Fall armyworm: identification, monitoring and resistance management

Fall armyworm (Spodoptera frugiperda) is native to the tropical and subtropical Americas. It was first detected in Africa in early 2016 and in India in 2018, its first record in Asia, and reached China in January 2019 [1][2][5]. Maize is its main host, but the larvae feed on more than 80 plant species, including rice, sorghum, sugarcane, vegetables, groundnuts and cotton, and the moth can fly up to 100 km in a night [1][2].

1. How to recognise it

Half-grown and fully grown caterpillars are the easiest to identify. Look for three yellow stripes along the back, a dark and then a yellow stripe along each side, and four dark spots forming a square on the second-to-last segment, each with a short bristle. The head is dark, with a pale, upside-down Y on the front. Young larvae are pale and harder to tell apart from related species [1].

Eggs are laid in masses of 100 to 200, usually on the underside of leaves near the base of the plant, and are covered with scales from the moth’s abdomen; when populations are high they are also laid higher up or on nearby plants. The moth is 3 to 4 cm across, with dark brown forewings and grey-white hind wings [1].

Stem borers cause similar leaf damage in maize, but their older larvae tunnel into the stem and leave holes with frass, whereas fall armyworm larvae stay in the whorl [1].

2. Damage symptoms

Young larvae feed on the leaf surface, usually on the underside, leaving semi-transparent patches called windows and small shot holes. Larger larvae feed in the whorl and leave ragged leaves with large amounts of fresh frass that dries to look like sawdust. Usually only one or two larvae are found per whorl, because larger ones are cannibalistic. In older plants they can bore through the husk into the cob [1].

Feeding is most active at night. Maize can compensate for a certain amount of leaf damage, so a low share of infested plants does not necessarily reduce yield much. Decisions should therefore rest on scouting rather than on the sight of damaged leaves alone [1].

3. Life cycle

Several generations can develop in one season. Resistance results from the repeated exposure of successive generations to the same mode of action, which is why rotation is planned by generation [3].

4. Scouting and when to act

FAO’s scouting method covers the field in a W pattern: at the start and at each turn, inspect ten consecutive plants in a row, five stations in all. Fresh leaf damage or fresh frass in the whorl indicates a live larva. Pheromone traps show when moths arrive in an area, which tells you when field scouting needs to intensify [1].

Action thresholds are set locally, by crop stage, product and cost [1]. China’s 2019 national plan, for example, set chemical-control thresholds by the share of damaged plants: 5% from the seedling stage to the 7 to 11 leaf stage and 10% from the 12-leaf stage in year-round breeding and migration-transit areas, and 5%, 10% and 15% across the same stages in the key prevention areas further north [5].

Treat young larvae. China’s plan names the first to third instar as the best treatment window and advises spraying in the early morning or evening, directed at the part of the plant under attack [5].

5. Prevention and natural enemies

FAO’s guidance puts prevention first. Diversity on the farm, through intercropping or push-pull systems, reduces egg laying on maize and supports natural enemies; staggered planting should be avoided; and seed treatment can protect seedlings from early damage. Heavy rain also washes young larvae off leaves and drowns those in the whorl [1].

In the Americas about 150 parasitoid species have been recorded on fall armyworm. The egg parasitoid Telenomus remus has been reported with parasitism rates above 80% in several South American countries, and Chelonus insularis is the most common parasitoid in the Caribbean and Central and South America. Earwigs, ladybirds, ants and spiders prey on eggs and larvae, and viruses, bacteria and fungi infect them [1].

6. Resistance is already documented

IRAC, the Insecticide Resistance Action Committee, lists known fall armyworm resistance to four mode of action groups: carbamates (Group 1A), organophosphates (Group 1B), pyrethroids (Group 3), and Bacillus thuringiensis proteins including Cry1F (Group 11A) [3]. A programme built mainly on these groups starts from a weak position.

7. Rotate by treatment window

IRAC’s guidance for fall armyworm uses treatment windows. A window is the period covered by one or more applications of products with the same mode of action. It should not exceed about 30 days, roughly one pest generation, and should contain no more than two applications of that mode of action. The next window switches to a different mode of action, successive generations are not treated with the same group, and over the whole crop cycle a single mode of action should cover no more than about half of the exposure period [3].

The table describes mode of action groups and one country’s current permits. It is not a list of registered uses. Which products may be used, on which crops and at what rate is set by each country’s registration and by the product label.

Some formulations combine two modes of action, such as emamectin benzoate with lufenuron, methoxyfenozide or spinosad. A mixture applies both groups at once, so plan the following windows around both of them. China’s 2019 emergency list included several mixtures based on emamectin benzoate [6].

8. Biological control in the same programme

Biological products work best inside the same rotation plan, and China’s plan relies on them, together with natural enemies, where pest density is low [5]. Entomopathogenic fungi infect insects through the cuticle. In China, oil dispersion formulations of Metarhizium anisopliae CQMa421 and Beauveria bassiana ZJU435 are documented for fall armyworm in maize; these strains are the basis of Metavia CQ421™ and Beauvia Z435™. Baculovirus products (Group 31) and Bacillus thuringiensis sprays are also used against the larvae.

Bt maize has been shown to reduce fall armyworm damage, but populations in the Americas have evolved resistance to some Bt maize varieties [1], and IRAC lists Cry1F among the documented resistances [3]. Bt maize therefore belongs within a broader programme, not in place of one.

9. Regional notes

Africa and Asia

The pest reached Africa without the natural enemies that hold it in check in the Americas, which allowed populations to build up largely unchecked [1]. By early 2018 all but ten African states and territories had reported it [2]. FAO promotes Farmer Field Schools that build monitoring, natural enemy conservation and local decision-making [1].

Latin America

Fall armyworm is native to the Americas, where it has been managed longest, and populations there have evolved resistance to some Bt maize varieties [1]. Biological control is established practice: in Brazil, releases of Trichogramma egg parasitoids at about 100,000 wasps per hectare, in three introductions three days apart, are used together with pheromone traps that monitor thresholds [1].

China

Fall armyworm migrated from South-East Asia into Yunnan in January 2019 and spread rapidly north [5]. In June 2019 the Ministry of Agriculture and Rural Affairs issued a national plan that combines monitoring, light, pheromone and food lures against the moths, treatment of young larvae, ecological measures, and rotation of insecticides with resistance monitoring [5]. Because no product was yet registered for the pest, the ministry approved 25 products for emergency use until the end of 2020, including emamectin benzoate, indoxacarb, chlorantraniliprole, lufenuron, chlorfenapyr, Bacillus thuringiensis, Metarhizium anisopliae and Beauveria bassiana [6]. Products currently registered are listed by the Institute for the Control of Agrochemicals (ICAMA).

10. A practical sequence

• Scout regularly from emergence using a W pattern, and record the share of damaged plants.
• Agree action thresholds by crop stage with local advisers before the season.
• Treat young larvae, in the early morning or evening, directed at the whorl.
• Choose one mode of action for the current window, and prefer selective options that spare natural enemies.
• Switch to a different mode of action for the next generation; with a mixture, count both of its groups.
• Check results three to four days after each application, and never repeat a failed spray with the same mode of action.
• Follow the local label for crop, rate and interval.

Frequently asked questions