Global experts are meeting in the Zimbabwean capital Harare to come up with a plan to combat it.
The name is a bit misleading. It is not actually a worm, but a hungry caterpillar that eats crops before turning into a moth.
It is a new pest, not to be confused with the similarly named "African armyworm", which has been present in the region for many years.
It is native to the Americas, but experts are not sure how it reached Africa.
One theory is that the eggs or the caterpillars themselves hitched a ride in some imported produce, or even made it on board commercial flights.
Why is it such a threat to farming?
- It is very hungry (and not picky) - This pest targets maize (corn) and other cereal crops, like its African namesake, but it also attacks cotton, soybean, potato and tobacco crops. When it does invade, up to three-quarters of the crop can be destroyed.
- Unknown enemy - Governments, communities and farmers have no previous experience of dealing with the new pest, which may be even harder to deal with than its native equivalent
- It is fast - According to the UN's Food and Agriculture Organisation (FAO), it has taken only eight weeks for the pest to spread to the six southern African countries where there are suspected infestations.
- It travels far and wide - The caterpillar stage does the damage but "it's the adult moth that migrates long distances and that's how it's managed to get round Africa," says Professor Ken Wilson, an expert on armyworms.
- It is not just targeting any old crop - Maize is the primary food staple in many of the areas where the pest has been identified.
- It is hard to find - The fall armyworm burrows right into the stem of maize plants, concealing itself from view and preventing farmers from spotting the problem early.
- Bad timing - It comes after two years of record droughts, which have already affected more than 40 million people in the region, making 15% less food available, according to the UN.
South Africa, Zimbabwe, Malawi, Zambia, Namibia and Mozambique are the chief suspects among southern African countries, according to the FAO.
The presence of fall armyworm in Africa was first reported on the island nation of Sao Tome and Principe in January 2016, it says.
Other research groups have also reported it in parts of West Africa, including Nigeria and Ghana.
But the governments of Zimbabwe and South Africa are the only ones to have publicly confirmed that they have a problem with this specific pest.