|Abstract or Summary
- The diamondback moth (Plutella xylostella L.) is a major
pest of crucifer crops worldwide. It is a particularly
important problem in Southeast Asia where climatic
conditions favor a short generation time and rapid
proliferation. In countries such as Thailand, the
diamondback moth has apparently become resistant to all
classes of insecticides, even the newest ones.
The first part of this thesis is the development of a
computer database of all the available information on
diamondback moth toxicology. About 1,550 records were
extracted from more than 235 published or unpublished
documents. These records include information on the
geographical location of experiments, the insecticide
evaluated, the testing method and the results observed.
Most records in the database are from the last decade, and
are predominantly from Taiwan, Thailand, the USA and
Japan. Trends in the use of various classes of insecticides
can easily be followed. Thirty three records have been
found of insecticide resistance of more than a thousandfold.
An the known mechanisms of resistance are found,
and the diamondback moth has become resistant to all the
classes of insecticides, including Bacillus thuringiensis but
excluding abamectin. Case studies made possible by the
use of the database showed a rapid succession in the use
of 29 insecticides in the last 26 years in a single country
(Thailand). Although this rapid succession is usually
ascribed to the development of resistance, a case study on
the regional differences in resistance to five insecticides
showed marked variations in the level of resistance over
distances of 20 miles or more. The possibility of managing
insecticide resistance at the local level, already suggested
by studies in Hawaii, is thus supported by this analysis.
In the second part of this thesis, the toxicity of Bacillus
thuringiensis var. aizawai and of the bacterial toxin
abamectin was tested by two bioassay techniques on
populations of the diamondback moth from two Chinese
kale fields at locations in Thailand at which the highest
levels of resistance to most insecticides have been
reported. The leaf-dip bioassay was found to be superior to
a residue-vial assay or to the FAO-recommended assay.
More than 70-fold resistance to Bacillus thuringiensis var.
aizawai was observed in the populations of DBM collected
from these two fields, suporting the theory that resistance
resulted in the control failures reported by growers.
Resistance did not explain control failures with abamectin
as no resistance to this compound could be detected.
The third part of this thesis analyzes the intraplant
distribution of the diamondback moth larvae in an effort to
explain the control failures with abamectin on Chinese kale.
Extensive sampling of the various larval instars of the
diamondback moth revealed that a majority of second instar
larvae were sheltered on the young folded leaves of
Chinese kale. The majority of fourth instar larvae and
pupae were found on mature (lower) leaves. Inefficient
spraying practices observed in the field with high volume,
low pressure hydraulic pump sprayers may allow the young
larvae to escape pesticide exposure possibly explaining
control failures in the absence of physiological resistance.
This study shows that resistance in the diamondback
moth is a worldwide problem of major proportions. The
rapid development of resistance to Bacillus thuringiensis in
the field is of practical concern. Resistance management
programs in developing countries should include
mechanisms to optimize insecticide delivery to the
protected habitats of diamondback moth on Chinese kale.