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Cassava is a staple food for people across sub-Saharan Africa. Over the last 20 years, there has been an increased frequency of outbreaks and crop damage in this region caused by the cassava-adapted Bemisia tabaci putative species. Little is known about when and why B. tabaci adults move and colonize new cassava crops, especially in farming systems that contain a mixture of cultivar types and plant ages. Here, we assessed experimentally whether the age and variety of cassava affected the density of B. tabaci. We also tested whether the age and variety of the source cassava field affected the variety preference of B. tabaci when they colonized new cassava plants. We placed uninfested potted \"sentinel\" plants of three cassava varieties (Nam 130, Nase 14, and Njule Red) in source fields containing one of two varieties (Nam 130 or Nase 14) and one of three age classes (young, medium, or old). After two weeks, the numbers of nymphs on the sentinel plants were used as a measure of colonization. Molecular identification revealed that the B. tabaci species was sub-Saharan Africa 1 (SSA1). We found a positive correlation between the density of nymphs on sentinel plants and the density of adults in the source field. The density of nymphs on the sentinels was not significantly related to the age of the source field. Bemisia tabaci adults did not preferentially colonize the sentinel plant of the same variety as the source field. There was a significant interactive effect, however, between the source and sentinel variety that may indicate variability in colonization. We conclude that managing cassava source fields to reduce B. tabaci abundance will be more effective than manipulating nearby varieties. We also suggest that planting a \"whitefly sink\" variety is unlikely to reduce B. tabaci SSA1 populations unless fields are managed to reduce B. tabaci densities using other integrative approaches.

The fall armyworm (FAW) Spodoptera frugiperda is thought to have undergone a rapid ‘west-to-east’ spread since 2016 when it was first identified in western Africa. Between 2018 and 2020, it was recorded from South Asia (SA), Southeast Asia (SEA), East Asia (EA), and Pacific/Australia (PA). Population genomic analyses enabled the understanding of pathways, population sources, and gene flow in this notorious agricultural pest species. Using neutral single nucleotide polymorphic (SNP) DNA markers, we detected genome introgression that suggested most populations in this study were overwhelmingly C- and R-strain hybrids (n = 252/262). SNP and mitochondrial DNA markers identified multiple introductions that were most parsimoniously explained by anthropogenic-assisted spread, i.e., associated with international trade of live/fresh plants and plant products, and involved ‘bridgehead populations’ in countries to enable successful pest establishment in neighbouring countries. Distinct population genomic signatures between Myanmar and China do not support the ‘African origin spread’ nor the ‘Myanmar source population to China’ hypotheses. Significant genetic differentiation between populations from different Australian states supported multiple pathways involving distinct SEA populations. Our study identified Asia as a biosecurity hotspot and a FAW genetic melting pot, and demonstrated the use of genome analysis to disentangle preventable human-assisted pest introductions from unpreventable natural pest spread.

Projected climate changes are thought to promote emerging infectious diseases, though to date, evidence linking climate changes and such diseases in plants has not been available. Cassava is perhaps the most important crop in Africa for smallholder farmers. Since the late 1990’s there have been reports from East and Central Africa of pandemics of begomoviruses in cassava linked to high abundances of whitefly species within the Bemisia tabaci complex. We used CLIMEX, a process-oriented climatic niche model, to explore if this pandemic was linked to recent historical climatic changes. The climatic niche model was corroborated with independent observed field abundance of B. tabaci in Uganda over a 13-year time-series, and with the probability of occurrence of B. tabaci over 2 years across the African study area. Throughout a 39-year climate time-series spanning the period during which the pandemics emerged, the modelled climatic conditions for B. tabaci improved significantly in the areas where the pandemics had been reported and were constant or decreased elsewhere. This is the first reported case where observed historical climate changes have been attributed to the increase in abundance of an insect pest, contributing to a crop disease pandemic.

Background The Fall armyworm (FAW) Spodoptera frugiperda (Lepidoptera: Noctuidae) is now established across the African continent and is a highly polyphagous and destructive pest of many crops. In Uganda, FAW has become the major maize pest, causing heavy damage especially on shoots and growing points. The objectives of this study were to: (i) document local farming practices that have been useful to manage FAW, (ii) establish farmers’ perspective on the time of FAW’s arrival to their localities, (iii) investigate the economic impact (yield) of FAW to maize farmers, (iv) establish the farmers’ perception on the current status of the FAW, and (v) document alternative practices used to manage the FAW and the perceived efficacies. Methods A questionnaire survey was undertaken in November 2020 in Kamuli and Namutumba districts of Uganda and 99 farmers were interviewed to understand their profiles and perceptions about the FAW. A descriptive analysis of this data was undertaken to establish the socio-economic profiles and perceptions of the farmers. Results Farmers’ education levels in the two districts ranged from basic (completed primary education) to advanced (completed University degree), with most farmers having 10–30 years experience in growing maize (F = 20.8; df = 3,7; P  = 0.0067), and with mainly small- and mid-sized production scales (F = 436.2; df = 2,5; P  = 0.0002). Famers in Kamuli (98%) and Namutumba (96%) reported 25–50% yield losses due to FAW infestation that negatively impacted their income. We found a significantly higher percentage of farmers (84% and 92% in Kamuli and Namutumba districts, respectively), could correctly identify the FAW by its appearance ( P  < 0.0001). While FAW was officially reported in Uganda in 2016, farmers confirmed noticing damage symptoms similar to those caused by FAW as early as 2013 and 2014 in Namutumba and Kamuli districts, respectively. 98% of the farmers in Kamuli and 96% of those in Namutumba strongly agreed that FAW infestation reduced their income, while 74% in Kamuli and 86% in Namutumba also strongly considered the FAW as a threat to maize production ( P  < 0.0001). The majority of farmers (64% in Kamuli, 82% in Namutumba) still considered the FAW to be a very serious challenge to maize production in their localities, six years since officially being reported in Uganda. To manage the FAW, 84% and 90% of Kamuli and Namutumba respondents respectively, predominantly use chemical control methods. Other methods used also included cultural control practices (i.e., by regular weeding and handpicking), while the use of biological extracts (pepper, tobacco, Aloe-vera, Lantana, sisal) was evident though not common. Pheromones and biological control methods to manage FAW were not reported, although a farmer in Kamuli district reportedly observed weaver birds ( Ploceus spp.) predating on the FAW in maize. A small number of farmers (ca. 4%) in both districts reportedly took no intentional action against FAW. Conclusions The farmers believe they can manage FAW if they have the appropriate and efficacious chemical insecticides as they are able to correctly apply them and follow recommended procedures. The farmers advocated for an area-wide approach as one of the best alternatives to manage this invasive pest.

Cassava (Manihot esculenta Crantz), an important commercial and food security crop in East and Central Africa, continues to be adversely affected by the whitefly Bemisia tabaci. In Uganda, changes in smallholder farming landscapes due to crop rotations can impact pest populations but how these changes affect pest outbreak risk is unknown. We investigated how seasonal changes in land-use have affected B. tabaci population dynamics and its parasitoids. We used a large-scale field experiment to standardize the focal field in terms of cassava age and cultivar, then measured how Bemisia populations responded to surrounding land-use change. Bemisia tabaci Sub-Saharan Africa 1 (SSA1) was identified using molecular diagnostics as the most prevalent species and the same species was also found on surrounding soybean, groundnut, and sesame crops. We found that an increase in the area of cassava in the 3–7-month age range in the landscape resulted in an increase in the abundance of the B. tabaci SSA1 on cassava. There was a negative relationship between the extent of non-crop vegetation in the landscape and parasitism of nymphs suggesting that these parasitoids do not rely on resources in the non-crop patches. The highest abundance of B. tabaci SSA1 nymphs in cassava fields occurred at times when landscapes had large areas of weeds, low to moderate areas of maize, and low areas of banana. Our results can guide the development of land-use strategies that smallholder farmers can employ to manage these pests.

The whitefly, Bemisia tabaci, is a major pest of cassava in Africa. Developing whitefly-resistant cassava can control both whiteflies and viral diseases. The main aim of this study was to identify cassava genotypes resistant to four B. tabaci populations, sub-Saharan Africa 1—subgroups 1, 2, and 3 (SSA1-SG1, SSA1-SG2, and SSA1-SG3) and sub-Saharan Africa 2 (SSA2) that colonize cassava, as well as understand the mechanisms of resistance. Utilizing the antixenosis and antibiosis techniques in the choice and no-choice tests, respectively, to screen for whitefly resistance, we tested 46 cassava genotypes. Of these, 11 (Njule Red, Nase 3, Nase 1, Kibandameno, Sagonja, Aladu, Kiroba, Magana, 72-TME-14, Sauti, and PER 415) exhibited antixenosis, as they were least preferred for oviposition by all four whiteflies population in choice tests. Ten genotypes exhibited antibiosis (nymph mortality) against SSA1-SG1 and SSA1-SG3 in no-choice tests, and these were, Pwani, Nase 14, Kalawe, Eyope, NGA11, CoI2246, Mkumbozi, KBH2002/0066, Yizaso, and PER 608. Eight genotypes—Tongolo, Mbundumali, Colicanana, Orera, Ofumbachai, Nam 130, Tajirika, and MECU72—exhibited both antixenosis and antibiosis mechanisms against SSA1-SG1 and SSA1-SG3. And these can be considered the best sources of resistance for the potential development of whitefly-resistant cassava varieties in African countries.

Bombyx mori densovirus type 2 (BmDNV-2), a parvo-like virus, replicates only in midgut columnar cells and causes fatal disease. The resistance expressed in some silkworm strains against the virus is determined by a single gene, nsd-2, which is characterized as nonsusceptibility irrespective of the viral dose. However, the responsible gene has been unknown. We isolated the nsd-2 gene by positional cloning. The virus resistance is caused by a 6-kb deletion in the ORF of a gene encoding a 12-pass transmembrane protein, a member of an amino acid transporter family, and expressed only in midgut. Germ-line transformation with a wild-type transgene expressed in the midgut restores susceptibility, showing that the defective membrane protein is responsible for resistance. Cumulatively, our data show that the membrane protein is a functional receptor for BmDNV-2. This is a previously undescribed report of positional cloning of a mutant gene in Bombyx and isolation of an absolute virus resistance gene in insects.

ContextAfrican production landscapes are diverse, with multiple cassava cultivars grown in small patches amongst a diversity of other crops. Studies on how diverse smallholder landscapes impact herbivore pest outbreak risk have not been carried out in sub-Saharan Africa.ObjectivesBemisia tabaci is a cryptic pest species complex that cause damage to cassava through feeding and vectoring plant-virus diseases and are known to reach very high densities in certain contexts. However, the factors driving this phenomenon are unclear.MethodsBemisia density data in cassava across a large number of sites representing a geographic gradient across Uganda, Tanzania and Malawi were collected. We tested whether in-field or landscape factors associated with land-use patterns underpinned Bemisia density variability and parasitism.ResultsWe found the B. tabaci SSA1 species dominated our study sites, although other species were also common in some cassava fields. Factors associated with the surrounding landscape were unimportant for explaining variability in adult density, but the in-field variables of cassava age and cultivar were very important. The density of nymphs and the parasitism of nymphs was heavily influenced by a diversity of landscape factors surrounding the field, including the size of focal cassava field, and area of cassava in the landscape. However, unlike the trend from many other studies on drivers of natural enemy populations, this pattern was not solely related to the amount of non-crop vegetation, or the diversity of crops grown in the landscape.ConclusionsOur findings provide management options to reduce whitefly abundance, including describing the characteristics of landscapes with high parasitism. The choice of cassava cultivar by the farmer is critical to reduce whitefly outbreak risk at the landscape-scale.

A key constraint to cassava productivity in Africa is the lack of adoption of improved cassava varieties tolerant to pests and diseases. To understand the drivers of adoption behavior, we examine the simultaneous adoption of improved cassava varieties and intercropping by 1200 smallholder farmers in Malawi, Tanzania, and Uganda. Using a linear model, we find that varietal characteristics, such as yield and early maturity, are critical drivers of adoption. Access to extension and credit is associated with an increase in the attractiveness of yield‐improving characteristics. We conclude that a more targeted extension approach would increase technology adoption in these countries.

The fall armyworm (FAW) Spodoptera frugiperda is thought to have undergone a rapid ′west-to-east′ spread since 2016 when it was first identified in western Africa. Between 2018 and 2020, it was also recorded from South Asia (SA), Southeast Asia (SEA), East Asia (EA), and Pacific/Australia (PA). Population genomic analyses enabled the understanding of pathways, population sources, and gene flow in this notorious agricultural pest species. Using neutral single nucleotide polymorphic (SNP) DNA markers, we detected genome introgression that suggested most populations were overwhelmingly C- and R-strain hybrids. SNP and mitochondrial DNA markers identified multiple introductions that were most parsimoniously explained by anthropogenic-assisted spread, i.e., associated with international trade of live/fresh plants and plant products, and involved ′bridgehead populations′ in countries to enable successful pest establishment in neighbouring countries. Distinct population genomic signatures between Myanmar and China do not support the ′African origin spread′ nor the ′Myanmar source population to China′ hypotheses. Significant genetic differentiation between populations from different Australian states supported multiple pathways involving distinct SEA populations. Our study identified Asia as a biosecurity hotspot and a FAW genetic melting pot, and demonstrated the use of genome analysis to disentangle preventable human-assisted pest introductions from unpreventable natural pest spread Competing Interest Statement The authors have declared no competing interest. Footnotes * Co-author name list including missing ORCID added and up-dated. Acknowledgement statement up-dated to include DAWE and NAQS, and funding agencies for French collaborators. Table S1: revised co-author name list Table S2: revised co-author name list Supplemental Figures: revised co-author name list