By Jack Keyser

Leon, Guanajuato, Mexico- - In the fertile irrigated fields surrounding this city 235 miles northwest of Mexico City, farmers raise two potato crops each year for the country's metropolitan markets. Of the two, it is the winter crop of white- skinned tubers preferred by Mexican consumers that has spelled the financial difference between a so- so year and a good one.

But this off- season crop is vulnerable to a wide range of pests and diseases, particularly the potato tuber moth, rated as the top enemy of the potato in the developing world. Until recently, the first line of defense against the moth was increasingly heavy doses of chemical pesticides. But continuing crop losses have led area farmers to adopt integrated pest management measures that rely on a mix of environmentally safe methods and minimal chemical sprays.

The potato preferred by Mexican consumers is Alpha, a variety developed in Europe more than 50 years ago. Despite its popularity, growing Alpha in a two- crops- a- year cycle is a highly risky venture. "Because it's late maturing," says Jose Luis Fox Quesada, a major grower, "we have about a 10- day window at the beginning and end of each of the two growing seasons to escape frost or rains. It's a risk."

Alpha is also highly susceptible to pests and diseases. Increasingly heavy infestations and subsequent crop damage from pests, particularly the potato tuber moth, led farmers to increase the use of pesticides to control them. In 1990 farmers were applying a kind of "pesticide cocktail," a mixture containing as many as three different chemical compounds, many of them banned in more industrialized countries.

Problems Create Pesticide Treadmill

But the moth problem, rather than improving, only got worse. In a counterattack, growers increased the number of sprays to as many as 20 a season- - running up a collective pesticide bill of about $7.5 million a crop. By the winter crop of 1991 the indiscriminate use of pesticides reached a point where spraying was having virtually no effect on the moth. Aphids and whiteflies also began attacking the crop because the insects that were once the natural enemies of these pests were either killed or rendered harmless by the sprays.

The Leon potato growers were caught in a situation of using more chemicals, more often - - some as many times as once every three days- - with steadily diminishing results. The situation is referred to by entomologists as being on a "pesticide treadmill." In desperation, the Guanajuato Potato Growers Association (Union Agricola Regional de Productores de Papa del Estado de Guanajuato) asked the International Potato Center (CIP) for assistance.

CIP sent entomologist K. V. Raman and agronomist Jose Luis Rueda to assess the situation. What they found were potato farmers with capital investments of up to $500,000 in a state of frustration.

"They were virtually spraying at the sight of a moth," notes Raman, who now works at Cornell University. "The situation was so bad that potato production in the area was on the verge of collapse had farmers continued unrestricted spraying."

Sex Pheromones To The Rescue

Although most farmers were aware of integrated pest management (IPM) as a method to check potato pests and diseases, they lacked confidence in the system. The assumption of IPM is that no single pest control program is totally successful, but that a variety of methods can provide environmentally safe, long- lasting, and money- saving control. Among these are the use of pest- and disease- resistant varieties, farming practices to reduce pest losses, and measures to preserve natural enemies of the pests. Chemical pesticides are used only when absolutely necessary.

In April 1991, scientists from CIP and the Mexican Institute of Forestry, Agricultural and Livestock Research (INIFAP) met with 70 area potato farmers to assess the Guanajuato situation. During a 10- day period the group mapped out a set of IPM recommendations, ran training sessions instructing farmers how to snuff out the life cycle of the potato tuber moth safely using low- cost sex pheromone traps, and practical farming practices such as removal of dead vines and hilling up soil around the plant.

Sex pheromones, naturally occurring compounds common to most insects, can be extremely effective in detecting pest infestation levels. Female pheromones of the potato tuber moth are used in plastic traps to attract male moths. This allows growers to monitor potato tuber moth infestations so insecticides are only applied when absolutely necessary. This process, in turn, has led to the restoration of the natural biological balance in many fields whereby other potato pests are controlled by beneficial insects that were previously killed by the sprays.

Spraying of Chemicals Cut By 75 Percent

Before he established an IPM program, Fox says he was spraying pesticides from 14 to 20 times during the winter crop season. This year, he notes, he has slashed pesticide applications by 75 percent, to four or five passes a crop.

Leon farmers say the cost of producing a hectare of potatoes used to be about $8,000. Before they incorporated integrated pest management practices, 18 percent - - or $1,500 - - of the $8,000 cost was for pesticides. Today, with a combination of IPM practices, the pesticide cost has been reduced to $560 a hectare.

Ramiro Rocha, an entomologist with INIFAP who has worked with potato farmers in establishing integrated pest management programs in the area, says a 1992 survey at three Guanajuato locations showed that where farmers used IPM measures during the irrigated winter crop, they were able to cut their pesticide spray schedule from 20 passes in 1991 to six in 1992.

Growers Ignacio Gonzales Alvarez and Ricardo Romero say that at first many growers kept away from IPM from fear that it wouldn't work. They didn't want to gamble on losing part or all of their crop. Now growers are gaining confidence in the methods. Gonzales notes that not all growers are using IPM. "Once we have widespread IPM practices in use in the Leon area by members of the potato association," he says, "others will follow. Then the natural control of the potato tuber moth and other pests will increase in effectiveness."

Jack Keyser, a freelance writer, produced this story on assignment in Mexico for the International Potato Center.