Studies on whiteflies living on herbaceous plants and their parasitoids in tropical and sub-tropical environment through combined morphological, molecular and geostatistical approaches

Abstract

In tropical and sub-tropical regions, whiteflies are common pests causing severe losses on food-crops and a threat to food-security. I the sub-tropics, a critical issue is the lack of spatial information about whiteflies. Moreover, outdated information is available about the current status of their parasitoid fauna. On the contrary, in the tropics no or incomplete information is available regarding the diversity of whiteflies and their parasitoids. Methods: Morphological, molecular and geostatistical methods were used to assess the whitefly and parasitoid fauna on herbaceous plants in a subtropical environment (Sicily) and in the tropics (Tanzania). Morphological techniques were adopted first to identify specimens. Moreover, molecular markers were used to confirm cryptic species. Bio-climatic aspects have been taken into account in the aim to understand which factors influence the spread of whitefly pest species. For this reason, diversity and distributional information 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 Principal findings: In Sicily, a measure of the current status of whitefly and parasitoid fauna was given. Nine whitefly species were identified: Aleurolobus marlatti, Aleurothrixus floccosus, Aleyrodes elevatus, Aleyrodes proletella, Bemisia afer, Bemisia tabaci, Dialeurodes citri, Trialeurodes lauri and T. vaporariorum. Moreover, three cryptic species were identified in the B. tabaci species complex: B. tabaci Mediterranean, B. tabaci Middle East-Asia 1 and B. tabaci Italy. Modelling the potential distribution of these major whitefly pests at regional scale revealed suitable areas where they can occur. A striking finding in this study was the replacement of the exotic invader B. tabaci Middle East Minor 1 by the B. tabaci Mediterranean species, which is at present the predominant. Another important finding was the fact that the elevational gradient functions as a filter, selecting the species of B. tabaci group. As for the parasitoid fauna, nine species were identified belonging to the genera Encarsia and Erermocerus. 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: Aleurodicus dispersus; Aleurothrixus floccosus; B. tabaci species group (B. tabaci Sub-Saharan Africa 1, B. tabaci Mediterranean and B. tabaci Indian Ocean); B. afer, Bemisia sp. (formerly Asterobemisia sp.), Dialeurodes citri, Paraleyrodes bondari and T. vaporariorum. The potential distribution of super-abundant B. tabaci was modelled, revealing the presence of suitable areas extending also further to neighbouring countries such as Zambia. Moreover, unsuitable area was identified serving as an environmental barrier and avoiding the spread of the pest to the south-eastern part of the country. Six Encarsia, four Eretmocerus and one Cales species were identified, adding more species to the previous list of parasitoids recorded in East-Africa and discovering of a probable new species of Eretmocerus. For the first time En. mineoi and En. sp. pr. circumsculpturata were recorded in Tanzania. 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 the two sites chosen, respectively tropical and sub-tropical regions. Combining the methods mentioned above was useful in understanding the diversity and distribution of these insects.