By Luis Jaime Acosta

MANIZALES, Colombia (Reuters) - Colombian farmer Jairo Morales is worried. His coffee trees are speckled with crimson as tiny red spider mites attack his plantation, posing a threat not only to his livelihood but to output in the world's No. 3 coffee growing country.

The mites cling to the leaves of coffee plants and gradually turn them reddish until they wither and die.

The threat comes at a time in which Colombia is struggling to increase annual coffee output to 11 million 60-kg sacks, the country's long-term average.

The tiny arachnids have always been a menace to coffee crops in the Andean country, but other predator insects have usually kept them at bay.

"This has been a surprise. I'd never seen anything like this in the many years that I've been growing coffee. I often see small areas by the side of the road, but never an attack like this," Morales said.

Red spider mites have attacked many plantations in Caldas, the No. 4 coffee producing region in Colombia, contributing about 10 percent to the country's total coffee output.

Morales suspects that the increasing number of spider mites could be a consequence of the ashes that covered the area after the Nevado del Ruiz volcano eruption in June, which apparently killed the insects that prey on the arachnids.

"The risk is that they 'burn' the leaves and it takes a long time for the plants to recover," said the farmer at his plantation on a mountain slope in the Caldas region.

"If the coffee trees fail to grow branches and flower we'll lose the crop that we're about to harvest and we can lose next year's because they will not flower," he said.

Crops in the Quindio, Risaralda and Valle del Cauca regions have also been hit, although less severely, according to the coffee growers federation.

Colombia, the world's top producer of high quality arabica beans, has missed its annual coffee production goals for three consecutive years due to torrential rains brought on by the weather phenomenon La Nina.

Heavy rains prevent flowering, which last year resulted in a reduced output or 7.8 million sacks, the lowest in three decades. Production this year is expected to be around 8 million bags.

MOUNTING TROUBLES

Farmers are fumigating plantations to combat the red spider mites. The additional expense raises production costs at a time of lower revenues due to a drop in global coffee prices and compounded by the appreciation of the Colombian peso.

The stronger local currency adversely affects growers who must cover their costs in pesos while receiving U.S. dollars for their exports.

Marcelo Salazar, a member of the Caldas coffee growers committee said that despite efforts to contain the pest, spider mites will likely diminish total output in the region.

"It could cut output by as much as 30 percent, it could cause damage because the leaves fall, the trees do not grow beans, the coffee could have lower quality," he said.

"And that would be awful now that we've got problems with the prices and hardships stemming from global warming," Salazar added.

The pest will likely cut into the income for some of the 560,000 families that depend on growing coffee for a living in Colombia.

Of the roughly 900,000 hectares of coffee plants in the country, about a third have recently been replanted with fungus-resistant varieties that will gradually become productive starting in 2013.

The national coffee growers federation does not yet have an estimate of how large an area has been affected by the mites, but says that if farmers act quickly the threat will be under control.

"There's a risk that this could become a plague if the number increases because it affects the leaves and hampers photosynthesis ... which could affect output," said Carlos Alberto Saldias, an agricultural expert at the federation.

"From an overall point of view it won't have a big impact if we tackle it now."

(Writing by Eduardo Garcia; editing by Gunna Dickson)