Sterile Insect Technique

Definition of the problem

The Asian Tiger Mosquito has been present in our country for over 15 years, during which time it has earned notoriety among millions of Italians and foreign tourists for its aggressiveness and the irritation caused by its bites. It is evident that the results achieved thus far through collective efforts to combat this mosquito are insufficient to guarantee a good quality of life in infested areas.

Alongside these considerations, there are others of a biological nature which make the species in question a suitable target for the application of SIT technology (Sterile Insect Technique):

  • it is an accidentally imported exotic species and hence there exist no ecological reasons to preclude its possible eradication;
  • most likely an exiguous number of insects were imported and it may thus be reasonably assumed that the genetic base of Italian populations is rather narrow;
  • its spread is linked to urban areas, with pockets of infestation involving a by now indeterminate number of cities and towns, the active dispersal capabilities of the species are limited to a few hundred metres while passive transport via motor vehicles that move from one area to another appears inevitable;
  • the species lends itself well to breeding in artificial structures.

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

 

Current topics of investigation

Techniques for maintaining the quality of reared males.

Sexing systems for the separation of males.

Methods of aerial distribution (Drone) of sterile males in the field.

Integration of the SIT strategy into a package of control measures.

Construction of a mass breeding module.

 

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 somministration: 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. Breeding thus takes place without the use of host animals.
  • Collection and counting of eggs: eggs are counted directly on the substrate (rough paper) by means of an automatic digital system. This makes it possible to introduce the desired number of eggs into each tray.
  •  Larva rearing: the standard diet presently used is a mixture of bovin liver powder, brewer’s yeast and fish food and vitamins administered in programmed doses.
  •  Sexing: it is necessary to separate the sexes so that only males will be released in the field. Separation is performed at the pupal stage by exploiting the difference in size between males and females. Using specific separator screens it is possible to obtain a yield of 25% males with an impurity rate of 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 dosimetric trials, it has been possible to establish the minimum dose required to induce close to 100% sterility in males. 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 are conducted in cages and green houses to determine how the rearing conditions and irradiation affect male 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 mate with wild females and induce sterility.

 

The results show that the released sterile males are capable of mating with wild virgin females and induce sterility. However, at present the levels of sterility are too low to bring about a progressive suppression of the population and efforts are ongoing in order to improve males performances.