenty early-whorl plants were assessed on farm in each MGA for FAW infestation (%) and Foliar Damage Severity– FDS
(0=immune to 5=highly susceptible). Life-cycle characteristics were assessed in the
laboratory on Development Duration– DD (days), morphometrics of immature stages
(mm), longevity of fed and unfed moths (days) and FAW oviposition. Twenty-five maize
varieties were evaluated on the field for response to FAW during two consecutive early
and late seasons; plots were laid-out in a randomised complete block design with four
replicates. Egg-mass abundance, larva abundance and FDS at three, five, and seven
Weeks-After-Sowing (WAS) were determined. Data were analysed using descriptive
statistics and ANOVA at α0.05.
Maize farmers (88.7%) observed FAW attack within two months-after-sowing but only
30.8% reported total damage. Most MF (75.9%) could describe FAW larva correctly.
Also, 58.5% MF exclusively applied synthetic insecticides for FAW control. Insecticides
commonly used by respondents were organophosphates (37.2%) >pyrethroids (29.0%)
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>avermectins (18.2%). Percentage FAW infestation and FDS were significantly higher in
HF (86.25±3.90%; 2.63±0.14) than in SGS (56.88±3.90%; 1.66±0.12), respectively. Total
DD for egg, larva and pupa was 2–3, 11–12 and 7–10, respectively. Egg was spherical
(0.24±0.01); larva comprised six-instars with 1.64±0.03 to 26.45±0.44 body-length,
0.18±0.01 to 3.45±0.07 body-width and 0.12±0.01 to 2.45±0.02 head-capsule-width. Pupa
body-length and body-width were 14.38±0.14 and 4.21±0.04, respectively. Fed moths
(7.25±0.47) significantly lived longer than unfed moths (4.13±0.17). Eggs laid by FAW
paired at 1♂:1♀ (1354.00±168.16) were significantly higher than those paired at 3♂:1♀
(599.89±210.31). Also, eggs laid on abaxial (56.30±7.29) were significantly higher than
on adaxial (13.00±0.00) leaf surface. Egg-mass abundance in early-season was 0.47±0.04
(3WAS) >0.18±0.02 (5WAS) >0.00±0.00 (7WAS). Larva-abundance in early-season was
0.38±0.03 (3WAS) >0.10±0.02 (5WAS) >0.03±0.01 (7WAS). Seasonal FDS of maize
varieties ranged from 2.0 to 4.0.
Fall armyworm infestation was more severe in the humid forest and early planting season
and development from egg to adult occurred within 20–25 days. Field-scouting within
three weeks-after-sowing for eggs and larvae could guide appropriate timing for pest
management interventions.
