Insecticide resistance is a major obstacle to the control of agricultural and medical pests. Resistance is defined by the World Health Organisation as " The development of an ability in a strain of an organism to tolerate doses of toxicant, which would prove lethal to a majority of individuals in a normal (susceptible) population of the same species".

The development of resistance in insects to acutely toxic insecticides generally occurs by selection of rare individuals in a population that can survive the insecticide. It is pre-adaptive and not a mutational effect. This implies that it is an inherited trait. Most commercial insecticides are designed to be poor mutagens and their use results in an intense chemical selection (high dose, high toxicity) which is not conducive to genetic alterations, but allows survival of pre-adapted (i.e. resistant) individuals. Insects become resistant either: behaviorally (avoid exposure to a lethal dose) or physiologically (find ways to survive a normally lethal dose).

There are a number of different mechanisms by which physiological resistance can occur. Mechanisms of :

• reduced penetration (of the insecticide through the cuticle),
• altered target site (i.e. the target site is not effected by the insecticide),
• increased metabolic detoxification (so that it is detoxified before it reaches the target site),
• sequestration (i.e. stored in the body where it is not harmful)
• and possible increased excretion.

In a population of insects it is often a combination of factors, which greatly results in the overall expression of resistance.

The susceptibility of Cayman mosquitoes to various insecticides has been monitored for the past 30 years. The most commonly employed testing was carried out by larval bioassay, in which mosquito larvae (usually III or IV stage) are exposed to insecticides of different concentration so that a dose mortality curve can be produced. From this LD50 and LD90 values can be found.

The values for mosquitoes from different areas of the island can then be compared and values from the same areas over time, thus providing information on the overall resistance situation. However, over a period of 30 years many different staff and consultants have passed through MRCU and carried out mosquito susceptibility tests. Consequently, many different protocols and methodologies have been used making useful interpretation of results difficult.

Insecticidal treatments by MRCU began in about 1965, initially using Malathion through vehicle mounted thermal fogging equipment. Aerial applications began in the early 1970's and have continued since. Currently ground space spray application is carried out using Leco ULV application equipment and aerial application is carried out using Micronair atomisers.