Use of Symbiotic Bacteria to Reduce Mass-Rearing Costs and Increase Mating Success in Selected Fruit Pests in Support of SIT Application

Objective:

The objective of this CRP is to characterize the microorganisms associated with the major fruit pests targeted by the SIT and to harness these symbionts to decrease production costs and increase sterile insect quality. Specifically, the CRP seeks to resolve four key questions related to symbiotic microorganisms.

Activities:

The sterile insect technique (SIT) is a sustainable and environment friendly control method against some major pests of fruits and vegetables world-wide. One of the main obstacles to the widespread implementation of the SIT is the cost relative to other, less sustainable alternatives.

In recent years there has been a major paradigm shift in understanding the intimate relationship between microorganisms and their hosts, be they humans, animals or plants. Specifically, studies on insects have revealed seminal contributions of microorganisms to the nutrition, health and reproductive success of their hosts. Furthermore, there is evidence that during the mass-rearing and radiation processes, the native microflora of the insects is disrupted and its contribution to the host diminished.

Can symbionts help reduce the cost of production and increase mass-reared sterile insect quality? Under natural conditions, fruits used by larvae have extremely low amino acid contents. Ovipositing females inject their eggs into fruit along with bacteria that fix atmospheric nitrogen and others that break down the fruit to produce nutrients essential for larval growth. The mass-rearing process frequently disassociates the reared insects from their native microflora, allowing the proliferation in larval media of opportunistic microorganisms that may not be beneficial. Yeasts to provide nourishment, and chemicals to suppress opportunistic microorganisms, represent by far the largest cost of larval diets. Adding endogenous symbiotic bacteria to the artificial larval diet may significantly:

  • reduce mass-rearing costs by eliminating the need for yeasts and chemicals
  • prevent the growth of competing microorganisms
  • improve mass-rearing efficiency and quality of the insects produced.

How are they affected by radiation? Evidence suggests that radiation of mass-reared flies can disrupt the symbiotic community by favouring some bacterial species and suppressing others. Understanding the effects of radiation may enable us to design responses that address them in a manner that optimizes the SIT efficiency. In addition, radiation may also result in the development and isolation of mutant strains of endogenous symbiotic bacteria leading to novel insect symbiotic associations with desirable traits. Such development may eliminate the need of intensive genetic screens and/or the application of transgenic paratransgenic approaches.

Can they be used as probiotics during the pre-release period to improve sterile insect quality? In nature symbiotic bacteria become established in the gut of adult flies. These appear to play an important role in the reproductive success of males. The complement of bacteria present in released males following mass-rearing and irradiation may differ from their wild counterparts enough to impede their performance. There is preliminary evidence that restoring the symbiotic bacteria, prior to release, can significantly improve their sexual performance. In the Mediterranean fruit fly, enriching the sterile insect diet with naturally occurring bacterium Klebsiella oxytoca significantly improved sterile male mating competitiveness in the laboratory and in field cages. In addition, bacterially enriched sterile males inhibited female receptivity to re-mating more efficiently than sugar fed males and survived longer periods of starvation. These results suggest that inoculating mass-reared sterile flies with bacteria prior to their release is a valid approach to improve the efficacy of the SIT. It is worthwhile to validate this approach at an operational level and to extend it to other insects targeted by the SIT.

Can they be used to develop novel pest control tools, complementary to the SIT? Certain symbiotic bacteria are known to manipulate mating behaviour and reproduction of their hosts. Identifying these organisms and introducing them to target populations can effectively reduce pest populations and their economic impact. For example, the incompatible insect technique (IIT) employs cytoplasmic incompatibility, induced by insect symbionts such as Wolbachia species. In a Wolbachia-based IIT strategy, female sterility is artificially sustained by repeated releases of cytoplasmically incompatible males. Since Wolbachia is not paternally transmitted, the infection type present in the release strain does not become established in the field. Similar to the conventional SIT, the increasing ratio of incompatible matings over time can lead to population suppression. This strategy has been successfully tested under laboratory conditions for two major agricultural pests, the Mediterranean fruit fly and the olive fly. It is worthwhile that such approach to be validated and extended, alone and/or in conjunction with the SIT, to other target insect pest species.

Participants:

Sixteen participating countries: Argentina, Australia (2), Bangladesh, Brazil, Chile, China, Greece (2), Guatemala, India, Israel, Italy, Mauritius, Mexico, Spain, Turkey, and USA (2).

Reports:


Project Officer:

Carlos Caceres