Research Methods in Toxicology and Insecticide Resistance ...

An insecticide is a pesticide used to kill or eliminate insect pests in agriculture,
households, and industries. Judicious use of insecticides may be a factor in
the increase of agricultural productivity. But, by their nature of having high
toxicity to nontarget organisms and capability to develop resistance through
widespread use, most insecticides have high potential to signifi cantly affect and alter
ecosystems. Many are toxic to humans and animals (both domestic and wildlife), and
can accumulate as concentrates in the food chain and water resources, giving rise to
serious environmental contamination and pollution.
Toxicity of a chemical is usually expressed in relative toxicity. All chemicals,
even those generally considered nontoxic, can become toxic depending on the dosage
given to an organism. As such, even a common consumable substance such as water
has an LD 50 of just over 80 g/kg, sugar (sucrose) an LD 50 of 30 g/kg, and alcohol
(ethanol) an LD 50 of 13.7 g/kg, and these can be toxic above a certain dosage. Therefore,
most insecticides, like other toxic chemicals, have varying degrees of toxicity.
Toxic chemicals with relative toxicity of 50 mg/kg and below are considered highly
toxic and those within the 50–500 mg/kg range are generally considered moderately
toxic. Some examples follow:
Highly toxic chemicals (0–50 mg/kg) Moderately toxic chemicals (50–500 mg/kg)
Botulinum toxin 0.00001 (= 10 ng) Paraquat 95
Dioxin 0.1 Caffeine 200
Parathion 13.0 Carbaryl 270
Strychnine 30.0 Malathion 370
Nicotine 50.0 2,4-dichlorophenoxyacetic acid 375
Brief history of insecticide usage in pest control
A brief history of insecticide usage in the control of insect pests appears in Table 2.1. It
should be pointed out that, up to 1950, the dominant insecticide used was arsenic-based.
With the discovery of DDT as a potent insecticide after World War II, organochlorines
were mainly used for insect control until they were replaced by organophosphates
and carbamates by 1975. Pyrethrins extracted from plants were effective insecticides
but were quickly degraded by UV (ultraviolet) light in the fi eld and thus were ineffective
as agricultural insecticides. Based on the pyrethrin molecule, a pyrethroid,
permethrin (stable under UV light), was discovered and synthetized specifi cally for
use in agriculture in the late 1970s. In the early 1980s, several pyrethroids began to
be used widely.
Because of the widespread use of organochlorines, organophosphates, carbamates,
and pyrethroids, insecticide resistance (cross- and multiple-resistance) developed in
many species of insect pests. Insecticide resistance renders many insecticides ineffective
as a control measure. Consequently, many chemical companies involved in
the manufacturing of insecticides have been replacing them with new and less toxic
chemicals.
Research methods in toxicology and insecticide resistance monitoring of rice planthoppers 21