Deletion of the Lymantria dispar multicapsid nucleopolyhedrovirus ecdysteroid UDP-glucosyl transferase gene enhances viral killing speed in the last instar of the gypsy moth

The Lymantria dispar multicapsid nucleopolyhedrovirus (LdMNPV) is used on a limited basis as a gypsy moth (L. dispar) control agent. In an effort to improve the efficacy (i.e., killing speed) of the LdMNPV, we generated a recombinant viral strain (vEGT-) that does not produce the enzyme ecdysteroid UDP-glucosyltransferase (EGT). We compared the potency and efficacy of vEGT- to wild-type virus (A21-MPV) and the impact of vEGT- and A21-MPV on larval weight gain. Bioassay of L. dispar 1st, 4th, and 5th instars showed no significant difference in the LC(50) values in larvae infected with vEGT- or A21-MPV. The LT(50) values of 1st and 4th instar larvae infected with either virus were similar, but the LT(50) value of 5th instar larvae infected with vEGT- was significantly lower by ca. 33% compared with larvae infected with A21-MPV. Female 4th and 5th instar larvae infected with vEGT- gained significantly less weight than those infected with A21-MPV, whereas male 4th and 5th instar larvae infected with either virus showed no significant difference in weight gain. Larval weight gain was also used as an indicator of feeding activity through generation of FT(50) values (time when 50% of larvae ceased feeding). Fifth instar larvae infected with vEGT- exhibited a decrease in the FT(50) of ca. 32% compared with A21-MPV-infected larvae. Comparison of the total number of polyhedra produced in vEGT- or A21-MPV infected larvae showed that significantly more polyhedra were produced in 5th instars infected with A21-MPV. Deletion of the LdMNPV egt gene generated an improved virus strain since the killing speed of vEGT- is significantly faster than wild-type virus in 5th instar L. dispar larvae, and larvae infected with vEGT-stopped gaining weight earlier than larvae infected with wild-type virus. Because the majority of foliage consumption by L. dispar larvae occurs in the 5th instar, the use of vEGT- may offer better foliage protection than wildtype virus in the field.