Embu County Fao programme officer Patrick Marangu said the situation poses a threat to food security.
Ms Lucy Kagendo, a resident of Tharaka, said they had sprayed the pest using various pesticides without success.
Livestock Chief Officer Adan Jaldesa said farmers in Burat and Ngaremara in Isiolo have been affected by the infestation.
More than a quarter of maize plantations in Embu County have been destroyed by the armyworm.
County Director of Agriculture John Ndwiga said 5,500 out of 20,000 hectares of the crop had been affected.
He said the infestation is mainly in Ugweri and parts of Mbeere.
The official spoke during the launch of a programme to mitigate the impact of the worm.
Food and Agriculture Organisation (Fao) Embu County programme officer Patrick Marangu said the situation poses a threat to food security.
“We have embarked on a pilot programme involving 300 farmers in Kagaari, Runyenjes Sub-County, to assess the modality of control,” he said.
Some 50 scouts have been selected for a two-day training on a mechanical control of the worm.
The 50, who will be paid by Fao, will then train farmers at the local level on this method, which involves manual destruction of egg masses and larvae.
It is considered a simple and inexpensive option for affected farmers to curb the damage to their crop.