The native range of the Fall armyworm – Spodoptera frugiperda – is in South and North America. But it’s rapidly spreading across southern Africa. This follows the first reports of its arrival on the African continent in Nigeria in January 2016. Within a year it spread, reaching South Africa by January 2017.
The Fall armyworm is adding to the devastation already caused by the native African armyworm, Spodoptera exempta.
The Fall armyworm problem has massive implications for people in the region. The moth is a severe pest of maize and other grass family crops such as sorghum. It poses a serious threat to African agriculture and food security as well as international trade through quarantine restrictions. Its arrival is a particularly heavy blow for southern Africa which is just recovering from a severe drought.
There are 208 million people dependent on maize for food security in sub-Saharan Africa. Maize also provides crucial income for small-holder farmers in the region.
Understanding how armyworms breed, travel and feed is critical to managing the devastation they can cause. They have a number of characteristics that make them particularly hard to control. This includes the fact that the moths are strong flyers, the fact that they breed at an astonishingly high rate and that their larvae can feed on a particularly wide range of host plants. In addition, they tend to develop resistance to pesticides.
Biological invasions like these threaten biodiversity, the functioning of natural and agricultural ecosystems, and ultimately food security. Sub-Saharan Africa is considered to be particularly vulnerable to invasive species due to its high dependence on agriculture.
Usually the expansion of the geographical range of a species is hampered by barriers like oceans and mountain ranges. But an increase in international trade and travel has greatly facilitated biological invasions in recent decades. The larger grain borer, Prostephanus truncatus, another native of the Americas, was accidentally introduced into Tanzania in the 1970s. The beetle spread rapidly through infested consignments of maize and dried cassava. The species has invaded numerous countries since its first introduction into Africa.
There’s disagreement about how the Fall armyworm arrived in Africa. One suggested avenue is that it arrived on foodstuffs imported from the Americas. This is feasible as insects can readily cross borders with infested plant material. The species has been intercepted on shipments destined for Europe on several occasions.
It’s also possible that the Fall armyworm arrived over the Atlantic through wind currents. This is because wind-borne adult insects can move over vast distances. The Fall armyworm wouldn’t be the first insect species crossing the Atlantic in this way. The most famous example is the monarch butterfly, Danaus plexippus, crossing the Atlantic from America to the British Isles.
Whichever way the Fall armyworm arrived, its rapid spread across the African continent attests to its high dispersal ability. As strong flyers adult moths cross borders with ease. In the US the species has long been known to use jet streams for adult dispersal.
Biology of the armyworm
The scientific name, Spodoptera frugiperda, refers to the grey-patterned wings of the moths and the fruit destroying habits of the caterpillars. The common name, Fall armyworm, is based on the habit of mass movements of the caterpillars in autumn.
The Fall armyworm has several characteristics that make it difficult to control. Apart from being a strong flyer, adult females are highly fertile, laying in excess of 1000 eggs during their lifetime. The Fall armyworm has a large host plant range that spans nearly 100 plant species in 27 families. Although polyphagous – the ability to feed on many plant species – the preferred hosts are grass based plants such as maize, sorghum, millet, rice, and sugarcane. The most damaging reports from its native range are for maize.
Another reason the Fall armyworm is difficult to manage is because of its tendency to build up resistance to pesticides. There have been efforts to curb its devastating effect by planting BT-maize. But this remains highly contested territory in many African countries.
The Fall armyworm has been reported to cause annual losses of US$600 million in Brazil alone. Caterpillars also feed on other important crops, such as cowpea, potato, and soybean. At this stage we know little about the potential impact on crops in Africa. But the fear is that it could be devastating.
Quick action is needed
Given the severe economic threat that the Fall armyworm poses, governments and international bodies are putting in place emergency plans. These include monitoring with pheromone traps to determine the spread of the Fall armyworm, road shows to increase public awareness and emergency registration of pesticides.
Eradication of the Fall armyworm at this stage is unlikely. Control of the pest will be best achieved if managed on an international scale with southern African countries coordinating their efforts.
A combination of native African armyworms and Fall armyworms from the Americas are ravaging staple crops across southern Africa. If uncontrolled, they have the potential to cause major food shortages. The Conversation Africa’s energy and environment editor Ozayr Patel asked Kenneth Wilson to explain the threat and what can be done about it.
What are armyworms, where do they come from and how do they travel?
Armyworms are the caterpillar stage of moths belonging mainly to the genus Spodoptera. They are called armyworms because when they have ravaged a crop they march along the ground like a vast army of worms in search of more food. There are at least eight countries in southern Africa that have been hit by outbreaks of armyworms.
This sequence of outbreaks began in mid-December 2016 in Zambia and has spread rapidly ever since. It is now as far south as South Africa. Because armyworms feed on many of the staple food crops they have the potential to create food shortages in the region.
The recent outbreaks in southern Africa appear to be a combination of the native African armyworm (Spodoptera exempta) and a new invasive species called the Fall armyworm (Spodoptera frugiperda). This new species is endemic to tropical and subtropical regions of Central and South America, where it causes considerable damage to maize and other crops.
It is unclear how it reached Africa from the Americas but it’s likely it arrived on imported plants. It’s also possible that it migrated across the Atlantic on favourable winds over multiple generations.
It is not yet known whether the recent outbreaks in southern Africa are derived from the earlier West African ones. But Fall armyworms are known to be strong migrants in the Americas. Every year Fall armyworms fly from Mexico and the southern states of the US to Canada.
What makes them so devastating?
Both African and Fall armyworms do most damage to the staple cereal crops such as maize, wheat, sorghum, millet and rice. They also eat pasture grasses which has an impact on livestock production.
The African armyworm – they can be 3cm long – can reach densities as intense as 1000 caterpillars per square metre, quickly razing crops to the ground. On maize, the number of caterpillars per plant is, of course, much lower but it can cause just as much of an impact. The insects strip the leaves of even mature maize plants bare.
Unlike their African cousins, the Fall armyworm also feeds on a range of non-cereal crops. Nearly 100 different host plant species have been recorded. These include cotton, soybeans, groundnut, peanut, potato, sweet potato, spinach, tomato, sweet peppers, cabbage and tobacco.
Damage to maize is likely to have the biggest impact on farmers in southern Africa because it’s the main staple food crop in the region.
The impact of the Fall armyworm is likely to be devastating because it eats the leaves of the plant as well as its reproductive parts. This damages or destroys the maize cob itself.
Where have the most devastating attacks occurred? What was the result?
In neighbouring Zimbabwe, seven out of eight maize-producing provinces have had armyworm outbreaks, and in Malawi at least 9,000 hectares have been attacked. Figures are not yet available for the other five southern African countries currently affected.
What are the potential economic consequences if the problem is not arrested?
It is too early to say what the impact will be on food production in the region. Chemical pesticides have been mobilised in most countries, though their efficacy has been questioned. In Brazil, where armyworms can breed all year round, controlling them costs an estimated US$600 million a year. The cost of control in southern Africa hasn’t been determined yet.
But it’s likely to be substantial given that many litres of imported chemicals have already been bought by countries desperate to protect their crops. This means that even if control proves to be effective it will have been costly.
The economic consequences could be severe if the Fall armyworm persists and spreads throughout the sub-Saharan Africa region.
What is the best way to stop them damaging crops?
Chemical pesticides can be effective against both armyworm species. But resistance to many chemicals is an issue for the Fall armyworm throughout its native range. It’s not known whether there is pesticide resistance in the Fall armyworms blighting southern Africa.
The variable efficacy may be due to genetic resistance, or it might be as a result of the way in which the spray is applied. The Fall armyworms are often inaccessible to insecticides because of their tendency to hide in the whorls and reproductive parts of the host plant.
Research is needed to work out which chemical is the best to control the strain of Fall armyworm in southern Africa.
But there are alternative approaches.
In parts of their native range in the Americas, genetically-modified Bt maize is grown to combat the Fall armyworm. This may also be an option for South Africa and some other countries where GM crops are already grown. But many parts of Africa do not allow or welcome GM varieties. And Fall armyworm has also evolved resistance to some Bt toxins, with some evidence for cross resistance.
There are non-chemical, biological pesticides that could also be effective. These are pesticides derived from natural diseases of insects, such as viruses, fungi and bacteria. I have been involved in the development of a highly effective biopesticide against African armyworm in Tanzania. But this still needs to go through the commercialisation and registration process, which is both costly and time consuming.
A similar biopesticide has also been developed against the Fall armyworm, but again this is not yet registered for use in Africa.
Biopesticides tend to be effective against a much narrower range of species than chemicals, which is good for the environment. But it means that they can only be used for a limited number of pests, often making them more expensive than chemicals.
There are also some other indigenous approaches that could be effective. This includes the use of local plant extracts like Tephrosia vogelli and neem, to produce botanical pesticides, and the addition of sand to maize whorls where armyworms are feeding.
Only time will tell what the full impact of this armyworm invasion will have.