What has changed in the outbreaking populations of the severe crop pest whitefly species in cassava in two decades?

Hadija M Ally,Hajar El Hamss,John Colvin,Christophe Simiand,Christopher A Omongo,M N Maruthi,Helene Delatte

doi:10.1038/s41598-019-50259-0

Abstract

High populations of African cassava whitefly (Bemisia tabaci) have been associated with epidemics of two viral diseases in Eastern Africa. We investigated population dynamics and genetic patterns by comparing whiteflies collected on cassava in 1997, during the first whitefly upsurges in Uganda, with collections made in 2017 from the same locations. Nuclear markers and mtCOI barcoding sequences were used on 662 samples. The composition of the SSA1 population changed significantly over the 20-year period with the SSA1-SG2 percentage increasing from 0.9 to 48.6%. SSA1-SG1 and SSA1-SG2 clearly interbreed, confirming that they are a single biological species called SSA1. The whitefly species composition changed: in 1997, SSA1, SSA2 and B. afer were present; in 2017, no SSA2 was found. These data and those of other publications do not support the ‘invader’ hypothesis. Our evidence shows that no new species or new population were found in 20 years, instead, the distribution of already present genetic clusters composing SSA1 species have changed over time and that this may be in response to several factors including the introduction of new cassava varieties or climate changes. The practical implications are that cassava genotypes possessing both whitefly and disease resistances are needed urgently.

Highlights

Crop protection involves practices to manage the plant diseases, weeds and pests that damage agricultural crops and forestry
The SSA1 species is widely distributed in Africa; SSA2 is mostly found in the eastern, southern and central areas of Africa as well as in the south of Spain; while SSA3 and SSA4 have been reported in Cameroon and the Central African Republic; SSA5 has only been described in the Ivory Coast and South Africa[27,28,29]
Eight (61.5%) of the 13 fields visited in 2017 had superabundant whitefly populations, among which two fields did not show cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) symptoms (Table 1)

Summary

Introduction

Crop protection involves practices to manage the plant diseases, weeds and pests that damage agricultural crops and forestry. Epidemics of CMD and CBSD have been reported in several parts of Eastern and Southern Africa since the early 1990s and these were associated with unusually high numbers of whiteflies on cassava[21,23,24] The presence of these ‘superabundant’ populations has been responsible for the rapid spread and development of two disease epidemics[25], but the reason(s) for their upsurges remain uncertain[26]. Analysing the nuclear genetic diversity of whitefly populations had been performed in the past using several types of markers, among which neutral-codominant markers such as microsatellites gave reliable results Those markers allowed to distinguish B tabaci species and populations within those species, including Med Q1 and ASl34, Med and MEAM135–37, Med Q1/Q238, or between a wide range of species worldwide[39].

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Results

Conclusion