Insecticide Resistance

Definitions

Insecticide resistance is defined by the Insecticides Resistance Action Committee as; ‘A heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species’

Insecticides do not create resistance. The resistant traits must be present in the insect population already. Insecticides represent a selection pressure which allows resistant individuals to build up in numbers within a given population if the selection pressure is applied repeatedly. Selection for, and development of, insecticide resistance is a real and pending threat, especially in areas where insecticides are applied incorrectly, excessively or without proper monitoring.

Insecticide modes of action used in Vector Control

The biggest problem currently faced by the vector control community is the heavy reliance on only four different chemical classes of insecticides.

• organochlorines (OC) - eg. DDT
• organophosphates (OP) - eg. pirimiphos-methyl, malathion
• carbamates (CAR) - eg. bendiocarb
• pyrethroids (PY) - eg. deltamethrin, cyfluthrin, permethrin, lambda-cyhalothrin

These four groups of insecticides act on only two different target-sites and represent only three modes of action. OPs and CAR both inhibit the activity of the enzyme acetylcholinesterase (an enzyme of crucial importance in terminating nerve pulses) however carbamates differ from OPs in that the inhibition is reversible. Pyrethroids and DDT are modulators of voltage-gated sodium channels.


In the illustration above it can be seen that:

• There are three main enzyme groups associated with metabolic resistance
• There are two main target-site modifications which are associated with resistance to insecticides
• Certain mechanisms (eg. esterases) confer resistance against multiple insecticide groups
• Certain mechanisms are regarded as having a greater level of impact against some insecticide groups rather than others (as indicated by the size of the circle)

Developing knowledge about the types of resistance which may be present in a mosquito population is important in order to make informed decisions about the selection of an effective insecticide. For example, in situations where high levels of kdr resistance in mosquito populations are present, the rotation of pyrethroids with Ficam (a carbamate) has proven to provide successful disease control.

The science of Resistance Prevention and Management has become one of Bayer’s key considerations in developing new products and initiatives worldwide.

General publications