Inhibitory effects of dietary tannins on the infectivity of a nuclear polyhedrosis virus to helicoverpa zea (noctuidae: Lepidoptera)

The role of tannins in Helicoverpa zea (Boddie) larval mortality in H. zea nuclear polyhedrosis virus (HzNPV)-exposed third instars was examined in bioassays on semisynthetic diet. Mortality was observed in H. zea larvae not exposed to HzNPV when reared on diet that contained tannic acid at 0.025, 0.05, and 0.1% (w/v) but not on diets that contained Quebracho (condensed tannin) or catechin. The percentage of mortality (45.8%) of third instars fed HzNPV diet was significantly less when Quebracho tannin but not catechin was added to the diet (P ≥ 0.05). At the 0.025% rate of tannic acid, the percentage of mortality in larvae fed HzNPV was significantly less than that expected from the tannic acid-HzNPV mixture. The HzNPV-exposed larvae fed on Quebracho tannin diets also had significantly lower (corrected) mortalities than did HzNPV-exposed larvae fed on tannic acid and catechin diets. Larval mortality was delayed in all tannic acid treatments. When larvae were exposed to an aqueous suspension of HzNPV that was 0.025, 0.05, and 0.1% (w/v) tannic acid or Quebacho tannin prior to being placed on diets without a source of tannin, mortality was significantly reduced on the tannic acid diets but not on Quebracho tannin diets. Larval mortality was not altered significantly when larvae were exposed to HzNPV in tannin-free aqueous suspensions prior to being placed on the tannic acid or Quebracho tannin.