Inherited Sterility

Area-wide integrated pest management programmes using the sterile insect technique (SIT) as a component have been successful against a number of pest flies or Diptera such as the New World screwworm (Cochliomyia hominivorax), various species of fruit flies (Tephritidae) and against tsetse flies (Glossinidae) . However, most moths or lepidopterans are more resistant to radiation than dipterans, and as a consequence, the higher dose of radiation required to completely sterilize lepidopterans reduces their performance in the field.

One approach to circumvent the negative effects associated with the high radio-resistance of Lepidoptera pests has been the use of inherited sterility or F1 sterility, first documented in studies on the codling moth (Cydia pomonella). Male moths treated with sub-sterilizing doses of radiation, which are then mated with virgin fertile females, will produce fewer F1 offspring, most of whom will be completely sterile. Attributes that are common to inherited sterility in Lepidoptera are: i) F1 male and female offspring are more sterile than the irradiated parental generation, ii) more F1 male progeny than female progeny is produced, and iii) longer development time and reduced sperm quality of the F1 generation. Like with the SIT, inherited sterility involves the mass rearing and release of sterilized insects to insure that when mating occurs in the field, a significant proportion of mating involves a treated insect.

The unique genetic phenomena responsible for inherited sterility in Lepidoptera and some other arthropods, as compared with full sterility, provide advantages for pest control. Lepidopteran females generally are more sensitive to radiation than are males of the same species. This allows the dose of radiation to be adjusted so that treated females are completely sterile and males are partially sterile. When these partially sterile males mate with wild females, the radiation-induced deleterious effects are inherited by the F1 generation. The lower dose of radiation used to induce F1 sterility increases the quality and competitiveness of the released insects.

The F1 sterile progeny produced in the field enhance the efficacy of released partially sterile males, and improve compatibility with other pest control strategies. For example, the presence of F1 sterile progeny can be used to increase the build-up of natural enemies in the field. In addition, F1 sterile progeny can be used to study the potential host and geographical ranges of exotic lepidopteran pests.

Field programmes releasing irradiated moths under an SIT or inherited sterility approach have been in operation since the 1960s. The pink bollworm, Pectinophora gossypiella has been successfully contained since 1969 in cotton areas of the San Joaquin Valley in California and is being successfully targeted for eradication from cotton areas in the south-western USA and north-western Mexico. Since the early 1990's, the codling moth has been successfully suppressed in apple and pear production areas in the Okanagan Valley in British Columbia, Canada, and countries such as Argentina, Brazil and South Africa have plans or programmes against this pest. New Zealand eradicated outbreaks of the Australian painted apple moth, Teia anartoides. Mexico eradicated outbreaks of the cactus moth, Cactoblastis cactorum and the USA contains its advance along the Gulf of Mexico coast. South Africa has a programme to suppress the false codling moth, Thaumatotibia leucotreta in citrus orchards.

Control of most moth pests is hampered by the increased resistance to the most widely used broad spectrum insecticides; hence the potential for expanded implementation of inherited sterility as part of an area-wide integrated approach is considerable.