Fall armyworm: Methods for identification and control

Overview

• What is the fall armyworm?
• What is the impact of the fall armyworm?
• How do I know if I have a fall armyworm problem?
• How do I get rid of fall armyworms?
• Summary

The fall armyworm, sometimes abbreviated to FAW is a species of moth native to the Americas but is now widely distributed in warm tropical climates across the globe, including Brazil, Africa, the United States, Sri Lanka, and has recently spread to Europe. The name “armyworm” comes from the characteristic behaviour of fall armyworm caterpillars moving across entire fields in large numbers and destroying every crop along the way. This pest affects over 80 types of plants and is particularly damaging to corn and certain grasses. The fall armyworm severely impacts crop yield and causes billions of dollars of damage each year.  

This article will cover what the fall armyworm is, how to identify it, and biological control methods to manage its numbers and reduce its impact on crop yield. 

What is the fall armyworm?

The fall armyworm (Spodoptera frugiperda) is a species of moth that is typically grey-brown with a wingspan of around 3 to 4 cm. Male moths have distinctive white triangles on their wings that the females lack.  

Fall armyworm larvae can be black, green, or tan in colour, are smooth-skinned, and grow up to 4 cm in length. They have a characteristic “Y” shape on their heads and four dark spots that form a square shape on their second-to-last segment. 

Egg masses are light to dark grey and are covered in a fuzzy material, and each mass contains hundreds of eggs.  

Take a look at our comprehensive factsheet guide on identifying fall armyworms, to spot this pest in your crops and signs of an infestation. 

Life cycle

The fall armyworm’s life cycle can take 1-3 months, depending on the season, with faster cycles in the summer and longer cycles in colder months. Females prefer to lay their eggs on light-coloured surfaces, and egg masses are typically found on the underside of leaves of host plants close to the soil. Depending on the season and climate, eggs can hatch after just a few days, and larvae progress through six developmental stages (known as first instar, second instar, etc.) over 2-4 weeks. Fall armyworm larvae typically form a cocoon underground and emerge as adult moths between 8 and 30 days later, depending on the season. Visit this page for more information on the lifecycle of fall armyworms in Africa. 

What is the impact of the fall armyworm?

Fall armyworm larvae damage plants by feeding directly on leaves and other parts. There are over 80 plants that fall armyworms feed on, including many grasses. However, they are notorious for causing damage to corn crops. Fall armyworm larvae feed on the leaves and developing kernels of corn crops. Larvae also eat into the growing point (whorl) of the corn, impacting the growth of new leaves. Large fall armyworm infestations reduce the yield of many important crops, with figures reaching billions of dollars in economic losses annually. Costs associated with different control methods also contribute to the financial burden of fall armyworm outbreaks. Adult moths are not known to damage any crops.  

How do I know if I have a fall armyworm problem? 

Fall armyworm infestations can occur on a broad host range and have different symptoms depending on the host plant. 

Grasses

The fall armyworm feeds on the foliage of grasses, drying out the plant. This results in brown patches in the grass, which resemble drought damage. These brown patches will spread across the entire pasture or field as the infestation grows. In tall grasses such as sorghum and wheat, fall armyworm damage will result in defoliation (complete leaf loss), and damaged leaves will appear ragged. 

Corn

In the earlier developmental stages, fall armyworm larvae feed on the green parts of lower corn leaves and do not consume transparent (non-green) layers. This causes the “window pane effect” and may be a sign of damage early in the season. At later stages, fall armyworms eat holes in the leaves and prefer hiding in the whorl of corn plants. If the larvae feed on the corn whorl, it can produce a characteristic pattern of holes as the leaf grows. 

How do I get rid of fall armyworms?

Fortunately, many effective control methods exist for dealing with the fall armyworm, including sustainable biological options. The first step is to identify the species as the source of the damage and monitor the extent of the infestation. 

Monitoring

Fall armyworms cause damage to crops during the larval stage. Therefore, it’s important to scout your crops early for the presence of these larvae. Noting the extent of plant damage and the number and size of the larvae will give you a better understanding of the infestation.  

Sampling

This is a systematic way to determine the extent of the fall armyworm invasion. Adult moths are typically monitored using different trapping methods. If adult moths are found, we advise sampling for larvae and egg masses. A typical approach is to examine 10 plants across 10 locations within a field or 20 plants across 5 locations. You may need to increase the number of plants you check if it is early in the season or if the infestation is less established. 

Note that sampling should only be performed in a field where crop damage has been observed. 

Cultural control

Fall armyworm numbers typically grow as the season progresses, peaking in late summer. Planting crops early or using strains that develop more quickly (such as Dekalb DKC33-78) can help prevent damage to young, more vulnerable plants. 

Mechanical control

Traps using hormones to attract adult fall armyworms can be an effective way to track and control their numbers. Traps typically contain a sticky substance that prevents the moths from escaping. Hormones used to control pest numbers are a type of biological control known as semiochemicals. Sweep nets can also be used to control fall armyworm numbers. These are handheld nets used to catch and monitor fall armyworms in crops by sweeping through the plants. 

Biological control

For managing pests like fall armyworms, the biological control methods listed below offer a safer, environmentally friendly alternative to using chemical pesticides. 

Natural substances

These are biopesticides derived from natural sources such as plants. Neem oil is extracted from the neem plant and used to combat various pests, including the fall armyworm. Azadirachtin is found in the seeds of the neem plant and can prevent fall armyworm feeding and reproduction. These approaches are typically more effective on younger fall armyworm larvae as mature larvae are more likely to hide within the host plant and avoid contact with the substance. 

Microbials

Microbials are microorganisms that control pest numbers and include bacteria, fungi, and viruses. Many viruses cause lethal infections to fall armyworm larvae and can be used as an effective biocontrol method. The bacteria Bacillus thuringiensis is widely used to combat the larvae of common pests, including the fall armyworm. These are typically applied in a spray and thus may be particularly effective against immature larvae and less so against more developed ones. Furthermore, this pest is more susceptible to infection in the early stages of development. 

Macrobials

These small animals help control pest populations within their natural habitat. Parasitic wasps are an effective biological control agent against fall armyworms and are also natural enemies of this pest. It is important to use this approach when fall armyworm egg masses are present. High adult moth activity can indicate that eggs are being laid. 

Chemical pesticides

Chemical insecticides can be used to control fall armyworm numbers but can lead to pest resistance and are known to harm humans, with studies showing direct toxicity on human cell lines. 

Summary

Fall armyworm is a major threat to crops globally, including in North and South America, causing extensive damage to corn and other plants. This pest, native to the Americas, invades in large numbers and can consume entire fields. Effective control involves monitoring, sampling, and a combination of cultural, mechanical, and biological control. Biological controls, like natural substances and parasitic wasps, offer eco-friendly alternatives to chemical pesticides. Adopting integrated pest management strategies is crucial for mitigating fall armyworm damage and ensuring sustainable agriculture. Governing bodies such as the UN Food Agriculture Organisation also provide information on fall armyworm management. 

Visit our resources section for more information on methods of biological pest control. To find integrated pest management programs for your particular pest problem, visit the CABI BioProtection Portal.