BIOECOLOGY OF THE FALL ARMYWORM, Spodoptera frugiperda J.E. SMITH ON MAIZE, Zea mays L. IN THE SOUTH-WEST, NIGERIA

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%) vii >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.