Carboxylesterase and Glutathione-S-Transferase Quantification ... 99
to be the best inhibitor amongst chlorpyriphos,
monocrotophos, profenofos, quinalphos, and ph-
enthoate, as it showed 54.34, 72.54, 78.71, 80.82,
and 82.94 % inhibition of carboxylesterase titres
associated with cypermethrin resistance at 0.01,
0.1, 1, 5, and 10 mg/ml and suggested that DDVP
is the best synergist to mitigate cypermethrin re-
sistance in P. xylostella. Rashad ( 2008 ) reported
high titres of esterase activity in brain, foregut,
midgut, and ovary of 2-day old adults of Schis-
tocerca gregaria (Forsskal) while in 13-day-old
adults the hindgut exhibited high esterase levels.
Inhibitory studies with EDTA and profenofos de-
picted high levels of both carboxylesterase and
phosphotriesterases in the brain tissues of two
ages, attributed to play a role in insecticide re-
sistance.
In the present study, correlation between LC 50
value of monocrotophos of the third instar larvae
and carboxylesterase activity was similar to that
obtained by Kranthi et al. ( 1997 ) who studied
the seasonal dynamics of metabolic mechanisms
responsible for pyrethroid resistance in H. armi-
gera and assigned it to the involvement of micro-
somal oxidase and esterases. Young et al. ( 2005 )
reported pyrethroid resistance in H. armigera
and attributed to overproduction of esterase
isoenzymes that metabolize and sequester pyre-
throid insecticides and found out that pyrethroid-
resistance-associated esterases were inhibited by
piperonylbutoxide (PBO) and maximum inhibi-
tion achieved 3–4 h after dosage and again re-
stored by 24 h.
Esterase zymogram in the present study
showed an additional fast moving band by mid-
gut extracts in comparison with whole body ho-
mogenates; this is in conformity with the find-
ings in DBM (Mohan and Gujar 2003 a).
In the present study, the carboxylesterase
present in ICRISAT population was calibrated in
vitro to be 155.2 μmols min−^1 mg−^1 protein and
the esterase zymograms showed intensely stained
bands depicting resistance association. The me-
dian lethal concentration, LC 50 , for this strain
was 60 ppm though more than the discriminat-
ing dose for monocrotophos, 0.35 μg per larvae
(Anandan and Regupathy 2007 ). The base-line
LC 50 estimate for the ICRISAT population with
Cry1Ab toxin is 0.50 μg/ml, the matter of con-
cern in this regard is that indiscriminate usage
of insecticides for control of C. medinalis may
further bring an elevation in the esterase titre
which may sequester the toxin before it reaches
the target site as exemplified in the case of ‘Silver
strain’ H. armigera towards Cry1Ac expressed
by transgenic cotton Ingard ® in Australia, where
sequestration by esterases was recognized as a
potential resistance mechanism apart from previ-
ous resistance mechanisms viz. reduced binding
by the Cry toxin to BBMVs of midgut epithelium
and alteration in midgut proteases that cleave
protoxin to active toxin (Gunning et al. 2005 ).
This chapter emphasizes the importance of
monitoring carboxylesterase enzyme titre and
its characterization in RLF larvae that may also
mediate resistance to Cry toxins along with other
resistance mechanisms when transgenic rice gets
commercialized.References
Abbott WS (1925) A method of computing the effective-
ness of an insecticide. J Econ Entomol 18(2):265–267
Aldridge WN (1953) Serum esterases—1: two types of
esterase (A and B) hydrolysing p-nitrophenyl acetate,
propionate and butyrate, and a method for their deter-
mination. Biochem J 53:110–117Fig. 2 Characterization of C. medinalis carboxylester-
ase with specific substrates. Lane: 1—DDVP (10−^4 M),
Lanes: 2–4—Eserine (10−^4 M), Lanes: 5, 6—Eserine
(10−^6 M), Lanes: 7 and 8—Control