relationship between nuclear polyhedrosis virus susceptibility and larval weight in heliothis armigera

the population response of heliothis armigera larvae to nuclear polyhedrosis virus (npv) was investigated. the virus was introduced by permitting the larvae to feed on lettuce leaves. median lethal doses (ld50) were determined. the estimated ld50 values for the first, second, third, early, and late fourth larval instars were 5, 141, 1226, 5168 and 24553 polyhedra per larva respectively. in the fifth larval instar a degree of maturation resistance against virus infection was observed. an inverse relationship between mortality and larval weight was detected. expressing the results in terms of ld50 / mg of the larval weight eliminated the observed variation in the susceptibility of larvae. the estimated ld50 values for each of the larval instar groups were used to predict the response of the larval population to virus infection. this procedure provided a sound determination of the response pattern, from which 96% of the variation in the larval susceptibility could be described in terms of the log larval weight.