Features Crop Protection Inputs
Greenhouse Grower Notes: July 2007

Avoiding resistance.  We need to maintain the susceptibility of pests in order to prolong the useful life of pesticides.

When a grower notices that a pesticide no longer works as efficiently as it did in the past, it is likely that the grower is experiencing pesticide resistance, barring errors related to spray rate and coverage. It is estimated that worldwide, more than 500 species of insects and its relatives have shown resistance to at least one class of insecticides. Given the difficulty and cost of developing and registering new pesticides, we need to make every effort to minimize the development of resistance in the products that we currently have. That is, we need to maintain the susceptibility of pests in order to prolong the useful life of pesticides.

How does resistance develop?
n Resistance to a pesticide occurs when susceptible individuals in a population have been removed because of excessive exposure to the pesticide, leaving only those individuals that have the genetic capability to survive or resist the pesticide. Often, a few individuals in a pest population may carry resistance genes that enable them to survive. Alternatively, such genes may arise through mutation. When exposure is high, either through many repeated applications, or through persistent exposure, such as with systemic pesticides, selection for survival of resistant individuals occurs. Eventually, as the resistant individuals multiply, they become the majority in a population.

Resistance mechanisms
n Resistance in insects and mites may be demonstrated in several ways, including the following:
1.     Reduced penetration – Changes may occur in the outer covering of insects and mites such that pesticides cannot penetrate into their bodies.

2.     Behavioural changes – Insects and mites may change their behaviour in such a manner as to reduce their exposure to pesticides. For example, they may hide deep within flowers versus staying in the open on leaf surfaces.

3.     Detoxification – Pests may have enzymes that can break down toxins and render them harmless.

4.     Alternation of target sites – Some pesticides may “bind” to target sites within the pest’s biological system, and thereby disrupt proper functioning of their nervous system, or respiratory system, or reproductive abilities, etc. When there is an alteration of these “sites,” such binding is reduced or not possible, thereby reducing or nullifying the effects of the chemical.

IRAC and mode of action classification
n The Insecticide Resistance Action Committee (IRAC) was formed in 1984 to provide a coordinated industry approach to managing resistance. IRAC International is comprised of representatives from key agrochemical companies, and is recognized by The Food and Agriculture Organization (FAO) and the World Health Organization (WHO) as an advisory body on insecticide resistance issues. To provide a single global source of information that would assist growers in practising alternation or rotation of pesticides, IRAC developed a classification scheme for insecticides based on their mode of action. This list is based on the fact that in most cases, development of resistance to a certain compound results in cross-resistance or resistance to other chemically related compounds. To date, there are at least 27 groups of insecticides and miticides based on their mode of action. The groups to which some of the products registered for greenhouse vegetables belong are shown in Table 1.

Conclusion: Knowing the group to which a particular product belongs can assist growers in avoiding overuse of products from any single chemical group, provided there are sufficient registered options. Selective and minimal use of pesticides is only one strategy in our management of pesticide resistance. Delay in development of resistance ultimately depends on use of pesticides as a last resort measure, with emphasis on regular monitoring, and use of biological and cultural techniques.