Sterile Insect Technique (SIT) for pest control

“A method of pest control using area-wide inundative releases of sterile insects to reduce fertility of a field population of the same species”

The SIT relies on mass rearing and release of insects sterilized by gamma radiation.

The released males mate with wild females, preventing production of offspring.

Muller, H.J. (1927) discovered that ionization radiation can induce dominant lethal mutations

Knipling, E. F. 1955. Possibilities of insect control or eradication through the use of sexually sterile males. J. Econ. Entomol. 48:459-62

GOALS

·         Production of Competitive sterile males for release in wild population of target insect.

·         Repeated releases lead to population control and eventual eradication.

Sterile Insect Technique (SIT)

Definition of sterility

·         Structural or functional inability to reproduce

·         “An Insect that, as a result of an appropriate treatment, is unable to produce viable offspring” (FAO 2005)

·         Sterility of the male is caused by dominant lethal mutations in the sperm of the released males resulting from radiation.

·         A sterile male has the ability to mate and transfer viable sperm and necessary fluid of the appropriate quality and quantity to ensure appropriate female behaviour

·         Species specific

·         Ecologically sound

·         Cost Effective

·         Non polluting

·         Effective and Long lasting

Pre-requisites for SIT

·         Preliminary   Lab. studies on

·         Rearing, handling, and evaluation of proper sterility doses for the target species 

·         Studies on the proper age & stage for irradiation

·         Studies on the males longevity (field & lab studies)

·         Sexual competitiveness before & after irradiation

·         Large Field Cage Studies

·         Release rate/ sq Km

·         Application intensity

·         Release Mode (ground, aerial)

·         Mating compatibility

·         Post releases evaluation

Mosquito SIT Related Projects

·         SIT against An. albimanus  control during 1972 at El Salvador

·         (Umbrella Project of WHO/ IAEA)

·         SIT on Ae. albopictus in La Reunion France

·         SIT on Ae. albopictus  Bolognia, Italy

·         Current SIT for Malaria Control in Sudan

·         Current SIT for Dengue control in South East Asia

·         Exploring genetic molecular, mechanical and Behavioural tools of sex separation in mosquitoes

How Males are Separated?

In SIT program only males are released, therefore, there must be system for separating males from females. The efficiency of insect control programs and cost of artificial rearing will greatly reduce when females are removed at the early stages of rearing.

This can be exploited through various techniques including sexual dimorphism, behavior, developmental time, swarming, blood feeding and genetic sexing

Sexual dimorphism

·         Color:

·         Size:

·         Developmental Time

·         Behavioral Differences

·         Male Swarming in mosquitoes

·         Blood Feeding by the females

Genetic Sexing Strains

The classical genetic sexing strains (GSS’s) have been developed for various insects including anophelines that rely on the linkage of a dominant selectable marker to the male determining chromosome.

Linkage is accomplished by radiation-induced translocations followed by crossing and screening of the offspring. Resistance genes, e.g. temperature-sensitive lethal genes and insecticide-resistance genes have been used as selectable markers.

Examples of GSS strains in SIT

·         GSS of medfly is currently used in all SIT operational programmes.

·         A successful Anopheline GSS was the MACHO strain of A. albimanus used in El Salvador.

·         This strain was created by linking an insecticide (propoxur) resistance gene to the male chromosome and an inversion was induced to suppress further recombination and thus stabilize the strain.

·         Females were removed from the population by treatment of the eggs with a discriminating dose of insecticide. The effectiveness of this sexing strain was 99.9%

·         Engineer strains of mosquitoes that are refractory to Plasmodium parasites

Species		Sexing Strategy
Mediterranean fruit fly C. capitata		Y-autosome translocation, using a temperature-sensitive lethal (tsl)
Melon fly B. cucurbitae		Y-autosome translocation, pupal colour separation
Oriental fruit fly B. dorsalis		Y-autosome translocation, pupal colour separation
Culex tarsalis	Malathion resistance males
Anopheles albimanus	Pupal size and Y-autosome translocation, propoxur resistance
Glossina austeni	Manual, sex-specific time of emergence, infrared screening of pupae

Successful Stories

·         The successful eradications of the New World screwworm fly from North and Central America and panama (Wyss, 2000)

·         Mediterranean fruit fly from areas in North, Central USA (including Mexico, Hendrichs et al., 1983)

·         South America (including all of Chile, SAG, 1995),

·         Melon fly (Dacus cucurbitae)/from Okinawa Islands Japan, Taiwan; (Kuba et al, 1996)

·         Queensland fruit fly from Western Australia (Fisher, 1996)

·         Pink Bollworm (Pectinophora gossypiella Saunders)/USA (California);

·         Tsetse Fly (Glossina species)/Tanzania, Zimbabwe, Upper Volta

·         Mosquitoes (various) USA (Florida), East Africa, Venezuela

·         Boll Weevil (Anthonomus grandis)/Southeastern USA

·         Mexican Fruit Fly, (Anastrepha ludens)/ USA (Texas), Mexico

·         Gypsy Moth (Lymantria dispar)/ Northeastern USA, Canada

·         Stable Fly (Stomoxys calcitrans)/USA (St. Croix, Virgin Islands - experimental)

·         Horn Fly (Haematobia irritans)/ USA (Texas - experimental

·         Corn Earworm (Helicoverpa zea Boddie)/ USA (St. Croix, VI)

·         Tobacco Hornworm (Manduca sexta)/ USA (St. Croix, VI)

Uses of Nuclear Techniques Agriculture (Export)

·         Due to WTO regulations exporters have to use pest and disease free goods and fresh produce for export to other countries

·         Have to meet max. residue levels (MRL’s)

·         Pakistan is one of the world largest exporter of Kino

·         Pak Mangos are much liked in the world market

·         Export of rice, mango, citrus fruit and spices from Pakistan has to meet WTO regulations

·         D. citri is locally spread by natural dispersal. Citrus material (budwood, grafted trees, rootstock seedlings) from infected areas can carry eggs and/or nymphs over longer distances.

·         5th or 6th-instar nymphs, as well as the adults born from these nymphs, are capable of transmitting the greening agent to citrus.

Quarantine Pests

·         A pest of potential economic importance to the area endangered thereby and not yet present there, or present but not yet widely distributed and being officially controlled (FAO, IPPC)

·         Quarantine Pests inflict direct losses to the fruits and vegetables and pose trade barriers in the export of fresh fruits and vegetables to the pest free countries as the eggs and nymphs can be transmitted with the export commodity to the importing countries.

·         Irradiation as a phytosanitary treatment for agric. commodities is environment friendly control strategy

·         Irradiation can be applied for the control of arthropods and pathogens in fresh commodities, stored products and ornamentals

·         Agri. Produce treated with optimum doses has potential to meet WTO constraints

·         Radiation Treatment for export quality products (stored products, fresh fruit, spices)

·         Quarantine pests are controlled 

·         High doses in permissible limits are applied that would not affect  quality, wholesomeness and consumer acceptability, or health hazardous