|SPORE Bulletin of the CTA No. 57 (CTA Spore, 1995, 16 p.)|
Insects are the most widespread and costly pests of agriculture. While some consume agricultural products, others spoil produce. Several insects are vectors of debilitating and often fatal diseases; others cause extreme distress, and even death, amongst livestock. Control has been largely dependent on chemical pesticides but for some serious pests non-chemical control and even eradication can be achieved utilizing what is known as the Sterile Insect Technique (SIT).
Insect pests are successful because they are resilient: many have short life-cycles and potentially high reproductive rates.
Their small size and mobility can make them difficult targets for pesticides and they are even better able to evade and survive pesticide applications when eggs or larval stages develop within plant or animal tissue, or pupae hide in the soil. Rapid reproductive rates also predispose developing populations to become resistant to specific chemicals. As a result, most crop and animal protection has to be on a continuing basis and is limited to reducing pest damage to economically acceptable levels. Pests are rarely eradicated, but as human population growth demands ever greater production, and market pressures require improved quality of products, the need for improved pest control becomes more urgent. The rising cost of pesticides and the new awareness of potential risks of environmental contamination and residues in food products mean that non-chemical pest control methods must be used whenever appropriate and available.
The SIT option
The Sterile Insect Technique requires the sustained release into an indigenous target population of large numbers of captive-bred male insects, which have been made infertile by exposure to a carefully calculated dose of gamma radiation. All wild females mated by the sterile males will fail to breed. No other insect is affected and there is no adverse impact on the environment.
SIT has proved its effectiveness for controlling and even eradicating several pests in numerous countries, including a number of ACP states, and the technique appears to have further potential in countries and against pests not yet targeted. The major successes have been with the New World Screwworm (Cochliomyia hominivorax), [see Spore 28 pp5] several species of fruit flies and Tsetse (Glossina spp) [See Spore 17 pp1-4].
The Sterile Insect Technique was developed in the United States during the 1930s in order to reduce the devastating animal suffering and economic loss caused by the screwworm in the southern states from Florida to Texas. The pest was also endemic throughout Mexico, the Caribbean and Central America. The outbreak of the Second World War delayed the use of SIT until 1953-54 when it was first used successfully to eradicate screwworm from the island of Curaçao. Eradication from the United States, followed in the '60s and '70s and eradication has been achieved in Mexico, Guatemala, Belize, El Salvador and Honduras. The SIT campaign against screwworm is now continuing in Nicaragua and Costa Rica and is about to start in Jamaica.
The early example of successful eradication in Curaçao, and later Puerto Rico, demonstrates the technique's potential for other Caribbean islands where screwworm is currently endemic: Jamaica, Trinidad, Dominican Republic, Haiti and also Guyana and Suriname. The Caribbean also offers potential for using SIT against two damaging pests of fruit, the West Indian and Caribbean fruit flies, Anastrepha obliqua and A. suspensa. Eradication in Mendoza, Argentina, of Mediterrean fruit fly (Ceratitis capitata) from over half a million hectares of fruit orchards is now nearing successful conclusion with the assistance of the Joint FAO / IAEA Division.
The Sterile Insect Technique has also proved capable of eradicating tsetse from pilot demonstration areas of Africa and of eradicating a potentially disastrous introduction of screwworm into Libya in 1988. If the screwworm had spread it would quickly have become a major threat to livestock, wildlife and even human health. Africa would have been faced with livestock losses amounting to thousands of millions of dollars and on-going treatment and control costs of hundreds of millions more. The situation was saved by prompt international action involving the Libyan Government, UN Food and Agriculture Organization (FAO), International Fund for Agricultural Development (IFAD) and the technical services of the Joint FAD/IAEA Division of Nuclear Techniques in Food and Agriculture, and the Mexican-American Commission for Screwworm Eradication. The eradication, achieved in only two years of operation at a cost of $78 million, was assessed independently to have provided a benefit:cost ratio regionally of 50:1.
The Joint FAD/IAEA Division, based in Vienna with its laboratory nearby at Seibersdorf, has also played a lead role in the use of SIT against tsetse. In the 1980s 1500 square kilometres in central Nigeria were totally freed of tsetse and currently the Joint FAD/IAEA Division is engaged with the Tanzanian Government in a campaign to eradicate the one species of tsetse (G. austeni) endemic to Zanzibar.
The Sterile Insect Technique has proved very successful not only with screwworm, medfly and tsetse but also oriental fruit fly, melon fly, pink bollworm coaling moth in apples and gypsy moth. There is potential for control of cotton boll weevil, cotton bollworm, sugarcane borer and sweet potato weevil. The Peach fruit fly and two species of Medfly in Mauritius could be candidates for SIT and since both the Dominican Republic and Haiti have fruit fly the whole island of Hispaniola could benefit from SIT.
In Africa currently the greatest potential is undoubtedly against tsetse. However, there remains some misunderstanding of the role of SIT within an integrated anti-tsetse programme incorporating target traps, spraying, dipping and chemical pour-on techniques. SIT is not proposed as a sole option but as a follow-up to other approaches to reduce fly numbers, when SIT can provide the final element to an eradication campaign. SIT also offers a long term option for keeping eradicated areas pest-free by using sterile male release in buffer zones to prevent re-invasion.
In essence, SIT offers an additional option for IPM for an area-wide approach to the control and eventual eradication of several serious insect pests rather than the farm by-farm approach using chemicals.