Fall armyworm

Spodoptera frugiperda

Crops: Maize

Why is it a problem?

The fall armyworm (FAW) is a polyphagous pest that damages more than 350 plant species belonging to 76 plant families (Montezano et al., 2018). The higher damage is found in maize and sorghum, which are their primary hosts (Casmuz & Juárez, 2010). The average fecundity of one FAW female is 1000 eggs (Mura et al., 2006), and can fly over 100 km per night (Song et al., 2020). Furthermore, FAW do not enter diapause, so they can migrate to new places with suitable environmental conditions (Du Plessis et al., 2020). The high fecundity, high migratory ability, and wide host range of the FAW pose high threats to the maize crop.

Where and when is it a problem?

In Bhutan, FAW was first observed damaging maize in Dabchagang and Pepchu in Guma Gewog and Mendugang in Dzomi Gewog, Punakha, in September 2019 (Mahat et al., 2021). FAW has been detected damaging maize in 15 Dzongkhags (Figure 1). Maize is damaged by FAW from the seedling to the ear development stage. FAW defoliates, damages whorls, and feeds on ears, resulting in yield losses (Capinera, 1969).

Figure 1: Distribution of fall armyworm in Bhutan (as of Feb 2024)

IDENTIFICATION, BIOLOGY & SYMPTOMS

Refer to the following guides:

  1. A Guide for FAW Identification and Management https://pestsofbhutan.nppc.gov.bt/version-2-a-guide-for-fall-armyworm-identification-and-management/

2. Fall Armyworm photo guide: identification https://www.fao.org/3/ca4856en/ca4856en.pdf

Confusion with other pests:

FAW is often confused with beet armyworms, Spodoptera exigua (Hübner), and yellow-striped armyworms, Spodoptera ornithogalli (Guenée).

Management practices

Monitoring 

Monitor the presence of FAW moths in a specific location by using sex pheromone traps. This method facilitates the tracking and quantification of FAW incidence. Traps and lures are available for purchase from NPPC. 

  1. Deploy five pheromone traps per acre, spaced 50 meters apart, and adjust trap height based on maize growth stages to ensure it remains above the crop canopy
    • Place the traps within or at the periphery of a maize field to allow the wind to carry the scent of the pheromone

Refer to this guide for pheromone trap installation: https://pestsofbhutan.nppc.gov.bt/a-guide-for-fall-armyworm-pheromone-trap-installation-2024/

Important note: The presence of moth captures in pheromone traps serves as an initial sign of egg laying and is the primary indication that FAW is present in the area. One moth trapped per day or seven moths per week indicates a significant capture and should be confirmed through field scouting (Prasanna, B. M., Huesing, J. E., Eddy, R., & Peschke, V. M. (2018)). 

Mass trapping

Mass trapping using pheromone traps aims to disrupt the mating behavior of insects with many traps in a given area, thus reducing their overall population. Install pheromone traps @ 15/acre for mass trapping of a male FAW moth (FAO and DPPQS, 2021).

Scouting 

The main aim of scouting is to assess the field’s pest pressure and crop health following a prescribed protocol, expressed as a percentage of infested plants.

Scouting protocol: Seedling and Early Whorl Stage: Follow the “W” pattern:

  1. Avoid the border rows to avoid the edge effects.
  2. Take a walk approximately 5 meters into the field, zigzagging and stopping at five different locations.
  3. At each location, assess 10–20 plants for FAW damage. Observe the feeding damage on the newest 3–5 leaves on the plant. Repeat the process at the different locations in the field.
  4. Calculate the total percent infestation: Total plants infested/Total plants examined * 100 to determine the action threshold (i.e. insecticide application)
  5. Apply insecticides when 20 percent (range of 10–30%) of the plants are infested.

Scouting protocol: Late Whorl Stage: Follow the steps as indicated under seedling and early whorl stages.

Apply insecticides when 50% (40–60%) of the plants are infested.

Scouting protocol: Reproductive stage (tassel and silk stage): Follow the ladder pattern.

  1. Avoid the border rows to avoid the edge effects.
  2. At each location, walk in a back-and-forth manner to systematically cover the entire area, allowing for an accurate assessment of the extent of damage caused by Fall Armyworms.
  3. At each location, assess 10–20 plants for FAW damage. Examine the base of the developing ears and the leaf axils
    above and below the ears for larvae and any signs of fresh feeding. Repeat the process at the different locations in the field.
  4. Calculate the total percent infestation: Total plants infested/Total plants examined * 100 to determine the action threshold (i.e. insecticide application)
  5. Apply insecticides when 20 percent (range of 10–30%) of the plants are infested

Check the link

 Effect of variety

The commonly grown maize varieties in Bhutan, Yangtsipa, Bhur Ashom, Shafangma, and Wengkhar hybrid, are susceptible to FAW.

 Non-chemical management 

  • Handpicking of egg masses and larvae is widely used by farmers in Africa (Yigezu & Wakgari, 2020).
  • Applying sand to the whorls of maize plants causes larval desiccation (Hruska, 2019).
  • Burn crop residues to destroy eggs, larvae, pupae, and adults (Assefa, 2018).
  • Field plowing is used to expose pupae to natural enemies (Abrahams et al., 2017).

Biological control 

  1. Parasitoids like Trichogramma spp. and Telenomus spp. have been observed in Bhutan but have not been evaluated against FAW

    Figure 2: Trichogramma spp. parasitizing FAW eggs (Dzomlingthang, Punakha 2022)
    2. FAW larvae are frequently observed being infected by entomopathogenic fungus and nucleopolyhedrovirus

Figure 3: FAW larvae infected by fungus and virus

Chemical management

The application of insecticides should be determined by considering both the Action threshold and the Crop stage. The action threshold represents the pest population level at which intervention, typically through insecticide application, becomes necessary to prevent economic damage to the crop. Additionally, taking into account the crop stage is crucial, as certain growth phases may be more vulnerable to pest damage.

Maize stage % Action threshold Recommended insecticides
Early whorl stage
(2–5 leaves fully emerged)		 20 percent (10–30%) Chlorantraniliprole 18.5 % SC: 3ml/10 liters of water

Or

Emamectin benzoate 5% SG: 2 g/ 10 liters of water

Note: Consistently using a specific pesticide without rotation may result in a rapid rise in resistance. For instance, if chlorantraniliprole is applied in one instance, consider using Emamectin benzoate for the next application.
Late whorl stage
(8–12 leaves fully emerged)		 50% (40–60%)
Reproductive stage
(Tasseling/Silking)		 20 percent (10–30%)

Note: Refer to Fall Armyworm in Asia: A Guide for Integrated Pest Management by CIMMYT (https://repository.cimmyt.org/handle/10883/21658) for detailed information on the economic threshold level for vegetative and reproductive stages.

Ongoing research activities: 

  1. Evaluation of intercropping against fall armyworm

Check out the following link: 

Version: NPPC 2022. Fall armyworm V2.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Last modified: Feb 15, 2023. Contact: nppcsemtokha@gmail.com

 

 

 

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