Insect Control: Biological and Synthetic Agents - Index of

1996; Harrison and Bonning, 2000b), although
there are a few exceptions in which relatively small
(threefold or less) increases or decreases have been
observed (Chen et al., 2000; Harrison and Bonning,
2000b). Deletion of an endogenous baculovirus
gene such as egt also results in no significant
(O’Reilly and Miller, 1991; Slavicek et al., 1999)
or relatively small (Pinedo et al., 2003) differences
in pathogenicity. However, the pathogenicity of the
virus can be dramatically increased (i.e., lower LD50
or LC50) by 4- to 100-fold by genetic modifications
that improve the ability of the virus to penetrate
the midgut, i.e., by the expression of enhancins
(Hayakawa et al., 2000b; Popham et al., 2001) or
incorporation of Bt toxin into the polyhedra (Chang
et al., 2003). In these cases, it is believed that
the enhancin or Bt toxin helps the virus to pass
more easily through the midgut and establish a
more rapid systemic infection. In contrast, however,
there were no significant differences in the LC50
values of recombinant AcMNPVs expressing basement
membrane degrading proteases (stromelysin-1,
gelatinase A, of cathepsin L) under early or late
promoters (Harrison and Bonning, 2001).
Another component of the transmission of a virus
is the rate at which the virus and host come into
contact. This component is obviously more difficult
to quantify in the field. As described above, neurotoxin-induced
paralysis will cause the insect to fall
off the plant (Cory et al., 1994; Hoover et al., 1995;
Sun et al., 2004). This knock-off behavior results
in lower levels of foliage contamination, thus the
likelihood that a second host insect will come into
contact with the recombinant virus (at least during
feeding on the plant) will be reduced. In contrast,
the wild-type virus infected larvae will most likely
die on the plant, thereby increasing the potential for
contact between the virus and host. Lee et al. (2001)
have examined this behavioral effect in a greenhouse
microcosm. They found that the wildtype
AcMNPV out-competed recombinant viruses
(AcAaITor AcJHE.SG) for a niche in the greenhouse
microcosm. AcMNPV and AcJHE epizootics lasted
for 8 weeks after the initial release of the virus,
whereas the AcAaIT epizootic ended by the fourth
week after release. Additionally, AcMNPV polyhedra
also increased to greater numbers in the soil in
comparison to AcAaIT or AcJHE.SG after 8 weeks.
A reduction in wandering behavior resulting from
the inactivation of the ptp gene may also reduce
foliage contamination as discussed above. In both
of these examples, the transmission rate of the recombinant
baculovirus should be reduced because
the likelihood of a host insect and virus coming into
contact is reduced.
10: Genetically Modified Baculoviruses for Pest Insect Control 365
Dispersal is another parameter that should be
assessed in order to determine the fitness of a
virus. Abiotic and biotic agents that are known to
disperse baculoviruses include rainfall, air currents,
predators, parasites, scavengers, and grazing mammals
(Fuxa, 1991; Fuxa and Richter, 1994). Predatory
and scavenging insects have been found to carry
(within their digestive tracts) and disperse recombinant
virus (in excrement) at rates of up to 125 cm
per day over a period of up to 10 days (Lee and
Fuxa, 2000). Thus, recombinant virus induced
behavioral changes of the host larvae such as
knock-off effects that reduce predation and scavenging
will also reduce the dispersal of the virus. Additionally,
the ability of the biotic agent itself to disperse
the recombinant baculovirus may be reduced. For
example, parasitic wasps that develop within larvae
of H. virescens that are infected with recombinant
baculoviruses are significantly smaller than wasps
that develop within wild-type virus infected larvae
(McCutchen et al., 1996). Thus, the ability of the
smaller wasps to travel, and subsequently disperse
the recombinant virus, may be reduced because it
has less energy reserves and reduced flight capability
in comparison to wasps that developed on wild-type
virus infected larvae.
Persistence is the final parameter that should be
considered when determining the relative fitness
of a genetically modified baculovirus. In general,
inactivation by sunlight (ultraviolet radiation) is
the primary route of inactivation of polyhedra in
the environment (Ignoffo and Garcia, 1992; Black
et al., 1997; Ignoffo et al., 1997). In contrast, soil
and foliage are factors that can protect the virus
from sunlight and subsequently increase its persistence
(Peng et al., 1999). Knock-off or other behavioral
changes induced by the recombinant virus may
increase their relative concentrations in the soil such
that the recombinant’s persistence is increased. In
contrast, however, the transmission rate of this recombinant
should be reduced because the potential
for contact between the virus and host is reduced.
The cuticle is another factor that can protect the
virus from sunlight such that viral persistence is
increased. Fuxa et al. (1998) found that the cadavers
of T. ni that are killed by recombinant viruses
(AcAaIT, AcJHE.KK, or AcJHE.SG) take much longer
to disintegrate (4–7 days versus 1 day) in comparison
to AcMNPV killed T. ni. Another report by
Ignoffo and Garcia (1996) found, however, that
there is no significant difference in the time to cell
lysis following death by AcAaIT or AcMNPV (lysis
took 1.7 versus 1.5 days, respectively). Clearly, viral
fitness is dependent upon a large number of intricately
integrated factors. Finally, it is also important