Sterile Insect Technique and Biological Control: How Radiation Helps Fight Pests

The SIT is one of the safest and most eco-friendly insect birth control methods available, which ensures environmental protection through a reduced use of insecticides. In addition, since sterile insects cannot self-replicate, this creates a long-term solution for the pest problem without running the risk of introducing non-native species into the ecosystem.

Since the 1950s, SIT has been successfully used in the global control of some insects affecting the health of people and livestock, such as the new world screwworm, the tsetse fly and disease transmitting mosquitoes, as well as in the control of insect pests that destroy crops and affect trade, such as the fruit flies and moths.

Inherited sterility, also known as F1 sterility, is another type of SIT, very similar to the traditional method, as it involves rearing, irradiation and release of semi-sterile male insects into a target area to reduce the mating of their fertile counterparts. In this technique, however, the reared and released male insects have a certain degree of fertility, but all their offspring is born sterile.

This technique is used when males of particular types of pests, such as moths, cannot be fully sterilized unless very high doses of radiation are used. A high dose, however, may weaken the insect and hinder its ability to compete with wild moths for reproduction. Therefore, this technique involves using much lower and less debilitating doses, which do not hinder the insects’ opportunities to reproduce, but which induce inherited sterility in the moths' offspring.

This technique has a number of advantages. It enables the scientists to target species that require very high doses of irradiation to become sterile. However, the females are fully sterile after irradiation, since female moths tend to be more sensitive to radiation. The irradiated males are only partially sterile and give birth to fully sterile offspring. The release of partially sterile males with inherited sterility, often helps to suppress wild populations to a greater extent than an equal number of fully sterile males released in conventional SIT, since the males can produce multiple fully sterile copies in the next generation.

Biological control is a method that uses mass-production and release of the pest’s natural enemies, such as predatory insects who feed off the pest’s eggs and larvae, or parasites (known as parasitoids) that lay eggs into their host insect, killing the insect.

Unlike the SIT and inherited sterility methods, which are based on the use of nuclear techniques, in biological control nuclear techniques are only used for particular purposes. Radiation can be used to increase the applicability, cost-effectiveness and safety of rearing, shipping and deploying these natural pest enemies. It can also improve the results of the method and alleviate a number of constraints associated with it. For example, radiation can be used to reduce the cost of production of biological control agents. In case of parasitoids, for example, radiation can lower the host insect’s natural defences (i.e. immune response) to increase the productivity and survival rate of parasitoids. Moreover, radiation helps to prevent development of the host insects, assuring that only parasitoids emerge from the pupae. In cases where some host insects do survive, the use of radiation ensures they are sterile in order to eliminate the risk of releasing fertile host insects, which can become pests in the new environment (see infographic below). In appropriate contexts, low doses of radiation can also stimulate the reproduction of some predators, which could be used to improve the effectiveness of biological control in environments where these predators do not threaten non-target insect populations.

Similarly, irradiation can help in the transport of predatory insects and parasitoids, often referred to as biological control agents. Parasitoids, for example, may require host insects in order to survive transportation, and predatory insects may need to feed off other insects. For this reason, they are often transported together with their “prey insects” or their eggs, which serve as a source of food during transportation. However, not all the insects being used as hosts of the parasitoid are affected by the parasitoid, therefore, in case these hosts manage to survive transportation and are released into the environment they will become pests.  Irradiation of the host before transportation, will ensure that no new pests can be accidentally introduced to the area undergoing biological control.