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 i mprove the efficacy (i.e., killing speed) of the LdMNPV, we generated a rec ombinant 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 larva l weight gain. Bioassay of L. dispar Ist, 4th, and 5th instars showed no si gnificant difference in the LC50 values in larvae infected with vEGT- or A2 1-MPV. The LT50 values of Ist and 4th instar larvae infected with either vi rus were similar, but the LT50 value of 5th instar larvae infected with vEG T- was significantly lower by ca, 33% compared with larvae infected with A2 1-MPV, Female 4th and 5th instar larvae infected with vEGT- gained signific antly less weight than those infected with A21-MPV, whereas male 4th and 5t h instar larvae infected with either virus showed no significant difference in weight gain. Larval weight gain was also used as an indicator of feedin g activity through generation of FT50 values (time when 50% of larvae cease d feeding). Fifth instar larvae infected with vEGT- exhibited a decrease in the FT50 Of ca. 32% compared with A21-MPV-infected larvae. Comparison of t he total number of polyhedra produced in vEGT- or A21-MPV infected larvae s howed that significantly more polyhedra were produced in 5th instars infect ed with A21-MPV. Deletion of the LdMNPV egt gene generated an improved viru s 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, B ecause the majority of foliage consumption by L. dispar larvae occurs in th e 5th instar, the use of vEGT- may offer better foliage protection than wil dtype virus in the field.