| Summary: | Stictococcus vayssierei Richard, 1971 commonly known as the African root and tuber
scale (ARTS), was first identified as an agricultural pest in the early 1980s. Following
population outbreaks in Cameroon and the Democratic Republic of Congo in late 1980s – early
1990s, the insect is currently reported as one of the major pests threatening root and tuber crop
production in Central Africa. Outbreaks can merely be viewed as symptomatic of some
breakdown of internal organization and extrinsic factors and, understanding and manipulating
the processes causing outbreaks have been at the basis of integrated pest management.
However, data on ARTS’ biology are lacking, and its ecology is poorly understood. Further,
recent observation provided the basis for the hypothesis that ARTS is a species complex,
complicating the development of sustainable management options, particularly host plant
resistance. In order to enhance knowledge on ARTS biology and ecology and contribute to the
development of solutions aiming at minimizing pest infestations and reducing its damage on
crops, this research focused on the farmers’ perception, distribution, life history and diversity
among pest populations.
Interviews of farmers realized during field surveys using a semi-structured
questionnaire revealed that ARTS has been known by the local communities at least since the
early 1900s under 41 names of which Kop, Giga, Ognia, Kamsi, Nakandja and Mokanza are
the most frequent. But farmers still lack control measures to tackle the pest. Field surveys
conducted in cassava-based agroecosystems in Central Africa neighborhood from 2002-2015
showed that ARTS was present in Cameroon, Central African Republic, Congo, Democratic
Republic of Congo, Equatorial Guinea, Gabon and Uganda, at altitude above 277 m. It is likely
that the pest is expanding its range and invading new habitat as it occurred not only in the forest,
but also in the moist savannah where it was never recorded before. However, prevalence was
higher in the forest where cassava was the most infested crop, compared to the savannah where
aroids (cocoyam and taro) were the most infested crops. High densities (> 1000 scales/plant)
were recorded along the Cameroon-Gabon border. Scale cohorts reared on cassava cuttings and
cocoyam corms (females) and in plastic tubes (males) in environment chambers at six constant
temperatures showed a significant decrease of development time with an increase in
temperature in both sexes, with recorded times for females shorter on cassava than on cocoyam.
The pest can tolerate a broad range of temperature (13-29°C). But the highest survival rate was
observed at 21°C and females survived better on cassava than on cocoyam. At this temperature,
females developed in 15-17 days regarding the host plant and males developed in 12 days. The
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lower threshold temperatures were estimated to be 7.4°C and 9.8°C for female development on
cassava and cocoyam respectively, and 7.6°C for male development.
Morphological characters and phylogenetic trees based on DNA sequences of two loci
(28S, COI-COII) supported the hypothesis that ARTS is a complex of two cryptic species with
2.5% and 9.3% mean between-clade genetic divergence. One species is endemic to the western
savannah highlands of Cameroon and should be regarded as a new species. The other species is
widespread across the Nigero-Congolian forest block and refers to S. vayssierei sensu stricto
with six biotypes. The description of the new species is undergoing. Denaturing gradient gel
electrophoresis and diagnostic PCR performed on the 16S rRNA bacterial gene using total DNA
indicated that ARTS’populations were infected with a primary endosymbiont close to Uzinura
diaspidicola of armored scale insects, three secondary endosymbionts including Rickettsia,
Serratia and Wolbachia, and an uncultured gamma-proteobacterium. The cryptic species and
related clades differed by their endosymbiont diversity and prevalence, suggesting a likely
contribution of the symbiotic bacteria to the adaptation, evolution and diversification of the
scale. Implications of the results on the management of pest populations are discussed.
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