COFFEE, the principal cash crop, is being harvested in Central America now. The guerrillas in El Salvador and Nicaragua are doing their best to make sure the crop does not come in. It is a form of warfare -- attacks on coffee farms and mills and on other economic targets -- that has cost El Salvador perhaps $1 billion since the guerrillas took up arms in 1979. Nicaragua's costs are lower but substantial. Then there are the farmers and their families and other civilians killed in these attacks on civilian targets. This is happening in places that, without a war, were already miserably poor.

It is foolish for insurgents who hope to take over a country to cripple its economy and destroy its infrastructure and, meanwhile, to risk alienating the people by shredding the means of their livelihood. The Sandinistas were guided by this logic when they took over Nicaragua from the Somozas. The guerrillas they then set loose upon El Salvador, however, have had no similar sense or scruple. Nor have the Nicaraguan contras, whose principal sponsor has been the American government.

Sometimes an effort is made to say that one or the other group of guerrillas is more respectful of the common people and their need to make a living. It would be truer to say that both groups of insurgents, in El Salvador and Nicaragua, routinely inflict awful damage and hardship.

That both do it has a further, political impact on the treatment of this particular aspect of Central America's agony. It inhibits condemnation of it. True, the United States protests the economic damage done in El Salvador, and compensates for a good bit of it with aid. But it cannot speak with a loud and clear voice when it is sponsoring an insurgency that follows similar tactics in Nicaragua. This is one more reason to end that sponsorship. On their part, the Sandinistas are eager to tell the world of the havoc being wrought by the contras. Their complaints must necessarily be set against the havoc caused by the guerrillas they encourage in El Salvador.

In the Salvadoran peace talks, the Duarte government proposed to outlaw attacks on civilian economic targets. This was a humane and popular proposal -- even though the Salvadoran army has been known to destroy the crops, and do worse things, in areas regarded as being under guerrilla influence. But, at the talks, the guerrillas and their civilian comrades turned the government down, asserting a right of sabotage as a weapon in "people's war." The bishops plead in their homilies for an end to attacks on the people, and the insurgents' radio orders up more devastation against the "oligarchy's economy." Destroyers are never short of fancy rationales.