CABI Blog

Cochliomyia hominivorax, the New World screwworm is a serious pest that lays its flesh eating larvae into wounds of humans and any other warm blooded animals causing myiasis. The adult female fly lays batches of 200-400 eggs on the edge of fresh wounds and the larvae hatch and burrow into flesh 12-21 hr later. The larvae feed on the flesh for about 5-7 days before dropping to the soil to pupate. The screwworm used to be found throughout north, central and south America, but aggressive control programmes
eliminated the fly from USA and Mexico and it is now found only in parts of the Caribbean, and South and Central America.

Control programmes of the screwworm takes advantage of the fact that the
females breed only once in their lives. This requirement of ‘getting it right
first time’ has been exploited by releasing large numbers of infertile male
screwworm flies who still have the ability to mate. The female still lays eggs
after mating with an infertile male, but they do not hatch. The flies are reared
artificially and exposed to irradiation shortly before they emerge from the
pupae. This sterile-male-release approach to pest insect control—devised and
tested in the 1950s by ARS entomologists Edward F. Knipling and Raymond C.
Bushland—has also been used successfully to control fruit flies and other
agricultural pests.

The programme to eliminate the screwworm has been pioneered by the USDA,
Agricultural Research Service over the last 50 years. The screwworm was
eradicated from the USA in 1982 and then from Mexico in 1991. The programme is run from the Screwworm Research Unit Central Office in Panama City, and from a fly-rearing factory in Tuxtla, Mexico . After rearing at the factory, young males are sterilized and then transported
by air to other facilities for release in support of screwworm control programmes
that extend through Central America and the Caribbean.

Basic research into the screwworm biology has lead to great leaps in
efficiency in the programme. For example, a new egg powder and molasses diet
developed for adult flies replaces the smelly, costly horsemeat-and-honey diet
first used in the 1950s. The new diet is also easier to handle and more
cost-effective, and creates a more pleasant atmosphere for the staff working at
the factory unit.

The horsemeat was replaced with spray-dried eggs and the honey replaced with
molasses. This major improvement was developed in the late 1990s by a team led
by entomologist Muhammad F.B. Chaudhury at the ARS Screwworm Research Unit’s
fly-rearing factory in Mexico. Since being introduced in 1999, the new adult diet has saved the
screwworm-release programme as much as $100,000 annually. It’s been further
updated by Chaudhury’s research team by using fine cellulose fibres, instead
of shredded recycled paper or a gelling agent previously used to solidify and
texturize the diet. This saves the eradication program even more money.
Chaudhury tested this new fibre to determine its effectiveness in dispersing
food particles without hindering movement of larvae throughout the medium. After
4 years of testing, the fibre-enhanced version was put into use for mass rearing
at the Chiapas facility in October 2005. This latest refinement is expected to save an
additional $250,000 annually, at a production rate of 150 million flies per
week.

To find out more about Cochliomyia and its control check out the CAB
Abstracts Database, or search the CAB Abstracts Archive to follow the research during the
early days of the control programmes.

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