Insect Control: Biological and Synthetic Agents - Index of

332 10: Genetically Modified Baculoviruses for Pest Insect Control
et al., 1998; Vail et al., 1999; Moscardi, 1999;
Copping and Menn, 2000; Lacey et al., 2001).
Both NPVs and GVs have been successfully
registered for use as microbial pesticides by commercial
companies and government agencies. NPVs
of the velvetbean caterpillar Anticarsia gemmatalis
(AgMNPV) and Helicoverpa armigera (HaSNPV)
are being used with particular success for the protection
of soybean in Brazil (Moscardi, 1999) and
cotton in China (Sun et al., 2002), respectively. Natural
baculoviruses, however, are slower acting and
more target specific (i.e., their host specificity is
narrow) compared to synthetic chemical pesticides
such as the pyrethroids. The general use of natural
baculoviruses in developed countries has been limited
except against forest pests primarily due to their
slow speed of insect killing compared to chemical
insecticides, and partially due to their relatively narrow
host specificity, low field stability, and cost of
production. As a natural control agent the ‘‘slow
kill’’ characteristic allows the virus to replicate to
tremendous numbers while allowing its host to feed
for several days. Although an attribute in a natural
control strategy, this trait is a severe limitation in
modern agriculture. This trait and others such as
narrow host specificity can and have been addressed
by genetically modifying the baculovirus using
recombinant DNA technology.
During the 1980s, the birth of genetically modified
(GM) baculoviruses came along with exciting
new research in the laboratories of Summers
(Summers and Smith, 1987) and Miller (1988).
They simultaneously exploited a combination of
unique characteristics of NPVs to establish the baculovirus
expression vector systems (BEVS) that are
now in common use for basic research and commercial
applications. These characteristics include:
(1) the availability of the exceptionally strong
polyhedrin gene (polh) promoter to drive foreign
gene expression; (2) a selection system based
upon the visualization of the nonessential (in
cultured cells) gene product of the polyhedrin gene;
(3) a dsDNA genome that can be easily modified;
(4) a rod-shaped capsid that can extend to package
additional DNA; and (5) a eukaryotic cell line that
supports virus replication at high levels. Autographa
californica multicapsid NPV (AcMNPV), originally
isolated from the alfalfa looper A. californica
(Vail et al., 1973, 1999), is the baculovirus type
species. AcMNPV was used by the Summers and
Miller laboratories as the parental baculovirus for
BEVS. Another baculovirus, Bombyx mori NPV
(BmNPV), isolated from the silk moth B. mori,
was used by Maeda (1989a) as the parental
baculovirus in an alternative BEVS that used larvae
of B. mori for in vivo expression. The methodologies
for the construction and use of recombinant
AcMNPVs and BmNPVs for the expression of heterologous
genes have been thoroughly described
(Summers and Smith, 1987; O’Reilly et al., 1992;
Richardson, 1995; Merrington et al., 1999). These
methodologies, with slight modifications, have also
been used for the construction of GM baculovirus
pesticides.
The studies to date indicate that GM baculoviruses
can easily become an integral part of pest
insect control, especially in developing countries
and for the control of insects that have become
resistant to synthetic chemical pesticides (reviews:
Hammock et al., 1993; McCutchen and Hammock,
1994; Miller, 1995; Bonning and Hammock, 1996;
Wood, 1996; Harrison and Bonning, 2000a;
Inceoglu et al., 2001a; Bonning et al., 2002). Several
innovative and successful approaches have been
taken to improve the speed of kill of a baculovirus
by genetic modification. These approaches include:
(1) insertion of a foreign gene into the baculovirus
genome whose product alters the physiology of the
target host insect or is toxic towards the target
host; (2) deletion of an endogenous gene from the
baculovirus genome; and (3) incorporation of
active toxin into the OV. Combinations of these
approaches have also been successful in terms of
decreasing the time required to kill the host or
more importantly the time required to stop host
feeding. Safe and effective protection of the crop
from feeding damage should be the goal of a GM
baculovirus pesticide. Baculoviruses have been
transformed from natural disease agents to efficient
pesticides through the above-mentioned discoveries
and innovative approaches. Here, numerous innovations
that have been used to improve the efficacy
of baculoviruses for crop protection are discussed.
Additionally, studies that have addressed the safety
of natural and GM baculoviruses, especially in
terms of risk to humans, the environment, and
nontarget beneficial insects are also covered. The
individual sections of this chapter provide a detailed
and comprehensive summary of the field, especially
in terms of improving the insecticidal activity of
GM baculoviruses. In Section 10.7, the reader
will also find a discussion of the reasons why
GM baculoviruses do not receive the interest that
they deserve. The authors hope that after reading
this review, the reader will be convinced that the
currently available GM baculovirus pesticides are
effective and safe, and should be used as biological
pesticides.