Sterile insect technique

DERRIC NIMMO: Introduction to the sterile insect technique by Dr. Derric Nimmo, Senior Technical Manager at the Innovative Vector Control Consortium.

The sterile insect technique is the type of insect pest control that has been used for over seven decades. Tests and development were initiated after World War II, and a large scale area wide use of the sterile insect technique started in the 1950s with the New World Screwworm. This will be discussed in more detail later. The sterile insect technique is a highly successful, targeted, and environmentally friendly area wide pest control technique, especially for agricultural pests. But it has been used in several pests that effect human diseases independently conceived by three different scientists in the 1930s and ’40s. Alexandra Serebrovskii, Moscow State University, working on chromosomal translocations to induce sterility. F.L.

Vanderplank, tsetse field research station in Tanzania, working on hybrid sterility in tsetse flies. And, Edward Knipling, United States Department of Agriculture, using ionising and radiation to sterilise the New World Screwworm. The sterile insect technique is a simple theory for the control of pest insects, which uses a radiation, ionising or X-ray, to sterilise insects.

The sterile insect technique is a simple theory for insect population control. To achieve control, you must flood the area with sterilised males, usually at a much higher level than wild type densities. So the insects have to be matched red. The males are sorted from the females as it is usually the females that either vector disease or cause damage. In addition, the females can distract the males, if released together. Adults are sterilised. There are several methods to sterilise the insects, but it is usually by ionising and radiation. Although, X-rays and chemical sterilisation have also been used. Sterile males are released into the wild, usually at a ratio of between 10 to 500 to one sterile males to wild type males.

The ratio is dependent on the thickness of the released males. The less fit they are, the higher the proportion of males will be required. Any wild type females that mate with sterile males produce no offspring. The releases of sterile male insects needs to occur regularly over an extended period of time from months to years. But if enough males are released over a long enough period of time, the insect population can be significantly reduced or even eradicated from large areas. The sterile insect technique and every other insect control method can benefit from an integrated vector control approach. Traditional methods, such as insecticides, larvicides, and breeding site production, tend to be cost effective at reducing high density populations of insects.

However, as the population reduces, it becomes harder and more costly to apply insecticides to every area or find every breeding site. This is where the sterile insect technique has an advantage. The lower the wild type density, the more efficient the technique becomes. Because the over flooding ratio increases as the population declines. A higher over flooding ratio means that wild females are more likely to mate with a sterile male. There a several main advantages of the sterile insect technique. It is an environmentally friendly technique that targets only pest species of interest. The sterilised males will target females of their own species. Other species will not be affected.

It can be used effectively for area wide control of insects, even eradication over entire countries.

The sterile insect technique has had some notable successes in the past 60 years being deployed against over 20 different insect species. The list shown here are some of the more successful sterile insect technique control programmes in the past 70 years. Most of these species are agricultural pests, but the sterile insect technique has had some notable successes in controlling pests and vectors of human and animal diseases. These are highlighted in bold and will be discussed in more detail. Tsetse fly are vectors of trypanosomes. Trypanosomes cause sleeping sickness in humans and animals, as well as the human burden of disease.

Many cattle die, and those that do survive produce little milk and are not healthy enough to be used as draft animals for agriculture. This has huge economic impact. Direct costs over Africa are estimated at $4.7 billion annually.

In Zanzibar, a tsetse fly eradication campaign was initially attempted with insecticide impregnated cloth screens. But after seven years, they could not totally remove the fly. It was too hard to get the screens into forested areas, and a large residual population remained. In 1994, they initiated an integrated pest management plan using insecticide impregnated screens, directly treating cattle with insecticides, and using the sterile insect technique. They distributed the sterile male tsetse fly by air. The map shows the flight patterns they used over the island, releasing the sterile insects twice a week. They also had 55 monitoring stations around the island to monitor the releases and the suppression levels. Within three years, they achieved eradication of the tsetse fly from Zanzibar.

Removing the tsetse fly had many economic benefits over the next five years. All of these were directly related to the removal of tsetse fly from Zanzibar and significantly contributed to an increase in the quality of people’s lives. To repeat these results in Africa would be a huge logistical and financial challenge with an estimated cost of $20 billion. However, set this against the annual cost of tsetse fly of $4.75 dollars and within four to five years, the cost of an eradication programme would be offset by the removal of the burden.

One of the most successful applications of this sterile insect technique has been the eradication campaign of the New World’s Screwworm from Central America. After successful tests on Sanibel Island in Florida and the eradication of screwworm from the island of Curacao, which was achieved within 14 weeks, large scale deployment was initiated in Florida. A mass rearing facility was built, producing 60 million sterile screwworms a week. Within three years, the screwworm was eradicated from Florida. The control programme was expanded into Texas and then down through Mexico all the way to Panama, where there is still a mass rearing facility producing and releasing sterile screwworm in a barrier zone to prevent re-invasion.

However, the barrier is not 100%, and sometimes, there are three invasions. In 2016, there was a re-invasion of screwworm into the US in Monroe County, Florida. A local vet Douglas Maida identified the fly in Key Deer on Big Pine Key. Identification was confirmed when samples were submitted to the University of Florida School of Engineering Medicine and the United States Department of Agriculture. The USDA initiated an extensive local education campaign. Because very few people knew what the New World Screwworm was, how it could affect them, the wildlife, and their pets. They also described what the sterile insect technique is, how it worked, and the history of the screwworm eradication campaign to help ensure public acceptance.

Over seven months from October 2016, around 154 million sterile screwworm were released on several islands in the Keys. This map shows which islands were treated. The inspection of pets, key deer, and other wildlife were intensified during this period to monitor the spread and the success of control. From January 2017, no new screwworm were detected, and it was declared eradicated from Monroe County in April 2017. The sterile insect technique has been attempted in several mosquito species. In the 1970s, a lot of work and effort went into the development of mass rearing systems, the sterilisation methods, and release systems for adult mosquitoes.

These pictures show a rickshaw that was modified to automatically release adult male mosquitoes as the rider pedalled down the street. The rate of releases could even be adjusted to suit different release densities. There were some notable small scale successes. But generally, they either did not work, or it was deemed too costly to expand the programme. There were issues with mass rearing, enough insects, the ability and effort required to salt males and females, and the cost of distribution of the males. The general fitness cost to the males from irradiation was also too high, and they were not fit enough to compete effectively with wild males.

The international atomic energy agency has recently been optimising irradiation techniques for mosquitoes, and this may prove successful for future control campaigns. Given the successes of the sterile insect technique, why has it not been applied to every pest insect? Well, there are several requirements that must be met to be able to provide a cost effective control programme. The pest insects should be present in relatively low numbers or be able to be reduced by other techniques. Ideally, the ecology and biology of the insect needs to be well understood to know when and where to release sterile males most effectively. Artificial rearing in the laboratory is essential, as well as the development of mass rearing.

An efficient and quick method of sex separation is usually required. The method of sterilisation needs to be quick and efficient. In the case of Medfly programmes, some facilities were producing 500 million males a day. Fitness effects must be as low as possible. The less fit the sterile males, the more are required to be released and the higher the cost. The distribution of males needs to occur over large areas for an extended period of time. Aerial releases are generally the best approach. There needs to be an effective monitoring system in place to monitor population decline.

As you can see, there are lot of requirements for an effective sterile insect approach, and a lot of pest insects do not meet them and are therefore not currently suitable.

The sterile insect technique is a cost effective and environmentally friendly control method. The overview of the sterile insect technique presented in this course is brief, and I encourage you to do further reading and research. Expansion to area wide control is possible, but usually expensive. However, the cost benefit can be significant. It is important to know the cost benefit. Many sterile insect programmes are seen as prohibitively expensive. But when weighed against the economical benefit, it is clear that they have a place in insect pest control. Challenges remain in applying the sterile insect technique to mosquitoes, but new techniques are on the horizon that may make it more effective to mosquitoes and applicable to other insects.

Other sections of this course will cover these new techniques.

Presented in this slide are some questions that you may want to explore to further understand the sterile insect technique.