Research Methods in Toxicology and Insecticide Resistance ...

Insecticide research generally involves comparing the level of toxicity of different
compounds or comparing the susceptibility of different insect species or the
same species from different environments. A useful way to make comparisons
is to determine doses that have equal toxicity and there are three general ways
to bioassay compounds to obtain the critical doses (Finney 1964). First is through
direct assaying to measure the exact doses necessary to kill individual animals by
gradually increasing the doses up to the critical point. For insects, these methods are
not practical. The other two ways involve indirect assaying and this is performed by
exposing batches of individuals to standard doses and recording the responses, which
may be death, knockdown, deformity, or discoloration, depending on the expected
effects of the compound on the insect species. Bioassays may be based on quantitative
responses, such as time of survival, but there are technical diffi culties in determining
survival times and thus this method is not useful for testing insecticides. The third
method is to use quantal response bioassays. The binary quantal response with one
explanatory variable is the simplest and most common bioassay test used in insecticide
research. In such dose-response or concentration-response bioassays, the explanatory
variable is a range of dosages or concentrations and the response is an all-or-nothing
observation, such as dead or alive, knocked down or remaining standing, deformed
or not deformed, and discolored or not discolored. The other two quantal response
bioassays are more complex, time-consuming, and less frequently used. Details can
be found in Robertson et al (2007).
In experiments based on quantal response, the data needed are the proportions
of each batch responding to the compound in a particular way. The purpose is to estimate
the dose level that is just suffi cient to produce death (or a particular response)
within the given proportion of insects and to use the estimate to make comparisons. It
is generally easiest to estimate the median (50%) response level of the population.
The median lethal dose is a quantitative expression of tolerance of a particular
species under a given condition or location. It is a defi nitive biological characteristic
and depends on other physiological and physical characteristics such as age, sex, rearing
conditions, and temperature. In the older literature, it is often abbreviated as MLD,
but this can be confused with the “minimum lethal dose.” Usually, the abbreviation
LD 50 is used for a 50% lethal dose. The other levels are abbreviated LD 90 or LD 95
to refer to 90% and 95% lethal doses, respectively. For other dosage variables, the
abbreviations are LC 50 for concentrations, LT 50 for lethal time exposures, KD 50 for
knockdown dosages, and ED 50 for effective doses. LD 50 and other measures provide
estimates of the toxicity of the insecticide used and are expressions of the tolerance
of the insect. The higher the LD 50 value, the lower the toxicity.
Bioassays
Quantal response data are obtained using bioassays and each unit in the bioassay is
the entity that receives the treatment. In assays in which each insect is individually
treated, the unit is the individual insect. When a group of insects are treated by spray
or fed a treated diet, the group (not individuals) is the unit. For experimental precision,
each unit must be a constant, for instance, the insects are obtained from the
Research methods in toxicology and insecticide resistance monitoring of rice planthoppers 39