Abstract:
Over dependence on insecticides for the control of diamondback moth (DBM), Plutella
xylostella (L ) has resulted in development of resistant strains and health hazards due to toxic
residues in cabbage. The present study was undertaken to evaluate resistance in DBM and to
assess the contribution of residues of insecticides used for DBM control on resistance
development. A preliminary survey conducted in Accra and Mampong-Akuapem, revealed
inappropriate agronomic practices as the main cause of resistance development and health
hazards due to insecticide residues on cabbage. Three pyrethroids viz lambda-cyhalothnn
(pawa), cypermethrin (cypercal) and deltamethrin (deltaplan), an organophosphatechlorpyrifos
(dursban) and a biopesticide- Bacillus thuringiensis (B.t.) (dipel) were selected
for the study. Wild DBM pupae were sampled from three sites in Accra (Dzorwulu, Airport
and Madina) and a site in Mampong-Akuapem. These populations were used to establish a
DBM colony, which was reared on potted insecticide-free cabbage in a screen house. Early
4th instar larvae were used for leaf residue bioassays for B.t. and larvae immersion for
organophosphate and pyrethroids. Compared to the recommended dosage the L C 9 5 of
dursban were 106, 74, 193, 114 fold in DBM populations from Airport, Madina, and
Dzorwulu and Mampong respectively. Similarly, for Pawa the LC95 to the recommended
dosage were 103, 77, 100, 58 fold for Airport, Madina, Dzorwulu and Mampong
respectively. In contrast, only 3-fold tolerance to dipel was recorded in Airport and Madina,
2 fold in Dzorwulu and 4 fold in the Mampong DBM populations The study revealed that
most of the field-observed resistance among the conventional insecticides might be
attributed to cross and multiple resistance. There was however lack of cues for crossresistance
between the conventional insecticides and B.t. Molecular studies of B.L resistance
using a PCR based method and further resolution using polyacrylamide gel electrophoresis
(PAGE) showed the B. t. resistant geue 10 nave multiple bands, including the diagnostic band, as compared with bands from the susceptible strains. When the wild larvae were tested for
carboxylesterase activities using the naphthyl acetate-diazo blue coupling reaction,
significantly higher activities of a- and )3- naphthyl esterases were recorded in the Mampong
DBM population than the Accra samples. Compared to previous findings, the mean activity
of a-naphthyl esterase had doubled, while an eight-fold increase was registered for
p-naphthyl esterase. Although this result has enormous implications for cabbage farming,
there was no relationship between resistance levels obtained by bioassay and activities of aand/
or p - esterases. The involvement of other metabolic detoxification enzymes or
resistance mechanisms is suggested. Nonetheless, polyacrylamide gel electrophoretic
zymogram study using a-naphthyl acetate revealed presence of bands, which are associated
with higher resistance in DBM The Mampong population showed higher frequencies of
such bands than the Accra populations. Residues of chlopyrifos-methyL, pirimiphos-methyl
and pyrethroids (Cypermethrin, lambda-cyhalothrin and deltamethrin) were estimated on
cabbage samples using biotoxicity to brine shrimp nauplii after fractionation of the
insecticide residues using solid phase extractor (SPE). The residue levels of chlopyrifos were
found to be higher than the FAO/WHO recommended maximum residue level (MRL).
However, residues for pyrethroids could not be detected for most of the study sites due to
low brine shrimp mortalities, except for Dzorwulu and Airport B. Compared to earlier
findings the residue levels recorded were not only above the WHO/FAO recommended
MRL but a 12-18 fold increase was also recorded in residue levels of pyrethroids. Results
from this study have shown that the insecticide residue problem for cabbage should be taken
seriously in Ghana. Farmers need to be educated on proper use and handling of insecticides
in order not to compromise human health. The study suggests that a more integrated
approach using IPM principles, careful and selective use of conventional insecticides
coupled with judicious use of B. t. could help reduce insect pests and the associated problems
on cabbage.