What do you get when you dangle a battered beer can on a stick? It may sound incredible, but the answer is the single most important component in farmers’ fight to control the sweetpotato weevil in Cuba.

Grown on 60,000 hectares across Cuba, sweet­potatoes contribute precious calories and vitamins to a needy population. That’s why, when in 1993 Cuba was suddenly cut off from the pesticide imports it had depended on for years and weevils devas­­­tated the country’s crop, Cuban researchers began to search for natural, in­expensive and easy ways to manage the pest.

The problem started with the collapse of the Soviet Union in the early 1990s. Without Soviet-provided inputs, defenses went down in Cuban fields and the sweetpotato weevil — Cylas formicarius, an imported insect with few natural enemies — faced little opposition. In 1990 average losses for cooperatives, private farms and state enterprises in their sweetpotato fields had been less than 10 percent. In 1993, the figure jumped to more than 50 percent. But by 2000, the tables had turned again, the weevil problem was under control, and damages were virtually nil. What made the difference?

New age

In 1993 CIP and INIVIT, Cuba’s Instituto de Investiga­ción de Viandas Tropicales, joined forces to develop a comprehensive plan of attack against Cylas. A return to pesticides was not only unlikely — it was not desirable. It was the beginning of a new age for Cuban farmers, and pesticides were not to be part of it. Instead, natural solutions were the aim.

Farmer Pedro Saez reflects on the pesticide-dependent era as he contemplates the healthy sweetpotato fields on his farm in Manacas (Villa Clara Province).

“I don’t need pesticides any longer, and I won’t go back to them again,” he says. According to a joint CIP-INIVIT sur­vey conducted in 2000, virtually all the Cuban producers agree.

Saez began working with predatory ants, soaking bunches of dried grass in sugar water and leaving them in his plantain fields. Two days later, when the grass bundles were covered with ants, he took them to his sweetpotato fields, where the ants feasted on weevil eggs and larvae.

Over time Saez has discovered that around 25 of these ant deposits will do the job on one hectare of sweetpotato. Once the crop is harvested, the ants will move on to the nearest field where sweet­potato, a year-round Cuban crop, is grown.

Saez gets ad­di­tional support from the fungus Bea­uveria bas­siana, now produced all over the island. Although this fungus is totally harm­less to plants, humans and animals, a small dose spread over infest­ed fields can be fatal to a good part of the Cylas population within a week.

Farmer Alfredo Acosta, President of the Heroes de Yaguajay cooperative in the Alquizar munici­pality (Havana Province) remembers his worst harvest. “In 1992 our losses from weevil attacks amounted to 60 percent of our harvest. And our cooperative,” and the agricultural ministry officials around the table nod in agreement, ”is, by Cuban standards, one of the best.”

He found a few things to fall back on: traditional crop management techniques and INIVIT’s sound recommendations, among them, the use of irrigation. Dry, cracked soils make it easy for weevils to lay their eggs directly on sweetpotato roots. Acosta was also advised to rotate crops and never plant his crop next to another sweetpotato field. Even so, he — like many other Cuban farmers —  suffered terrible losses.

Acosta, nonetheless, was initially skeptical about the new devices, known as weevil traps, that INIVIT and CIP were promoting. “I saw them in other pro­ducers’ fields and there were lots of weevils around them. I was afraid they might actually harm my crops. It looked to me like a bad idea,” he recalls. But as the economic crisis persisted, he felt obliged to give the traps a try. Today, the co­operative’s 60 hectares of sweetpotato fields yield a healthy 30 tonnes per hectare, way above the national average of 6-7 tonnes.

Nilo Maza Estrada, an INIVIT economist, agrees that ‘trap’ might not be the best word to describe what the beer cans do. “The cans are bottomless and will catch nothing. But inside there’s a small, red plastic disc on a string. That disc contains a 0.25 mg dose of pheromones with a smell that resembles the mating signals of a female Cylas.” The beer can merely provides protection against the sun and the rain; in other locations it is substituted by a small piece of wood for roofing.

Pheromones are used in combination with the Beauveria fungus, which is heavily sprayed in the few square meters around the traps. When male weevils swarm around the traps in search of females and alight on nearby plants, they encounter the fatal fungus. The ideal number of traps is 16 per hectare, but farmers often use fewer, moving them around the field every week. The effect of the pheromones lasts three to four months, a whole sweetpotato-growing season.

Acosta and farmers like him are now convinced of the effectiveness of these traps. Even so, there remains one challenge: to find a means of guaranteeing stable supplies of pheromones, the most expensive of the control agents.

More to come

Meanwhile, other options are also being explored. Cristóbal Yera, in charge of one of the big state agricultural enterprises in Santo Domingo (Villa Clara Province), is staking his bets on an important component of the integrated pest management, or IPM, tool box: his seed bed. Sweetpotato is reproduced with stem cuttings. Traditionally, the cuttings are made at harvest time to ensure that the plant has completed its productive cycle. The risk of transplanting stems infested by weevils, however, is higher at this time.

Yera explains the alternative: “Now we plant a smaller area and make cuttings from plants only half way to maturity, when infection by weevils is not so advanced and the cuttings are very vigorous. By making the cuts at the top of the plants we reduce the risk of infection even further.”

A bonus to this system is that harvest and planting don’t coincide, meaning less of a labor problem. Previously, producers would get around this problem by storing their harvest in the soil until they could find time to take it to the market. Farmers now know that the longer the crop stays in the ground, the higher the risk of weevil damage.

The IPM tactics recommended by INIVIT and CIP  also include disinfection of cuttings, elimination of weeds that host the weevil, and removal of leftover roots and plants from harvested fields. Not all producers put all the components into practice. From among them, farmers pick and choose the set of techniques that works best for their farms, their labor capacity – and their pocketbooks.

To help make those choices simple, CIP and its Cuban partners have put together an easy-to-
understand booklet on the good and bad guys in the sweetpotato weevil drama, presented as cartoon figures. The villain is the black sweetpotato weevil. The cops cleaning the sweetpotato rows are two predatory ants (Pheidole megacephala and Tetramorium guineense) and the Beauveriabassiana fungus. The smoking gun is the ragged beer can.

Measures of success

INIVIT and CIP researchers gathered facts and figures about the impact of this work in a study published in 2000. The economists were cautious in defining the criteria used to measure success, but, even so, the figures spoke eloquently. The return on the research investment was cal­cu­lated to be at least 49 percent — by the most conservative standards — and as high as 73 percent. All in all, there was no doubt about the value of the work for Cuba’s economy, which has gained an estimated US$31 million in increased yields, reduced losses and market value.

Aside from the economic returns, there have been important health and environmental benefits. In 1990 farmers sprayed their fields with highly toxic imported pesticides 12-15 times each growing season. Today, they use none.

Meanwhile, scientists continue to breed new and better sweetpotatoes that can resist weevil attacks and boost yields. They are seeking to develop plants with deeper, harder-to-get-to roots and slimmer stems, which are less attractive to the weevils. A promising candidate from combined INIVIT-CIP germplasm has already been identified. In trials, without other control measures, it yields 34 tonnes, with weevil loss at only 4 to 5 percent.

One of the most rewarding aspects of this IPM work, according to INIVIT director Sergio Rodriguez Morales, has been the collaboration between CIP and INIVIT researchers: “It is a model of true partnership among my staff and the CIP researchers. Both sides can take pride in their results.” Cuba’s appreciation for their efforts, he points out, was officially acknowledged with a special award of ‘relevance to the nation‘.

— reported by Ebbe Schiøler