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Mass production of sterile New World screwworm flies in southern Mexico

Facilities for mass production
Production requirements
Problems of mass production
Methods development
Biological security
Radiation section

E.S. Babilonia and D.L. Maki

Rearing of the New World screwworm on a massive scale has been taking place in southern Mexico since 1976. In February 1991, after 15 years of production and the sterilization of 220 billion insects, Mexico was declared screwworm free. The screwworm rearing plant in Mexico, the only one of its kind, continues producing flies on a large scale for the eradication efforts under way in Central America and now provides FAO's Screwworm Emergency Centre for North Africa (SECNA) with sterile flies to combat the recent outbreak in North Africa. This article describes the operation of the plant, each step of the rearing process and the benefits of a recently developed gel diet. The pupae are sterilized by the use of atomic energy under strict monitoring controls. The plant is constantly involved in assuring quality control and in testing new rearing techniques and procedures. New strain development is a constant concern. Biological security is maintained to prevent escape of fertile materials.

The authors are Microbiologist and Entomologist, respectively, with APHIS, the USDA International Services Program, and are working with the Mexican/American Commission's Screwworm Eradication Program in Southern Mexico. Their address is Apdo. 544, Tuxtla Gutierrez, Chiapas, Mexico, or PO Box 3149 Laredo, Texas 73044, USA. The authors wish to acknowledge the assistance of T.R. Ashley, H.O. Hofmann and J. W. Mackley in reviewing this article.

By the beginning of 1991, over 1000 articles had been published on the New World screwworm, Cochliomyia hominivorax (Coquerel). Its life cycle, geographic distribution and economic importance have been well documented. This article highlights improvements in production rearing and discusses recent technological developments in the screwworm eradication programme.

Facilities for mass production

The screwworm mass-rearing facility is located in Chiapa de Corzo in southern Mexico, 10 km from the city of Tuxtla Gutierrez, the state capital of Chiapas. Since August 1976 the rearing facility has been operated by the joint Mexico-United States Commission for the Eradication of the Screwworms. The "Plant", as it is commonly referred to, is composed of a conglomerate of buildings comprising the following areas: insect rearing, irradiation, main warehouse, methods development and research laboratories, motor-pool and pupal transport facilities, maintenance and engineering installations, health clinic, and military and civilian security compounds. The Plant also operates its own water purification and waste treatment systems, which include two oxidation lagoons.

The largest building in the complex is the main production area. It is composed of the diet mixing rooms, larva rearing area, pupation and maturation rooms, adult colony area, laboratories, irradiation area, hot/cold rooms, maintenance, engineering facilities and offices. The building occupies 18900 m², which is about the size of two football fields in length and one in width. Approximately two-thirds of this building are designated as a biological security zone.

Production requirements

The current brood stock (OW-87) originated from a strain collected in Orange Walk, Belize in 1987 by the United States Department of Agriculture (USDA). A strain collected in Guatemala in 1988 (G-88) is maintained for backup purposes. In 1991 a new strain from Costa Rica (CR-91) was being developed by the USDA and field testing of the new strain commenced in El Salvador.

Diet systems

From 1972 until April 1990, screwworm larvae were reared on a hydroponic diet. Cotton or cellulose acetate mats were used to support larvae in rearing vats as they fed on the diet. Refeeding and removal of spent diet had to be performed every four hours, making this procedure labour intensive. Candidate agents to gel the liquid diet were screened by the USDA Agricultural Research Service (ARS) in 1984. One product, "Water Lock G-400", proved successful, and rearing of larvae on gelled diet commenced in April 1990. The new diet eliminates the need for support mats and rigorous feeding and waste-removal schedules, thus reducing labour costs and improving diet utilization. The gelled diet system also allows for a more efficient larva collection system. Comparative studies indicated that 30 percent less gelled than hydroponic diet produces the same number of pupae and significantly improves biological efficiency and adult emergence.

Diet formulation

The present diet formulation consists of 6 percent whole, spray-dried bovine blood, 3 percent milk substitute, 3 percent spray-dried egg, 1.2 percent gelling agent, 0.1 percent formaldehyde, and 86.7 percent water (see Figure 1). Feeding trays with a 22-litre capacity are initially infested with 1.25 g of eggs (23000 eggs per gram) on a 50 g ground horsemeat patty (see Figure 2) placed on 2.5 litres of gelled diet.

1. Gelled diet in tray; 1.25 g eggs are placed on diet - Régime gélifié sur plateau 1,25 d'œuf est placé sous ce régime - Dieta gelificada en una bandeja. En ella se depositan 1,25 g de huevos

2. 1.25 g eggs being placed on horsemeat patties for placement on gelled diet - 1,25 g d'œuf est placé sur des pâtés de viande de cheval en vue de leur passage à un régime gélifié - 1,25 g de huevos depositados sobre carne picada de caballo para colocarlos sobre la dieta gelificada

The infested trays (see Figure 3) are stored on racks and placed for 56 hours in larva rearing rooms (initiation rooms) at 98 to 100°F (36.7 to 37.8°C) and 65 percent relative humidity (RH). Six litres of fresh diet are added to the trays at 40, 64 and 88 hours post-infestation.

3. Immature larvae (second instar) feeding on gelled diet - Larves immatures (deuxième stade larvaire) placées sous régime gélifié - Larvas inmaduras (segundo estadio) alimentándose en la dieta gelificada

At 64 hours, the rearing trays on racks are moved to another larva rearing room (90°F [32.2°C], 70 percent RH). Then at 88 hours post-infestation they are placed in a larva collection area. A three-day larval crawl-off period begins. Mature larvae exit feeding trays, fall on to collectors and are collected every four hours.

Larvae are then transferred in bulk to a pupation room (82°F [27.8°C], 50 percent RH) where 3 litres of larvae (approximately 11000 larvae per litre) are combined with 6 litres of sawdust in a plastic pan (see Figure 4). Approximately 99 percent of the larvae pupate in 16 hours in the pupation room. The pupae are then transferred to a pupal maturation room (78°F [25.6°C], 70 percent RH), placed on screened-bottom trays and stored for five and a half days. Some pupae (3 to 5 percent of the total harvest) are diverted to the adult colony as brood stock.

4. Larvae that have pupated in sawdust medium - Larves qui se sont chrysalidées sur sciure de bois - Larvas transformadas en pupas en un medio de serrín

Once fully mature, pupae (excluding brood stock) are placed in 49 x 12.5 cm, 5-litre stainless-steel cylindrical canisters and irradiated (see Figure 5). The pupae are then stored at 50°F (10°C) for up to 72 hours before being transported to distribution centres for aerial dispersal. The brood stock is taken to the adult fly colony (78°F [25.6°C], 70 percent RH), where 6 litres of pupae are placed in each adult-fly cage supplied with water, 1.6 litres of honey and 2.4 kg of honey-meat mixture.

5. Mature pupae being loaded in stainless steel canister for irradiation - Pupes parvenues a leur plein développement placées dans des boîtes en acier inoxydable en vue de leur irradiation - Carga de pupas maduras en un recipiente de acero inoxidable para su irradiación

After emergence, the adults are held in these cages for about 5.5 days, during which time mating and egg development occur. When adult females are fully gravid, cages are equipped with an oviposition device and transferred to the oviposition room (85°F [29.4°C], 78 percent RH). Egging boards are placed on a special support containing spent diet and located immediately under two 7-watt incandescent lamps. The adult females are attracted to the egging boards by positive phototropism and the odour of spent-diet baiting. Oviposition proceeds for four hours, after which the egging boards are removed with the adhering egg masses. These eggs are harvested by scraping them off with spatulas for subsequent rearing (see Figure 6). The adult flies are removed from the cages and destroyed by cold room and cage-washing procedures.

6. Scraping off egg harvest from egging board removed from adult fly colony - Récupération des œufs par grattage d'une planche de ponte prélevée dans une colonie de mouches adultes - Raspando los huevos de un tablero retirado de la colonia de moscas adultas

The Plant has produced up to 550 million sterile pupae per week. The current weekly quota is 170 million, and this is accomplished with an average of 150 employees per day (three shifts) directly involved in insect handling. As programme needs expand, higher quotas are contemplated. Modernized equipment and augmented automation are expected to offset the need for more labour and to ensure that the goal of 350 million sterile pupae per week is reached.

Problems of mass production

In artificial rearing systems the requirements for a stable rearing environment can be met only in varying degrees but never entirely. There are also varying genetic responses in insect propagation. Maximum rearing efficiency ranges from 60 to 70 percent. Quality and quantity of diet, temperature and relative humidity, physical handling and higher than natural insect densities have the greatest impact on growth, development and survival of the insect. Dietary ingredients that do not meet minimum specifications or improper mixing can have an adverse impact on egg eclosion and larval growth, as can bacterial growth.

A larval crawl-off period of three days is provided for on the rearing floor. Approximately 60 percent of the harvested larvae crawl off on the second day, and 20 percent each on the first and third days. This uneven crawl-off causes unequal larval collections. Temperature and humidity disparities can have a significant impact on all aspects of the rearing schedule, from delaying or reducing egg eclosion and oogenesis to pupal age variations at irradiation. Quality of pupation media can affect maturation. Irradiation of pupae at an inappropriate age may affect aspects of adult field performance such as emergence, longevity and flight agility. At times, union demands and labour intransigence interfere with complete compliance in daily chores, which affects the insects rearing cycle.

Methods development

Essential to the programme is the Department of Methods Development. With a personnel force of 71, this department develops new technologies that increase programme efficiency while minimizing expenditures. The sections that make up Methods Development include Insect Quality Control, Quality Control of Essential Materials, Research and Development, and Strain Maintenance.

Insect Quality Control's primary responsibility is to assure the quality of mass-produced insects by continuous monitoring of various post-irradiation parameters of the adult fly, such as emergence, flight agility, sexual aggressiveness, sterility, longevity, sex ratio, mortality, pupal weight and appearance. Quality Control of Essential Materials is responsible for ensuring the acceptability of dietary and other raw materials used in the programme.

Research and Development is involved in testing new products and in designing innovative techniques that can be applied to mass rearing. A recent accomplishment was the implementation of gel technology in mass rearing. Another technology being studied is "naked pupation". This method induces the larvae to pupate in the absence of a pupation medium and may increase the efficiency of mass rearing.

Strain Maintenance is responsible for rearing, maintaining and field testing newly developed strains. After a successful field evaluation, the strain can then be used if the existing strain fails to perform adequately. At present, the new strain being developed is Costa Rica 91 (CR-91)

"Chilled fly", a concept that involves rapid cooling of newly emerged flies, will have a definite impact on the advancement of screwworm eradication. The adult flies are maintained in a lethargic state until dispersal, and this has proved to be a constructive and cost-effective eradication technique.

Biological security

In screwworm rearing, biological security is of optimum importance because of the risks of reinfestation caused by the escape of fertile screwworms. To achieve maximum security, the rearing areas are kept isolated from the rest of the production area. At the main entrance/exit all personnel are checked before entering or leaving the plant. There are no windows or other exits, with the exception of emergency exits.

Inside the biological security area, personnel are provided daily with clean clothes and boots. All personal belongings are kept in assigned lockers isolated from rearing areas. Employees take showers prior to leaving the security area. All equipment and material that enters and leaves the plant goes through a hot room for an established period of time to assure biological security. Traps and sentinel animals are maintained around the perimeter of the plant as surveillance for the possible escape of fertile flies.

In the event of fertile fly escapes, sterile adults are immediately released around the plant site. Biological security is also maintained by proper disposal of the waste material generated in the screwworm rearing process. The organic waste is steam-treated prior to leaving the plant and then placed in sedimentation tanks. The aqueous portion of the waste is decanted and pumped to an oxidation lagoon. The remaining solid waste is buried in a special landfill. Aqueous waste is aerated and eventually pumped into a second oxidation lagoon. The water is returned to the nearby river once the normal nitrification process is completed.

Radiation section

Mass-reared screwworms are rendered sterile in the pupal stage. This is accomplished by exposing mature pupae to ionizing radiation, namely gamma particles, in one of four Husman irradiators, designed in the early 1970s by Chester N. Husman, a former USDA-ARS engineer. In 1974, after minor external modifications to the exposure mechanism, the four Husman irradiators were utilized in the screwworm facilities in Mission, Texas. Later these irradiators were transferred to Tuxtla Gutierrez; they have been the only irradiators used in the eradication programme in Mexico.

The radioactive source, caesium-137, was produced at the Oak Ridge National Laboratory in Tennessee. With a half-life of 30 years, caesium-137 proved to be more reliable than the previous source, cobalt-60 (half-life of five years). These irradiators are considered extremely safe. They were originally loaded with a source of 46800 curies (Ci) distributed in three stainless steel capsules located 120° from the centre of the irradiation chamber. Because of normal degradation, the source averaged 31594 Ci as of February 1991. At present a dose of 8 heads is obtained with an exposure time of I minute, 48 seconds to 1 minute, 52 seconds. The four irradiators have a maximum operational capacity of sterilizing 600 million pupae per week. Their operation is controlled by strict monitoring procedures established by the United States and Mexican atomic energy commissions.


From the irradiation of the first pupae in Mexico on 25 August 1976 until the official declaration proclaiming Mexico screwworm free on 25 February 1991, a total of 220 billion pupae were irradiated. Releases of sterile adults continue in Guatemala and Belize. Arrangements are under way to begin eradication efforts in El Salvador and Honduras, with releases scheduled for 1991-92. Eradication efforts will continue after 1992 in the remainder of Central America.

In February 1991, the first weekly releases of sterile flies began over 25000 km² in the north of the Libyan Arab Jamahiriya in response to the introduction and subsequent outbreak of the New World screwworm in that country. Sterile screwworms for release in Libya are reared, sterilized and packaged in Tuxtla Gutierrez (see Figure 7). The African programme is directed by the FAO Screwworm

7. Loading irradiated pupae for transport from Mexico to Libya - Chargement des pupes irradiées qui seront transportées du Mexique en Libye - Carga de pupas irradiadas para su transporte de México a Libia

Emergency Centre for North Africa (SECNA). It is expected that these weekly dispersal flights will soon eradicate this outbreak of the New World screwworm in Africa.

Utilization of the sterile insect technique on the screwworm has been a success story in the eradication of a very detrimental pest from North America, ensuring a larger food supply while saving billions of dollars in domestic meat and dairy production. Incalculable dividends are also reaped by the positive impact on wildlife conservation. The success of this programme is a tribute to both biological control and international cooperation. As a model of scientific understanding, technological advancement and the peaceful use of atomic energy, the programme continues its work for the benefit of all.

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