Insecticide resistance is a heritable change in the susceptibility of a pest population to a particular insecticide, or insecticide group. In practice, insects that repeatedly fail to be adequately controlled by the registered rate of an insecticide are said to be insecticide resistant. Where insecticide resistance levels are high, pest populations may be unaffected by many times the normal rate of insecticide application.
Cross-resistance occurs when resistance to one insecticide or group, confers resistance to another insecticide.
Genetic resistance occurs naturally within any population, and can be transferred from one generation to the next. Continued use of an insecticide or insecticide group with a common mode of action, over several generations will select for resistance. While the insecticide will kill most pests, a small minority will survive and multiply and this proportion of survivors increases with each generation.
Selection pressure means that the proportion of resistant individuals compounds from generation to generation. If the pest had 3 generations in one growing season, over 5 years this would be 15 generations. The original 5% resistance level in a population would quickly compound over 5 years – if the treatment regime continues to rely upon a single group of chemicals.
Some pest species possess natural tolerances to chemicals. This is unrelated to insecticide resistance. Reasons for differences in tolerance levels between species are unknown.
- Balaustium and Bryobia mites are not controlled with some insecticide rates used to effectively control other mites.
- Red banded shield bugs (Piezodorus oceanicus) cannot be controlled with insecticides that control other pod sucking bugs.
- The smudge bug (a beneficial insect) has a tolerance to synthetic pyrethroids
These pests have not developed resistance following extensive exposure to insecticides, but rather have a naturally high tolerance to some chemical classes.
Insecticide resistance management strategies:
- Use recommended pest thresholds to minimise insecticide use
- Delay the use of broad spectrum insecticides for as long as possible
- these products reduce beneficial populations and can lead to flaring of other pests
- Do not apply a first foliar spray from the same insecticide group as contained in a seed treatment
- Avoid the continuous use of any one mode of action group – including Bt products
- rotate between chemical groups where possible
- Do not exceed maximum acceptable use limits
- Do not respray an apparent failure with a product of the same group
- Comply with any use restrictions placed on insecticides used on other crops
CropLife Australia has developed specific management strategies for a number of insects where insecticide resistance has been found. Look under resistance management for strategies that deal with: