(AFP) - Peeling back the maize plant's leaves reveals a small brown caterpillar -- an armyworm that writhes as it burrows into the heart of the crop, producing a sticky dark paste.

Eighty percent of the Prinsloo family's maize plants are under attack, as are those of other farmers in Haakdoringboom, a farming community 20 kilometres (12 miles) north of South Africa's capital Pretoria.

"These worms are eating everything that they touch," said farmer Jacques Prinsloo, who held up a damaged leaf to demonstrate the alarming speed at which the fall armyworms devour the crop.

Leaves are shredded and residue speckles the inside of the plants -- a tell-tale sign of trouble.

The recently-arrived pests, which are proving immune to existing pesticides, are devastating crops and threatening southern Africa's fragile food supply having spread through Zambia and Zimbabwe as well as South Africa.

Malawi, Mozambique and Namibia are also reported to be affected by the worms.

They originate from South America and are thought to have arrived in Africa in shipments of plants or on commercial airliners, with the first fall armyworms in Africa seen in Nigeria and Togo last year.

"I tried everything on the market. I spent 45,000 rand ($3,400) on pesticides alone," said Jacques, 24, who has been battling the pests for six weeks.

"Last year the drought, this year the worms, what next year? Everyone thinks it's easy to farm. It seems easy until you start doing it."

Jacques estimates that as many as four in five of his maize plants are affected.

- 'We're fighting' -

If the crop fails entirely, he estimates it will cost his family up to 700,000 rand ($53,000) this year alone.

Crops in neighbouring farmers' fields are also being ravaged by the pests, according to Adele who, along with her son Jacques, employs six staff on their roughly 100 hectare farm.

"We're fighting. The farm next door to us is fighting," said Adele.

Across southern Africa, fall armyworms are wreaking havoc with staple crops for the first time.

Key food sources like maize, wheat, millet and rice have all come under attack, raising fears of imminent mass food shortages.

Nearly 40 million people in southern Africa have been affected a two-year-long drought caused by the El Nino climate phenomenon reducing food availability by 15 percent, according to the UN Food and Agriculture Organisation (FAO).

David Phiri, the FAO's coordinator for southern Africa, warned experts at a crisis summit in Harare this week that the armyworm poses "a huge threat to food security".

- 'Hopeless, angry, heartbroken' -

Despite their proximity to South Africa's seat of power, the Prinsloos feel that their plight, and that of hundreds of other farmers, is being ignored by the government.

"I'm feeling hopeless, angry, heartbroken -- it feels like I could go and sit and cry myself to death," said 50-year-old Adele who has been involved in farming for nearly half her life.

"All the money and effort that's been put in there and I'm getting no help.

"We were promised a visit by the farming minister on Sunday but they cancelled. Nobody is doing anything. They said they would import a poison from abroad but we've heard nothing."

The region around the Prinsloo farm had only just begun to recover from one of the worst dry spells in recent history when the armyworms struck.

"A year ago we had the drought but then we had good rains. Now the worms are destroying the crop," said Adele.

In one badly affected field, nearly every plant is showing signs of damage.

Looking out over his family's fields, bordered by tracks of rich red earth and criss-crossed with mechanical irrigation systems, Jacques is doubtful there will be a quick solution to the crisis.

"(The plants) are not going to make corn because of the damage," he said of this year's crop. "The larvae is making new worms and you must fight them again. Burning it might be the only option."

© Agence France-Presse