Nuclear polyhedrosis virus of the European pine sawfly, Neodiprion sertifer (Geoffr.) (Hym., Diprionidae) retains infectivity in soil treated with simulated acid rain

The effect of simulated acid rain on the infectivity of the nuclear polyhedrosis virus of the European pine sawfly, Neodiprion sertifer (Geoffr.) was studied in southern Finland. Three‐year‐old saplings of Scots pine, Pinus sylvestris (L.), were sprayed once with diluted N. sertifer virus nuclear polyhedrosis ( Ns NPV) suspension or distilled water; after this they were irrigated during the growing season with ground water (pH 7) or with acidified water of pH 4 or pH 3, adjusted by adding both H 2 SO 3 and HNO 3 . The ability of virus‐killed N. sertifer larvae to shelter viruses was tested by killing larvae with Ns NPV on half of the virus‐treated saplings before the beginning of the irrigation treatments. The persistence of the virus on the foliage and in the soil was tested the following summer by two separate bioassays with European pine sawfly larvae. In the first bioassay, in which larvae were fed once with the foliage of the experimental saplings, no significant difference in mortality among the various treatments was detected. In the second bioassay, in which larvae were fed on pine foliage contaminated with soil from the virus‐treated saplings, virus treatment had a significant effect. In that case, only 4–7% successfully formed cocoons, compared to larval survival of 22% in the absence of the virus. Of the larvae fed with pine twigs contaminated with soil from saplings on which virus‐infected larval cadavers were retained, the mortality rate was 100%. Acid rain treatments did not affect larval survival in either bioassay. Thus, Ns NPV infectivity in the field is maintained in the soil but not on the foliage and appears to be unaffected by simulated acid rain.