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International Journal of Tropical Insect Science

, Volume 25, Issue 3, pp 218–222 | Cite as

A preliminary study on the pathogenicity of two isolates of nucleopolyhedroviruses infecting African bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae)

  • J. G. OgemboEmail author
  • E. C. Kunjeku
  • S. Sithanantham
Short Communication

Abstract

Two isolates of nucleopolyhedroviruses (NPVs) from Kenya and South Africa were compared to Gemstar® (a commercial NPV) for their pathogenicity against the first four larval instars of Helicoverpa armigera (Hübner). The larvae were fed on droplets with the three virus products in concentrations of 0 (control), 6 × 102, 6 × 103, 6 × 104 and 6 × 105 occlusion bodies/μL. The bioassays showed that the median lethal dose (LD50) values of 23 and 631 occlusion bodies for the first and second instars, respectively, were comparable to those of Gemstar®. The LD50 values for the third and fourth instars were 3981 and 39,810 for the Kenyan isolate and 1288 and 25,119 for the South African isolate. There was a linear relationship between the log LD50, the larval age and the lethal time (LT50), which appeared to be dose dependent. This correlation constitutes a useful index for estimating susceptibility of larval populations. The LT50 increased from 2.8 to 11.9 days and 2.8 to 6.8 days, respectively, for the Kenyan and South African isolates, suggesting a slight increase of resistance with age within infected larvae.

Key words

Helicoverpa armigera nucleopolyhedrovirus bioassay LD50 LT50 Gemstar® 

Résumé

La pathogénicité vis-à-vis des quatre premiers stades larvaires d’Helicoverpa armigera (Hubner), de deux isolats de virus à polyhédroses nucléaires (VPN) provenant du Kenya et d’Afrique du Sud a été comparée à celle du Gemstar®, un produit commercial à base de VPN. Les larves ont été nourries avec des gouttelettes contenant les trois virus à des concentrations de 0 (témoin), 6 × 10, 6 × 10, 6 × 10 et 6 × 10 capsules/μL. Le test biologique a montré que les deux isolats ont une virulence identique à celle du Gemstar®, avec des valeurs de la dose létale (DL50) de 23 capsules occlusives pour le premier stade et 631 capsules pour le second. Pour les troisième et quatrième stades larvaires, les DL50 pour l’isolat kenyan sont respectivement 3981 et 39,810 capsules contre 1288 et 25,119 capsules pour l’isolat sud africain. Ceci laisse penser que les larves acquièrent une résistance au virus à polyhédroses nucléaires, au cours de leur développent. Il existe aussi une relation linéaire positive entre le log DL50 de l’âge de la larve et le temps létal (TL50) et ceci constitue un bon indice pour évaluer la sensibilité des populations larvaires. Le TL50 est de 2,8–11,9 jours et de 2,82–6,8 jours, respectivement pour l’isolat kenyan et sud-africain, suggérant une légère augmentation de la résistance au sein des larves infectées au fur de leur développement.

Mots clés

Helicoverpa armigera virus à polyhédroses nucléaires test biologique LD50 T50 Gemstar® 

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Copyright information

© ICIPE 2005

Authors and Affiliations

  • J. G. Ogembo
    • 1
    Email author
  • E. C. Kunjeku
    • 2
  • S. Sithanantham
    • 3
  1. 1.Laboratory of Biodynamics of Insect-virus InteractionsGraduate School of Bioagricultural Sciences, Nagoya UniversityChikusa, NagoyaJapan
  2. 2.Department of Biological SciencesUniversity of ZimbabweMt Pleasant, HarareZimbabwe
  3. 3.International Centre of Insect Physiology and Ecology (ICIPE)NairobiKenya

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