Contents - Faperta

260 Biotechnological Approaches for Pest Management and Ecological Sustainability
High selectivity of scorpion neurotoxins makes them an ideal choice to improve the effi -
cacy of baculoviruses. The AaIT neurotoxin, specifi c to insect larvae, is associated with a
single protein, which is distinct from those associated with toxicity to mammals (Zlotkin
et al., 1971b). The scorpion venom toxins are neuropeptides of 60 to 70 amino acids crosslinked
by four disulfate bonds (Bernard, Courard, Rochat, 1979; Anderson, 1992). The AaIT
causes fast and reversible paralysis in insects, similar to pyrethroids (Zlotkin et al., 1991).
Maeda et al. (1991) expressed the cDNA of AaIT in NPV from silkworm, Bombyx mori L.,
and achieved a signifi cant increase in biological activity. However, the larvae had to be
infected by injecting the baculovirus instead of by feeding because the recombinant baculovirus
was polyhedron-negative. A polyhedron-positive baculovirus (AcMNPV) has now
been developed that expresses the AaIT from a synthetic gene under the control of the p10
promoter (McCutchen et al., 1991; Stewart et al., 1991). Bioassay with the chimeric baculovirus
vAcUW2 (Carbonell et al., 1988) has shown a signifi cant decrease (40%) in time to kill
the larvae of Southern armyworm, Spodoptera eridania (Cramer), cabbage looper, Trichoplusia
ni (Hubner), and tobacco budworm, Heliothis virescens (Fab.). The recombinant virus is
active in lepidopteran larvae when fed orally, and is expected to be stable under fi eld conditions
(Kuzio, Jaques, and Faulkner, 1989).
Wild-type A. californica nuclear polyhedrosis virus (WT AcNPV) has been modifi ed to
encode for an insect-selective toxin derived from the venom of the scorpion, A. australis to
produce the recombinant virus AcAaIT (Hoover et al., 1995). Larvae of tobacco budworm,
H. virescens, infected with the baculovirus AcAaIT fell off the plant 5 to 11 hours before
death, and were unable to climb back on to the plant to continue feeding. These larvae
were capable of feeding if placed on diet or confi ned to a leaf with a clip cage. Larvae
infected with AcAaIT consumed signifi cantly less foliage than the larvae infected with WT
AcNPV or uninfected controls. Absence of liquefi ed cadavers on plants following treatment
with AcAaIT may be more desirable to growers and consumers relative to treatment
with wild-type virus. Du and Thiem (1997) developed a new recombinant A. californica
nuclear polyhedrosis virus, vAcLdPD, bearing only the gene for host range factor 1 (hrf-1)
controlled by its own promoter. Recombinant AcNPV with the enhancin gene from T. ni
granulovirus, AcEnh26, has been propagated in Sf9 cells (Hayakawa et al., 2000). The
infected cultured cells were combined with either AcNPV occlusion bodies (OBs) or
Spodoptera exigua (Hubner) NPV (SeNPV) OBs and fed to third-instar larvae of S. exigua.
Feeding larvae with AcEnh26-infected cells resulted in a 21- and 10-fold enhancement of
infection by AcNPV and SeNPV, respectively, as compared to the controls.
Insecticidal properties of AcAaIT and corn earworm, Helicoverpa zea (Boddie), NPV
(HzAaIT) genetically altered with toxin from A. australis have been evaluated against
selected Heliothine species by Treacy, Rensner, and All (2000). The LD 50 based on dietoverlay
bioassays showed AcAaIT and HzAaIT to be equally virulent against larval tobacco
budworm, H. virescens, but HzAaIT showed 1,335-fold greater biological activity than
AcAaIT against corn earworm, H. zea. The HzAaIT killed the larvae at a faster rate than the
nontransformed HzNPV (LT 50 2.5 and 5.6 days, respectively). In the greenhouse, foliar
sprays of AcAaIT and HzAaIT were found to be equally effective in controlling H. virescens
on cotton. However, HzAaIT was superior to AcAaIT against H. zea on cotton. Cotton
treated with AcAaIT or HzAaIT at 10 10 11 occlusion bodies (OB) ha 1 averaged 2.5 and
16.2 nondamaged fl ower buds per plant, respectively. Quicker killing speed exhibited by
HzAaIT led to more improved protection than with HzNPV. Field trials indicated that
HzAaIT at 5 to 12 10 11 OB ha 1 provided better control of Heliothine complex in cotton than
Bt, and equal to spinosad, but slightly poorer than pyrethroid and carbamate insecticides.