Comparison of the activity enhancement of a baculovirus by optical brighteners against laboratory and field strains of Choristoneura occidentalis (Lepidoptera : Tortricidae)

The enhancement of Choristoneura fumiferana (Clemens) multicapsid nuclear p olyhedrosis virus (CfMNPV) activity by 4 optical brighteners was compared i n the laboratory between nondiapausing laboratory and diapausing field stra ins of the western spruce budworm, C. occidentalis. The laboratory strain w as 2.7 times as susceptible to the virus as the field population in terms o f LD50 (dose at which 50% of the tested larvae died). When a 1% concentrati on of optical brightener was added to the virus, lethal doses for both stra ins were significantly reduced, indicating that all 4 brighteners acted as activity enhancers for CfMNPV. In terms of LD50, brightners enhanced viral activity against the field strain more (7.6-11.0 times) than the laboratory strain (2.5-3.5 times). Although there were differences in larval suscepti bility to CfMNPV between the 2 strains, LD50 values for both strains were r educed to the same levels when 1% brightener was added to the virus. Furthe rmore, the larvae died more quickly with the addition of brighteners. The t ime required to kill 50% of the larvae was reduced by 39-60% for the field strain, and by 43-57% for the laboratory strain by the addition of 1% brigh tener.