By David N. Leff
The screwworm isn't a worm, it's a fly. The Mediterranean fruitfly is a far greater scourge in Florida and California than around the Mediterranean. The sheep blow fly doesn't blow, it buzzes - very loudly. All three of these insect agricultural pests have one thing in common: Attempts to eradicate them are playing the sterile-insect card.
Furthest along in this effort to finesse sex instead of blunt-instrument pesticides is the New World screwworm fly (Cochliomyia hominivorax), which owes its common name to the circular spines that girdle its maggots, looking like screw threads around a worm. Its larvae invade the living tissues of livestock animals, which the winged insect enters via open wounds or nostrils.
"Thanks to aerial bombing of vast areas with sterilized male and female winged adults," recalled molecular biologist Maxwell Scott," starting in the late 1950s, screwworm has been eradicated from all of North and Central America." He added: "The program continues up until the present day. I believe the flies are pushed down to Panama now. This is one of the big success stories of the U. S. Department of Agriculture." Scott is a senior lecturer in genetics at Massey University in Palmerston North, New Zealand.
The Mediterranean fruit fly (Cerititis capitata) - better known as the medfly - still wreaks mayhem on the citrus crops of Florida, where it first surfaced in 1929, and California, which experiences sporadic outbreaks. The black and white insect lays its eggs on citrus and other fruits. When the larvae hatch, they munch their way inward to render the produce inedible.
"It's still a major pest," Scott observed, "because it has such a wide host range, in all warm climates. If you have medfly in your orchards, then other countries can refuse to import your fruit. You've got the damage the fly causes, and then the added trouble of losing markets.
"The ongoing sterile-insect campaign against the medfly," Scott said, "is science on a large scale. It typically involves insect factories, and employs male releases only. That makes common sense. Sterile males have to work a little harder to find a female to mate with. "The males will survive because they carry a normal wild-type form on their Y chromosome of a gene called tsl, for which females are mutant.
"So that's the system that works right now for medfly," Scott continued, "and there are factories around the world producing millions and millions of medflies each week in this system. It's taken a long time to develop it, and it works quite well. The medflies," he related, "are raised in huge cages, probably 6 inches wide by 4 or 5 feet tall and at least that wide. These frames are covered with a sort of mesh. Flies are flying around in these big containers, and the females lay their eggs through the mesh. They back up to it and poke their eggs out. The eggs drop down into troughs of water, which skim them up and transfer them to a food source where the hatched larvae can eat.
"If you want to use this method for other insects," Scott suggested, "trying a sterile-male-only release, you need another system for separating males from females. That's what we've done. We've developed what we think is a general system that could be put into any insect that can be genetically manipulated."
Dealing Blow Flies A Mortal Blow
"In the last few years it's become very clear that we can manipulate most insects that people have tried. One of the things others are considering is putting into mosquitoes a system like the one that we've developed. We'll be putting it into blow flies eventually."
Blow flies (Calliphorid species) belong to the same taxonomic family as the screwworm flies. "In Australia and New Zealand," Scott said, "blow flies cause primarily sheep-strike. The flies lay their eggs on the sheep, and they hatch into larvae. When these get sufficiently big, they burrow into the animal and start eating - much like screwworm does in the States. It makes the sheep quite sick, and eventually kills it. Flystrike costs the New Zealand sheep industry about $40 million in damage and loss each year."
"The first thing we have to do in the blow fly," Scott observed, "is get the transgenic system going. No one has yet made transgenic blow flies. We think we've pretty much figured out how to do that now. We'll probably have something to say about it in a couple of months. But once we've got the ability to make them transgenic, the first thing we'll test in blow flies is just the same system we put into common fruit flies - Drosophila melanogaster."
Scott is senior author of a report on that proof-of-principle project in the current Proceedings of the National Academy of Sciences (PNAS), dated July 18, 2000. It's titled, "A repressible female-specific lethal genetic system for making transgenic insect strains suitable for a sterile-release program."
"As we report," he told BioWorld Today, "we have devised and tested a way to reliably trigger the death of female fruit flies, leaving sterilized males alive."
He genetically modified the insect so that a tetracycline-sensitive protein, encoded by a tTA gene, turns on a cell-death apoptosis gene called hid. That protein is made only in the female's fat tissues, a ploy operated by a female-specific yolk protein gene. "When the flies are raised on normal fly food," Scott explained, "expression of the apoptosis gene kills the fat cells, so females die. Adding small amounts of tetracycline to their food inhibits tTA, so they survive."
U.S. Collaborator Takes Aim At Mosquitoes
Scott is collaborating with research entomologist Stephen Dobson, at the University of Kentucky in Lexington.
"We are sharing DNA constructs and transgenic animals," Dobson told BioWorld Today. "We also have plans to do future experiments. We're both addressing a similar problem through somewhat different means. He has a purely transgenic approach, whereas I am using a naturally occurring bacterium, Wolbachia pipiens, which accomplishes the same thing. It sterilizes the male. My current focus," Dobson went on, "is mosquitoes. Culex pipiens is the mosquito implicated in transmission of West Nile virus. It's the first insect in which Wolbachia was discovered."