The problem:
Olive is an ancient ubiquitous crop of considerable socioeconomic importance, being a major agro-ecosystem in the Mediterranean basin. Olive products are premium food products. Through time, a fruit fly has evolved to exploit this exquisite resource: Bactrocera oleae (Rossi) (Diptera: Tephritidae) developed strategies that allows them to use unripe olives, and thus to complete several generations before ripe fruit become available. For more than 2000 years this fruit fly has specialized to become monophagous and is now the most important olive tree pest. Production losses are estimated to average more than 15% yearly (B.oleae has been responsible for losses of up to 80% of oil value and 100% of some table cultivars; [1]).
The current PM situation:
Conventional control strategies include the use of baits, attracting the olive fly by colors and/or pheromones, but mainly the use of insecticides. Particularly, over the last decades, the management of olive fruit fly has been based on the use of organophosphate insecticides, mainly dimethoate [e.g. 1,2]. Resistance to dimethoate had evolved, involving mutations of the Ace gene, which codes for ace-tylcholinesterase, the target enzyme of OPs and other insecticides. Resistance-associated Ace alleles were found in high frequency (up to 0.90) throughout the pest distribution range, and are spreading (if in Portugal, the lower frequency reported was of 0.2 [2] we know now that for an equivalent location the frequency is up to 0.8 [Nobre, unp. data]). B.oleae resistance to pyrethroids (i.e., alpha-cypermethrin) was also encountered [3] and even to spinosad, a relatively new insecticide, derived from a bacterium [4]. It is thus clear that B.oleae is capable of fast developing resistance to the commonly used insecticides.
The development of non-insecticidal pest management methods for the olive fly is crucial to reduce the selection pressures on the resistant populations and thus limit development and generalization of resistance.
Alternatives:
Non-insecticidal alternatives that have worked in some situations or shown potential include (a) mass trapping programs, (b) sterile insect technique (SIT), (c) particle film, and (d) biological control using natural enemies [1, 5-8]. The future of pest management lies upon integration: the combination of non-chemical methods that may be individually less efficient than pesticides can generate valuable synergies. Hence, the more tools we have the better.
The current project aims to introduce a novel strategy into the management of B.oleae: a symbiosis-based strategy. This type of strategies implies the disruption of microbial symbioses required by insect pests or symbiont-mediated manipulation of insect traits.
The story behind…
The notion of biological-individual is crucial to all fields of life sciences, and is undergoing revision [e.g. 9-11]. Each individual is better seen as a group of genetically different entities and nature seems to be selecting at the level of holobiont and hologenome rather than individuals or genomes. Through the synergy of combined abilities, symbionts find faster solutions than individual organisms, changing the shape of their adaptive landscape for evolution (eg. symbiogenic origin of eukaryotes [12]; plant land colonization [13]; herbivore dispersion allowing the use of new food sources [14, 15]).
The olive fly is no exception. B.oleae evolved to harbor a vertically transmit and obligate bacterial symbiont -Erwinia dacicola- which allows the insect to cope with the olive-plant produced defensive compound oleuropein (a bitter and otherwise toxic phenolic glycoside). Additionally, traditional microbiological approaches have identified other bacteria of the genera Bacillus, Lactobacillus, Micrococcus, Pseudomonas, Streptococcus, Citrobacter, Proteus, Providencia, Enterobacter, Hafnia, Klebsiella, Serratia, and Xanthomonas as associated with the olive fruit fly. Molecular analyses have established the consistent presence of Acetobacter tropicalis in olive fruit fly adults [16]. Indeed Erwinia dacicola is often accompanied by other bacteria, probably ephemeral symbionts and/or transient inhabitants of the gut, some of which might, nonetheless, play a role on the host homeostasis. The gut of larvae seem to show a reduced bacterial population suggesting that bacteria other than E.dacicola are poorly adapted to colonize the eggs or to propagate in the larval gut [17]. Even though B.oleae is rather unlike other fruit flies (in that it shows co-evolution with specific obligate symbiotic bacteria), it is also likely that the olive fly has retained some of the features of other close related Bractocera species. The sparse existing data suggest a succession of associated bacterial species depending on the insect maturation. In B.dorsalis, the bacteria diversity was similar across immature stages (eggs and larvae), but greatly differed from the pupal and adult stages; and closely related congeneric flies seem to share similar bacteria species [18]. Insect symbiotic bacteria have been shown to be able to play different roles than nutritional alone, from boosting host immune system to courtship and reproduction, going through roles such as supplementing oxygen or conferring resistance to chemicals [15, 18-21].
Needless to say that many microorganisms cannot be classified exclusively as beneficial, harmless or deleterious because their impact on the insect can depend on circumstance. And it is that (in)balance that symbiosis-based strategies should explore. As an example of a possible target-specific strategy, the manipulation of engineered natural symbionts (bacteria) which will act as a Trojan horse is highly promising as these symbionts are capable of surviving inside the insect while carrying and expressing toxins. No doubt that a pre- requisite for developing a symbiotic control approach is the knowledge of the microbiota associated with the insect pest and its determinants.