Quantitative and Qualitative Differences in Gypsy Moth (Lepidoptera: Lymantriidae) Nucleopolyhedrosis Virus Produced in Different-aged Larvae

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Abstract
As gypsy moth, Lymantria dispar (L.), larvae matured, the concentration of nuclear polyhedrosis virus (NPV) required to kill 50% of the insects also increased. Based on LC50’s, first instars were ca. 2-,3-,23-, and Ill-fold more susceptible to NPV than second, third, fourth, and fifth instars, respectively. When LC50’s were estimated on the basis of polyhedral inclusion bodies (PIB) per milligram of larval body weight, however, fourth instars were most susceptible, followed by third, second, fifth, and first instars. Insects infected as first instars produced the least amount of NPV (ca. 4 × 103 PIB per larva), and NPV yield increased with larval age. NPV yield per milligram of larval tissue increased from ca. 1.45 × 106 PIB (first instar) to ca. 3.2 × 106 PIB (fourth instar) and then fell to ca. 2.2 × 106 PIB (fifth instar). On the basis of LC50’s, NPV activity increased in a linear fashion as the larvae developed from the first to the fourth instar. Virus activity then decreased among fifth-instar donors. These data suggest that NPV yield estimated on a whole-insect basis and on a milligram of larval weight basis are not equivalent, but such parameters as yield per milligram and virus activity appear to be similar. Use of fourth-instar gypsy moth appears to be more advantageous than use of fifth instars for NPV production. Although fourth instars produced only ca. 80% of NPV produced by fifth instars, NPV from fourth instars was ca. 3-fold more active than NPV from fifth instars.