Fall army worm (FAW)

Fall Army Worm (FAW), is a new emerging invasive pest that is wreaking havoc in Kenya and many other parts of the world, causing huge losses to farmers and impacting on food security.  FAW or Spodoptera frugiperda, is a caterpillar native to tropical and sub-tropical regions of the Americas.  FAW was first detected in West and Central Africa early in 2016 and quickly spread across Sub-Saharan Africa.  It got to Kenya early 2017.  In July 2018 it was confirmed in India and early this year it reached parts of China.  It has even been picked up as a quarantine pest on plant material entering the EU (exporters be aware).

The larvae of the FAW feeds on many different crops, but particularly affects our maize, sorghum and sugar cane farmers, damaging the crop stand and substantially reducing yields.  In the Americas, farmers have been dealing with FAW for centuries and much research has been done on this insect.  We can learn a great deal from their previous knowledge.

In Kenya, as a new and emerging pest, we were caught unaware.  Not only did we not know how to identify and deal with it, but, as a new species, the natural enemies of the FAW had not built up. So, the first outbreak of the FAW was disastrous.

By understanding the nature and lifecycle of the pest, we can formulate better, more effective ways to deal with it.

  • The FAW adult moth is very mobile! Adult moths can fly 100 km in a night, and over 1000 km in a lifetime. It can spread very fast, over 2000 km in a mere few months!
  • The FAW is polyphagous, which means that it can feed and reproduce on many different crops and plant species. It can spread across many different cropping systems and migration is not limited by diet.
  • FAW is a prolific breeder! The lifecycle is about 30 days in warm weather, and the adult female lays about 100-200 eggs per egg mass, and 1500 – 2000 eggs in her lifetime, 12 generation a year.
  • Unlike our native army worm and locusts, that hang around in gregarious groups and are easier to control on a county level, FAW populations are diffuse and hard to hit. They spread out, with thin population cover over wide areas/crop types.  This makes them very difficult to eradicate or control on a large scale.  Thus, control must be done at farm level.

The Fall Armyworm (FAW) Lifecycle

Day 1-3 – the females lay egg clusters containing 100-200 dome shaped eggs underneath the leaves close to the stem junction at the base on the plant.  If FAW populations are high they will lay eggs higher up the plant and on surrounding vegetation. The eggs are protected by a layer of scales between the eggs and over the egg mass.

3-5 days after the eggs are laid, the larvae emerge, begin feeding and migrate to the whorl (in maize).  Young larvae can spin silken threads that catch the wind and transport them to new plants (ballooning, which can create 100% infestation in fields).  The very young larvae only partially eat through the leaf creating transparent feeding windows in the leaves.  They are greenish with a black head.  As they get bigger, they can eat through the leaves, growing points and protective leaf bracts on the cobs, causing major destruction.  Mature larvae are 3-4 cm long and vary in colour from light brown to green and black, they have a light-coloured inverted Y on their faces.

After 14-22 days the larvae mature and drop to the ground to pupate, forming a reddish-brown oval cocoon, 2-3 cm in length, that is hard to see.  In soft soil they can burrow down 2-8 cm, in hard soil they can hide under leaf debris spinning a webbed cocoon.

8-9 days later the adult moths emerge.  Their bodies are about 2.5 cm long, with a wingspan of 3-4 cm.  They are mottled brown/ grey and difficult to spot.  Adult moths are nocturnal and most active during warm humid evenings.  The female starts laying eggs 3-4 days after emerging and can continue laying eggs for up to 21 days, although adult life duration is 10 days.

The length of the FAW life cycle is affected by temperature, diet and humidity.  Rising temperatures speed up the life cycle, cooler temperatures slow it down.  Optimal temperatures are 28C for the larvae.  In Kenya the life cycle is about 30 days, which means we can have 12 generations in a year!  Frost kills the FAW, rain can wash the young larvae off the leaves and windy conditions will aid dispersal of the moths.

FAW control

Fall army worm Life cycle: FAW eggs are usually tucked away under the leaves and the larvae hidden in leaf whorls

To spray or not to spray?

Spraying may not be economically viable due to high cost of sprays and low farm gate returns.  FAW is an emerging pest, so finding approved registered pesticides is always a challenge.  KALRO have approved a shortlist of effective pesticides.  Maize is a food staple and risks of exceeding MRL’s in the crop are high, affecting food safety.  The FAW are difficult to kill with contact pesticides (eggs tucked away under the leaves, larvae hidden in leaf whorls, pupae are encased and below ground and adults are nocturnal).  The most important reason not to spray (especially small-scale farmers!), is that the pesticides are more likely to kill emerging natural enemies of FAW, than they are the FAW itself, resulting in continued high pressure from this new pest.  Spraying is more effective at the early vegetative stage, where the FAW can eat the growing tip and kill the plant, and at silking – where FAW can affect reproduction and cob quality. Spray in the evening or early morning when the FAW are active.  Target spray into the whorls.

Early Warning and Scouting.

Early detection of FAW is very important.  Pheromone traps are readily available, and capture the male moths, well before the damage is seen in the crops.  There are several Early Warning Systems, that collect and share information.  (CABI, FAO, KALRO).  Talk to your local extension worker.  Search and install the FAO-FAMEWS app on your phone.

Scouting your crops is very important.  Eggs tend to be laid on plants near grassed areas or on the edges of large fields, and hatch after three days.  Reduce the scouting frequency and concentrate more often on these areas.  FAW, eggs and larvae, are often confused with native caterpillars.  Get to know the pest and download the poster off CABI or the FAO website. 

Field scouting

Cropnuts’ Agronomists George (left) and David (right) doing a routine field scouting in a wheat field in Timau, Kenya

Biological control, cultural control and habitat management

Even though it is a new, emerging pest, FAW has many natural enemies in Kenya:  Predators, birds, bats, ants, earwigs, beetles, kill and feed off FAW.  Parasitoids, tiny wasps that lay eggs in the eggs, larvae & pupae.  Pathogens infect and kill the FAW include bacteria (BTs), viruses, fungi (Beauveria spp) and nematodes.  Farmers would be well advised to be extra observant in the field so as to spot, conserve and promote the natural enemies.  The cumulative effect of natural enemies can be very effective in FAW control and much longer lasting & more environmentally sound.

Plant early at recommended time taking advantage of the first effective rains.  FAW populations build up during the season.  Critical crop stages for maize are early vegetative growth and silking.  Avoid late planting and choose a fast maturing variety to be ahead of the crowd.

Improved crop health will very much reduce the impact of FAW on yields.  Good quality seed, appropriate plant spacing, good soil management and crop nutrition will boost vigour and reduce yield losses.

Hand picking and destroying the eggs masses and larvae is very effective in small areas.  Reducing immediate crop damage and appearance of 1500-2000 more larvae in a month!  Chickens are great at finding the pupae and eating any larvae that fall off the plants.  Don’t waste the larvae, an effective age-old natural remedy for caterpillar infestations is to collect the larvae, stress them for a few hours, and grind them up with water and spray on the crop, to promote the spread of natural pathogens.

Remove volunteer plants and infested crop residues.  Eliminate grassy (and other) weeds in and around the crop, these can perpetuate high populations of FAW.

Intercropping with legumes will repel and confuse the adult female and deter her from laying eggs on the crop, AND benefit with nitrogen fixation to produce a more vigorous plant, as well as providing alternative food source for natural enemies.  Crop rotation with non-grass crops such as cassava will reduce populations, but they can quickly build up.

Conservation agriculture: no till, reduced till, residue retention, crop rotation and cover cropping all increase biodiversity and numbers of natural enemies, as well as improving soil health, water infiltration and water storage of soils, to mitigate damage and yield losses.

Habitat management using a push plant that repels FAW e.g. Desmodium inside the maize plot, with a pull crop (Napier or Brachiaria) that attracts FAW on the outside, can be effective for small plots.  Always remember to scout the pull crops to make sure they are not breeding up the pest!

Remember that the best way to deal with any new, emerging pests it to be prepared.  Good soil health, increased biodiversity, promotion of natural enemies, Integrated Pest Management (IPM) and Good Agricultural Practices (GAP) are paramount in cushioning the negative effects of these outbreaks.

For more information on Fall army worm control or to join our agronomy newsletter list please contact us on [email protected].

Till next time

Kindest regards,

Ruth

ruth vaughanRuth Vaughan is the Technical Director at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines such as the HortFresh JournalHortiNews and Floriculture. Ruth is a great believer in soil health, organic matter, biochar and carbon sequestration as a way to alleviate climate change and increase food security. Loves visiting farmers and seeing all the different farming methods