Fall Armyworm
The fall armyworm, Spodoptera frugiperda, is one of the most devastating agricultural insect pests in the world.
Fall Armyworm Update
The fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), feeds on a broad range of important food and feed crops including maize, sweetcorn, sorghum, potatoes and many other crops. It affects the economy and agricultural trade of entire regions and is a threat to food security for millions.
This pest is native to the tropical and subtropical regions of the Western hemisphere, where its defoliating damage to maize can lead to more than 70% yield reduction. Under heavy infestations, larvae may disperse to new emerging adjacent fields with reports of 100% plant-cutting damage to newly planted maize. Biosecurity New Zealand says fall armyworm is thought to have been carried on storm fronts from Australia and arrived in New Zealand around February 2022. There are now confirmed findings of fall armyworm through the upper half of the North Island.
Overview
1. Impact to the industry
The larval stage of the fall armyworm (FAW) is a pest that can damage and destroy a wide variety of crops causing large economic loss. Outbreaks usually occur during the early part of the summer.
Destruction of crops can happen almost overnight, because the first stages of a larva’s life require very little food, and the later stages require about 50 times more. Because of this rapid change in food consumption, the presence of larva will not be noticed until they have destroyed almost everything in as little as a night.
• The fall armyworm attacks >350 plant species from 76 plant families.
• Examples of targeted crops include: sweet corn, sorghum, grasses, cereals, fruits and many more. The list of possible food sources for the larvae is extensive. Because the larva eat so much of the plant, they are extremely detrimental to crop survival and yield.
• It is most commonly recorded from wild and cultivated grasses; from maize, rice, sorghum and sugarcane.
• It occurs in two races: a ‘rice strain’ (R strain) and a ‘corn strain’ (C strain).
• While both strains have been detected in Africa and Asia, work is currently underway to determine which strain is attacking in New Zealand.
2. Why is fall armyworm so devastating?
Key pest status – Maize yield losses of 70% were reported in Central America when the pest was not controlled. Crop destruction (100%) in Brazil was reported when late stage larvae acted as a seedling cutworm. Damage across the globe estimated in billions of dollars.
Highly polyphagous – Reported feeding on over 350 host plant species from 76 plant families. FAW has already been reported feeding on maize and sweetcorn in northern New Zealand.
High reproductive rate – Females capable of producing 2000 eggs in their lifetime.
Highly migratory pest – Adults able to travel 100 km per night and 500 km prior to oviposition when assisted by strong winds. Larvae occasionally relocate to nearby suitable crops. Persist year-round in tropical environments – Eight to ten generations per year in some regions and lack of diapause allow this pest to infest crops at any time of year.
Cryptic feeding and behaviour – Adults hide during the daytime; ground migrating larvae may hide under crop cover; larvae feed deep in the whorl. These behaviours allow this pest to go undetected, avoid predation and make it difficult to reach with foliar sprays.
Multiple feeding behaviours – Defoliator, ear and flower feeder, seedling cutter. The fall armyworm feeds on almost any above-ground plant part.
Difficult to control – Natural levels of tolerance to some insecticidal proteins and demonstrated ability to develop resistance to insecticides and transgenic traits.
Transboundary pest – Reported in more than 90 countries, capable of dispersing further via commercial trade or strong migratory flight behaviour.
3. Fall armyworm spread in New Zealand
Adults are capable of flying long distances. Their migration rate is remarkably fast, estimated at almost 500 kilometres per generation. They can also spread through people movement, with it believed that their arrival in Africa was via a passenger flight.
• Fall armyworm is adapted to the warmer parts of the World; the optimum temperature for larval development is reported to be 28°C.
• In the tropics, breeding can be continuous with four to six generations per year, but in cooler regions only one or two generations develop; at lower temperatures, activity and development cease, and when freezing occurs all stages are usually killed.
• Fall armyworm can survive north of Latitude 35 south and thrive in warm climates.
- Absolute minimum temperature (ºC) 8.2
- Mean annual temperature (ºC) 17-35
- Mean maximum temperature of hottest month (ºC) 30-35
- Mean minimum temperature of coldest month (ºC) 7-17
It is anticipated that the geographical spread will be across the northern north island of New Zealand while migration further south MAY occur, especially over the summer months.
Biology & Identification
Life Cycle
Identification of adult through to eggs
The larva is the damaging stage of the fall armyworm. The larval cycle last 14 days in warmer temperatures and about 30 days in milder or cooler conditions. The larva undergoes six growth stages called “instars” which are identified as L1, L2 , and up to L6.
Learning to locate and identify egg masses and their colour is a valued skill in the scouting process to predict larval eclosion and take appropriate actions.
Eggs
- Eggs laid in clusters of 50 to 200 called “egg masses” attached to plant foliage.
- Egg masses are covered with a protective layer of whitish hairs from the female’s abdomen. A female can deposit 1500 to 2000 eggs in her lifetime.
- In maize, egg masses are deposited on the underside of leaves, next to the stalk. Under heavy population pressure, eggs may be deposited on the upper side of leaves and stems, but rarely on silks.
- Individual eggs are dome-shaped, 0.3 mm tall and 0.4 mm wide. New eggs are greenish and turn cream-coloured, brown and almost black as they mature. Eggs hatch occurs in 2-4 days under normal warm conditions.
Learning to locate and identify egg masses and their colour is a valued skill in the scouting process to predict larval eclosion and take appropriate actions.
Young larvae - growth stage L1, L2 & L3
Description:
- 1½ mm in newly eclosed L1 (neonates) to about 6-9 mm at L3.
- Pronounced hairs and bumps along the body, body colours of pale green and yellowish are common in L1-L2, which help them camouflage in green vegetation.
- As larva grows, it takes on grayish brown to dark brown colouration. Head colour varies from orange to dark brown.
- Colour variation and the small size of young larvae make it difficult to differentiate from other larva species. Seeing these distinctive markings often requires a hand lens.
Behaviour:
- Newly eclosed larvae wander around briefly and quickly move to the whorl leaves where their exposure to chemical sprays is reduced.
- Neonates may disperse by wind to nearby plants by spinning silk threads, a technique called “ballooning”, allowing an infestation to spread to nearby plants.
- As larvae grows, they exhibit increasing cannibalistic behaviour.
Older larvae - growth stages L4, L5 and L6
Description:
- 15 mm at L4 to about 38 to 40 mm on fully grown L6.
- Colour patterns and spots in mid to large-sized larvae easier to observe for proper identification.
- Common colour is brown. Greenish and blackish colouration are less common.
Pupae
Description:
- 15 mm in length and shiny reddish brown colour.
Behaviour:
- Fully-grown caterpillars normally drop or crawl down the plant to the ground to pupate. Occasionally, they may pupate in plant parts, especially after ear tissue feeding.
- Larvae burrow 2 to 8 cmsinto the soil and spins a loose cocoon made of silk impregnated with fine soil particles and soil debris.
- Pupation lasts 8 to 9 days under optimal warm conditions.
Adult
Description:
- Medium-sized moths, 32 to 40 mm wingspan, with gray or brownish forewings and dull white hindwings.
Behaviour:
- Highly active at night, hide during the day. In maize, adults can be found hiding in between leaves or inside whorls.
- Strong nocturnal flyers, capable of long distance migration up to 100 km on a single night and an estimated of 500 km in the few days prior to oviposition assisted by strong wind currents.
- Females release pheromones to attract males. Oviposition takes place two to three days after mating.
- Adults live two to three weeks.
Key Identifiable features
Prevention
Effective fall armyworm management requires multiple approaches; including prevention and avoidance.
Tactics to prevent or avoid the arrival of the pest to a particular area include regulatory strategies and cultural methods.
1. Strategies by multiple government agencies
- The spread of fall armyworm across large regions is facilitated by its high-migratory behaviour and by international trade and human travel.
- Regulatory control refer to the role played by multiple government agencies in conjunctions with other regulatory agencies and governments to stop the spread of fall armyworm via inspection and destruction of infested material.
- It includes the critical role of governments in implement scientific review panels and streamlined processes to review and approve new tools to manage the fall armyworm in a new country.
2. Cultural crop management
Cultural crop management is critical to enable a healthy crop, minimising plant stress and avoiding crop or field characteristics that invite fall armyworm infestations:
- Early planting to avoid heavier pest densities in late season.
- Weed management – Eliminate weed hosts that sustain larval populations before their migration to a new crop.
- Avoid adjacent sequential planting to prevent migrations of larvae from a maturing crop to new fields.
- Avoid plant stress – Proper fertilization, irrigation, cultivation, etc. Healthier plants tend to recover faster and recover from some yield loss.
Note: The practice of crop rotation is of limited use for fall armyworm management due to the pest’s high mobility, migratory status and polyphagous behaviour.
Monitoring
1. Traps
Importance:
•Pheromone traps alert growers of the arrival of fall armyworms adults to an area.
•An increase in adult fall armyworm trapping does not trigger an insecticide application and should not be used as an action threshold. However, this information helps farmers understand when to increase field scouting.
Notes:
•There are numerous lures and trap designs for fall armyworm. Instructions for use vary by manufacturer. It is important to standardize trapping methods when the intention is to use for community-wide forecasting.
•Adult data collected over several season provide an indication of when peak adult trapping are normally expected to occur in an area.
•When handling lures, wear disposable rubber gloves or wash your hands immediately. Not doing so may cause contamination of bait stations.
•Do not leave used lures or lure wrappings near traps. The small amount of lure left in them may compete with baited traps.
•Never place more than one lure at a time in a trap.
2. Field Scouting
It is designed to detect fall armyworm presence in a field and assess feeding injury:
•Field scouting is one of the most important activities a farmer can do to manage infestations based on informed decisions.
•It consists of entering the maize field and looking for presence of fall armyworm eggs, larvae, etc. and feeding signs.
•Relies on the ability to identify correctly the fall armyworm, understand its biology and behaviour and recognize early feeding.
•It helps farmers understand ecological factors that favour or delay a potential infestation such as natural insect mortality, plant stressors, alternate hosts, plant phenology, etc.
•Fields should be “scouted” at a minimum of once per week or more often when conditions favour infestations, e.g. crops in the vegetative stage, increase in male moth trapping, presence of egg masses, etc.
Control
1. Biological
- The multiple life stages of fall armyworm are affected by predators, parasites and diseases.
- Biological control encourages the conservation of natural enemies and the augmentation of natural enemy populations to maintain fall armyworm feeding below an economic threshold level.
2. Chemical
Chemical control depends on the rational and responsible use of selected approved chemical insecticides to manage the fall armyworm.
- Includes baits and foliar insecticides.
- The use of foliar insecticides should be based on recommended economic thresholds and the implementation of an Insecticide Resistance Management Strategy (IRMS) plan.*
- Target smaller larvae (L1, L2, L3) to prevent heavier injury caused by more difficult to control large larvae. Also targeting instar larvae before they become entrenched.
- Make foliar applications before larvae burrow deeper into the whorl in vegetative stage (or before they enter the ears in mature maize plants).
*Note: Known fall armyworm resistance has occurred for carbamates (MoA Group 1A); organophosphates (Group 1B); pyrethroids (Group 3A); Bacillus thuringiensis and Cry1F protein (Group 11A); diamides (Group 28). An IRMS plan requires the rotation of different MoA for season management of an insect pest.
Solutions
Corteva Agriscience has successfully secured a registration for Sparta insecticide to help control Fall Armyworm in a number of crops. Always refer to the label prior to application to understand the full directions for use.
Important points for effective control
Sparta insecticide works in two ways,
- via direct contact
- via ingestion
It is important to remember that it is not systemic, this means that it does not travel through the plants vascular system, thus new untreated growth remains unprotected. It also means that grubs entrenched deep within the plant foliage are protected from the spray deposit and may not be controlled.
It is for this reason it is recommended that application of Sparta are made early in the crops growth stage – apply during the vegetative crop stages to ensure thorough contact with the pest. Follow label directions ensuring that the correct label rate is applied with sufficient water to achieve excellent coverage and penetration into the crop canopy.
Larval size and chemical control
