Comparative Infectivities of Gypsy Moth (Lepidoptera: Lymantriidae) Nucleopolyhedrosis Virus Isolates from North America, Europe, and Asia

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Abstract
Biological activities of 19 geographical isolates of Lymantria dispar (L.) nucleopolyhedrosis virus (NPV) were determined in the laboratory. LC50 values varied from 1.7 × 103 polyhedral inclusion bodies (PIB)/ml to greater than 5 × 106 PIB/ml. Twelve statistically different activity levels were observed. The North American isolates were generally most active, whereas the least active isolate originated from Asia (Japan). The North American isolates varied in activity by 16-fold, whereas the European isolates varied by only 6-fold. Five isolates were more active than the Connecticut standard. NPV isolates from Romania, Spain, Yugoslavia,the Soviet Union, Japan, and the United States were passed serially through L. dispar larvae. Although the activity of the American isolate did not change, activities of all other isolate increased significantly by the fourth passage. In general, activities became stabilized from the second to the fourth passage and were comparable to that of the American standard.