Insect Control: Biological and Synthetic Agents - Index of

larva) was reduced by about 30% (88.0 versus
125 h) in comparison to control larvae infected
with AcMNPV. By droplet feeding assays of neonate
H. virescens, the ST50 of AcAaIT infected larvae was
reduced by up to 46% (Inceoglu and Hammock,
unpublished data). McCutchen et al. conducted
additional experiments in which third instar larvae
of M. sexta (an unnatural host of AcMNPV) were
injected with AcAaIT (5 ml containing 1 10 6 pfu).
They detected AaIT specific symptoms as early as
72 h post injection and found a 29% decrease (120
versus 168 h) in the speed of kill. Additional observations
made during this study showed that larvae
infected with AcAaIT typically were paralyzed and
stopped feeding several hours prior to death. Furthermore,
the yields of progeny viruses (polyhedra
per cadaver) in AcAaIT infected larvae (third, fourth,
or fifth instar T. ni) were only 20–32% of those
of control larvae infected with AcMNPV (Kunimi
et al., 1996; Fuxa et al., 1998). This suggested that
the recombinant virus will be quickly outcompeted
in the field by the wild-type virus (see below).
Hoover et al. (1995) have further characterized
the paralytic effect of AaIT by bioassay using
third instar H. virescens that were inoculated with
either AcAaIT or AcMNPV. When the AcAaIT or
AcMNPV infected larvae were placed on greenhouse
cultivated cotton plants, it was found that the
AcAaIT infected larvae fell off the plants approximately
5–11 h before death. This ‘‘knock-off’’
10: Genetically Modified Baculoviruses for Pest Insect Control 343
effect occurred before the induction of feeding cessation.
As a consequence, the amount of leaf area
consumed by the AcAaIT infected larvae was up to
62% and 72% less than that consumed by the
AcMNPV and mock infected larvae, respectively
(Figure 3). Knock-off effects have also been
observed in field trials (on cotton) to assess the
efficacy of recombinant AcMNPV (Cory et al.,
1994) or HaSNPV (Sun et al., 2004) expressing
AaIT. On the basis of this knock-off effect, Hoover
et al. (1995) suggested that median survival time is
not necessarily predictive of the reduction in the
amount of feeding damage that results from the
application of a recombinant baculovirus. Cory
et al. (1994) and Hoover et al. (1995) also emphasized
that the knock-off effect should reduce foliage
contamination because unlike AcAaIT infected larvae
that fall off the plant, wild-type virus infected
larvae tend to die on the plant. Consequently, they
suggested that the reduction in virus inoculum on
the foliage should decrease the spread and recycling
of the recombinant virus in comparison to the wildtype
virus. A discussion of the fitness of a recombinant
virus in comparison to the wild-type parent is
given later in this chapter.
The AaIT gene has been expressed using other
baculovirus vectors, for example, the NPVs of the
mint looper Rachiplusia ou (RoMNPV; Harrison
and Bonning, 2000b) and cotton bollworms Helicoverpa
zea (HzNPV; Treacy et al., 2000) and
Figure 3 Damage to cotton plants by third instar larvae of Heliothis virescens that were inoculated with an LD 99 dose of AcMNPV or
AcAaIT or mock infected. On average, the leaf damage caused by AcAaIT infected larvae was reduced by 62% in comparison to
AcMNPV infected larvae and 71.7% in comparison to mock infected larvae (Hoover et al., 1995). (Reprinted with permission from
Hoover, K., Schultz, C.M., Lane, S.S., Bonning, B.C., Duffey, S.S., et al., 1995. Reduction in damage to cotton plants by a recombinant
baculovirus that knocks moribund larvae of Heliothis virescens off the plant. Biol. Control 5, 419–426; ß Elsevier.)