Survey and evaluation of nuclear polyhedrosis virus (NPV) for the control of Helicoverpa armigera (Hubner) (Lepidoptera: Nocduidae) on selected vegetable crops in Kenya

Abstract

Samples of helicoverpa armigera larvae obtained from several sites and crops in kenya in surveys carried out during june 1998 to october 1999 were examined at icipe, nairobi. They revealed natural infection by nuclear polyhedrosis virus (npv). Sixteen isolates were collected altogether. In laboratory bioassays on fieldcollected third instar larvae fed on contaminated diet with two local npv isolates and the indian (icrisat) isolate, the cumulative percent larval mortality did not differ significantly but the local isolates were apparently superior. Percentage larval mortality recorded 9 days after inoculation for the kenyan isolates were 78.3±7.38% and 68.3±5.68% respectively, compared to 55.9±2.10% for the indian isolate. Larval mortality was found to be dose-dependent and increased with the larval instars. The kenyan isolate 1 (kibwezi isolate) had a higher slope of 0.685±o.1480 compared to 0.0981±o.14680 for the icrisat isolate while the ld50 values were 1.59976 x 1010obs/ml and 3.178 x 1013obs/ml respectively by probit analysis at 95% fiducial limits. In age-response studies with laboratoryreared larvae, mortality was found to decrease with later instars. Cumulative percent mortality of second instar larvae reached 78.3±2.2% by the third day, 93.3±1.36% by day 6 and 100% by day 8. In pot culture tests, when aqueous suspension sprays of native hanpv were applied on pigeon pea and snowpea potted plants artificially infested with third instar h armigera larvae, significant reduction in larval damage to pods and seeds were obtained. The grain yield per vi plant was also at par with yields of plants sprayed with two chemical pesticides, larnbdacyhalothrin (karate@) and bifenthrin (brigade@). Replicated small plots testing on vegetable pigeon pea also confirmed the potential of native hanpv in providing satisfactory reduction in pest damage leading to grain yield benefits comparable to the chemical pesticide (larnbdacyhalothrin) spray. Molasses (1%) increased viral activity both in the laboratory, pot-culture and field tests compared to 1% charcoal powder. Further efforts should be made to test the biological potential, characterize, improve and demonstrate the potential of native hanpv for augmentation biocontrol of h armigera in kenya.