Sterile Insect Technique

The sterile insect technique (SIT) is a biological pest control method based on area-wide inundative releases of sterile insects to reduce the reproduction in a field population of the same species (FAO). The SIT has been employed all around the world as part of area-wide integrated pest management programs (AW-IPM) over the past 60 years to contain, reduce, eliminate or prevent the establishment of insect pests of agronomic, veterinary and medical importance (IAEA).

Definition of the problem

The Asian Tiger Mosquito has been present in Italy for over 25 years, during which time it has earned notoriety among millions of Italians and foreign tourists for its daily aggressiveness and the irritation caused by its bites. It is evident that the results achieved with the current mosquito control strategies are insufficient to guarantee a sensible reduction of the mosquito population below epidemic risk level or nuisance threshold.

Alongside these considerations, this species is particularly suitable for application of the sterile insect technique (SIT) because:

• it is an accidentally imported exotic species and hence there are no ecological reasons to preclude its possible eradication;
• its spread is mainly linked to urban and suburban areas, with an island-like distribution with pockets of infestation involving an indeterminate number of cities and towns;
• the active dispersal capabilities of the species are limited to a few hundred metres from breeding sites while passive transportation via motor vehicles that move from one area to another appears inevitable;
• the species is relatively easy to rear under artificial conditions.

On an international scale renewed attention is being focused on the SIT strategy, applied both as a defence against agricultural and for public health pests.

Progress of the SIT project targeting Aedes albopictus

Over the years all aspects relevant for SIT strategy optimisation have been addressed. From a technological perspective the current state of the art may be summed up as follows:

• Development of a new pilot mass rearing facility: at the CAA in Crevalcore a new mass rearing pilot unit has been set up which can potentially produce 1,000,000 males per week when operating at full capacity.
• Blood meal management: the system adopted involves the use of thermostat-controlled device capable of maintaining the blood at a temperature of 37 ± 1 °C. The blood is obtained from a nearby slaughter facility and adequately treated so as to render it suitable for this specific purpose. Mass rearing of mosquitoes thus takes place without the use of host animals.
• Collection and counting of eggs: eggs are estimated based on their weight after being brushed off from the ovipositional substrate (rough paper). This makes it possible to introduce the desired number of eggs into each tray for an effective larval development.
• Larva rearing: the standard diet presently used is a mixture of bovine liver powder, brewer’s yeast, fish food and vitamins administered daily in programmed increasing doses.
• Sexing: it is necessary to separate the sexes so that only males will be released in the field. Separation is performed mechanically at the pupal stage by exploiting the difference in size between males and females. Using specific separator plates, it is possible to obtain a yield of 25% males with an impurity rate less than 1 % females (these females will be sterilized by irradiation but are nonetheless able to bite, so their number must be kept to a minimum).
• Sterilisation: males are sterilized by gamma o X irradiation of the pupae in water. Based on the results of dosimetry trials, it has been possible to establish the minimum dose required to induce close to 100% sterility in males while maintaining high quality performance in the field. The current reference dose is 35 Gy.
• Sterile male competitiveness: a crucial element of the strategy is the degree of competitiveness of sterile males versus wild males during mating. In nature, the males of this species form numerous mating swarms which are made up of few individuals and gather in shady areas to attract virgin females. The females are monogamous. Trials in laboratory cages, green houses and in the field are conducted to determine how the rearing conditions and irradiation procedures affect the sterile male’s performance.
• Experimental field releases of males: sterile males are released into the environment as young adults. The results show that sterile males are able to survive, disperse and effectively mate with wild females and induce a decrease of fertility and fecundity into the wild target population.