Présentation de thèse

Abstract:

Background: In tropical and sub-tropical regions, whiteflies are common pests causing severe Losses on food-crops and ornamental plants. Many of these species are tropical in origin, nowadays adapted also to sub-tropical and temperate regions. Their control involves great effort and attention, as they have also been a threat to food-security in several tropical countries. Whiteflies have been well documented in the sub-tropics; however, a critical issue at present is the lack of information regarding their spatial distribution or abundance. Moreover, scarce or outdated information is available about the current status of their parasitoid fauna. On the contrary, in several tropical regions no or incomplete information is available regarding the diversity and distribution of whiteflies and their parasitoids. The value of a geographical perspective to harmful pests and their control has long been recognized. However, it requires several steps such as identifying correctly the species occurring in a given area, studying their spatial diversity and distribution, producing maps and keeping these data updated.

Methods: This study combined morphological, molecular and geostatistical methods to assess the current diversity and distribution of the whitefly and parasitoid fauna occurring on herbaceous plants in a sub-tropical environment (Sicily)and in a tropical one (north-western part of Tanzania).Generally, morphological techniques were adopted to identify specimens. Moreover, molecular markers where used to confirm doubtful or cryptic species. The phylogenetic relationships were then reconstructed through Bayesian analysis. A noteworthy aspect of this thesis is that in both tropical and sub-tropical environments, various bio-ecological aspects have been taken into account in the aim to understand which factors influence the spread of common whitefly pest species, such as the species of Bemisia tabaci group. For this reason, diversity and distributional information of both whitefly and parasitoid species was first accumulated to build a database. The database was then analyzed by Geographical Information System for (i) mapping richness and abundance, (ii) modeling the potential distribution of a pest species and (iii) revealing zones for parasitoid reserve selection. Finally, we showed how elevation gradient can be an additional instrument to study the cryptic species, as in the case of B.tabaci species complex.

Principal findings: In Sicily, a measure of the current status of whitefly and parasitoid fauna was given and the species richness and evenness were mapped. Nine whitefly species were identified on herbaceous plants and these include Aleurolobus marlatti, Aleurothrixus floccosus, Aleyrodes elevatus, Aleyrodes proletella, Bemisia afer, Bemisia tabaci, Dialeurodes citri,Trialeurodes lauri and Trialeurodes vaporariorum. Moreover,three cryptic species were dentified in the B.tabaci species complex: B. tabaci Mediterranean, B.tabaci Middle Easth Asia  and B. tabaci Italy .The study revealed aggregated spatial distribution for the two major pests in the region, B.tabaci (s.l.) and Trialeurodes vaporariorum, in cultivated lands along the southern and eastern coasts. On the contrary, no aggregated spatial distribution was observed for these species in natural or semi-natural areas or nearby surroundings. In the latter areas their population size decreased sometimes reaching almost nil. Modelling the potential distribution of these major whitefly pests at regional scale revealed the main suitable areas where they can occur. Contrary to previous studies, a striking finding in this study was there placement of the exotic invader B.tabaci Middle East Minor 1 by the B. tabaci Mediterranean species, which is at present the predominant species. Another important finding of this research was the fact that the elevational gradient functions as a filter, selecting the species of B.tabaci group. In mild altitude, the three B.tabaci species were found to occur contemporarily whereas B.tabaci species Italy seemed to be the only one adapted to high gradient. As for the parasitoid fauna, nine species were identified to parasitize the various whitefly species collected. Encarsia pergandiella,En. Lutea and Erermocerus mundus were widespread. On the contrary, other species such as En. formosa, En. inaron  and En. tricolor exhibited a narrow distribution range.A reserve selection model permitted to identify two main areas that seem to function as reservoir for whitefly parasitoids.

In Tanzania the diversity of whitefly species infesting cassava and other herbaceous plants was investigated. Ten whitefly species were identified in the cassava growing regions which include Aleurodicus dispersus; Aleurothrixus floccosus; Bemisia tabaci species group(B.tabaci Sub-Saharan Africa1, B.tabaci Mediterranean and B.tabaci Indian Ocean); Bemisia afer, Bemisia sp. (formerly Asterobemisia sp.), Dialeurodes citri, Paraleyrodes bondari and Trialeurodes vaporariorum. As the study area is affected by super-abundant B. tabaci populations, the potential distribution of this species was modelled. The model revealed the presence of suitable areas not only in the study area but also extending further to neighbouring countries suchas Zambia. Moreover,the overlay of this model with environmental data showed clearly the existence ofabroad and unsuitable area serving as an environment albarrier and avoiding the spread of super-abundant population to the south-eastern part of the country. Six Encarsia species, four Eretmocerus species and one Cales 2 species were identified in the surveyed areas, adding more species to the previous list of parasitoids recorded in Easth Africa and leading to the discovery of a probable new species of Eretmocerus. For the first time in Tanzania, this study recorded En.mineoi and En. sp. pr. circumsculpturata. The occurrence of whitefly parasitoids in the study areas in north-western Tanzania collided with that of whiteflies. However, parasitoids have been detected at very low numbers, showing the occurrence of a relatively scarce natural enemies complex.

Conclusions:The research presented here documents the whitefly and parasitoid fauna colonizing herbaceous plants in two sites chosen as case studies for respectively tropical and sub-tropical regions, focusing on the common species that occur in both environments and that represent a threat for agriculture. The combination of morphological, molecular and bio-ecological traits was useful in understanding the diversity and distribution of these insects.