|Dr. Raymond Cloyd.
PHOTOS COURTESY RAYMOND CLOYD, KANSAS STATE UNIVERSITY
Resistance develops at the population level and is an inherited trait. In addition, any surviving pests can pass traits (genetically) on to their offspring (young) or next generation, thus enriching the gene pool with resistant genes.
The amount of selection pressure or frequency of applying pesticides is the primary factor that influences the ability of an insect or mite pest population to develop and sustain resistance to pesticides.
Insect and mite pests, in general, develop resistance to pesticides more rapidly in greenhouses because immigration of susceptible individuals is relatively low and pesticide applications are intense due to the frequency involved.
Furthermore, the rapid life cycle and reproductive potential of many insect and mite pests results in intensive use of pesticides due to multiple applications.
LIFE STAGES OF PESTS MAY INFLUENCE THEIR ABILITY TO DEVELOP RESISTANCE
■ The life stage of insect and mite pests (eggs, larvae, nymphs, pupae and adults) may influence their ability to develop resistance, which is associated with the level of exposure and feeding rate.
However, is it possible for resistant pest populations to become susceptible to a given pesticide? In general, once selection pressure is reduced, then pests may revert to being susceptible (again).
However, this is an over-simplification of the process. For example, the rate of reversion (time required for a pest population to reach a susceptible level) is typically slower in greenhouses compared to the field due to a lack of – or too few – susceptible individuals migrating and the intense selection pressure.
Also, widespread use of the same pesticide (and mode of action) may deplete the susceptible gene pool, thus increasing the frequency/proportion of resistant genes.
DILUTING THE GENE POOL AFFILIATED WITH RESISTANCE
■ Therefore, it is important to leave a reservoir of susceptible individuals to dilute genes required for resistance. Fitness differences, as well as immigration (which dilute the gene pool affiliated with resistance) may strongly affect the rate of reversion, and thus increase susceptibility to pesticides.
|The importance of crop scouting in early detection and IPM response.
However, again, the rate of reversion depends on the amount of selection pressure placed on a given pest population, which is associated with the frequency of applying pesticides.
The length of time required to revert back to “normal” susceptibility varies considerably depending on the insect/mite pest species or strain. In general, the frequency of alleles (one of two or more alternative forms of genes) conferring resistance may decrease rapidly or very slowly following a reduction in selection pressure.
A DECREASE IN PESTICIDE USE MAY RESULT IN DECLINE IN RESISTANCE LEVELS
■ Moreover, the number of generations required to revert to the original level of susceptibility varies based on the initial level of resistance to a particular pesticide. As such, a decrease in pesticide use may result in a decline in resistance levels and increasing potential for susceptibility. The level of resistance may affect how rapidly pest populations (based on the number of generations) revert to being susceptible.
For example, strains of insect and/or mite pests possessing “low levels” of resistance to a given pesticide may revert more rapidly to a susceptible level compared to individuals or strains with a higher level of resistance to the same pesticide (and mode of action).
However, highly resistant strains may never revert to previous susceptible levels or it may take an extended period of time or many generations, and minimal pesticide exposure, for these populations to revert to being susceptible again to the same pesticide. In fact, strains of insect or mite pests possessing a “high level” of resistance to a particular pesticide may never revert to being completely susceptible.
In general, field populations of pesticide-resistant pests may revert to a susceptible level when reared in a laboratory, in the absence of selection pressure.
ASSESSING THE STABILITY OF RESISTANT GENES
■ Another consideration is the stability of resistance. Stability refers to how “fixed” in the pest population are resistant genes, which typically results in a high proportion of resistant individuals.
Furthermore, stability of resistance is governed by the type of selection pressure or mode of action of pesticides applied.
In addition, the stability of resistance may be influenced by the presence or absence of migrating susceptible individuals and fitness costs.
Immigration of susceptible individuals from untreated areas is important in reducing the rate of resistance developing and enhancing the reversion rate to susceptibility; however, this is contingent on selection pressure also being reduced.
The unstable nature of resistance in a pest population oftentimes results in reversion to susceptibility when selection pressure is relaxed.
Moreover, this will only occur if migrating individuals originate from untreated weeds or crops, whereas if migrating individuals come from treated crops, this would likely cause (or delay) reversion to proceed more slowly.
The immigration of susceptible individuals might enhance the rate of reversion although the presence of “actual” susceptible individuals may be rare. This is especially true in greenhouses compared to the outdoor field environments, because field populations of insect and mite pests typically don’t reach the extreme levels of resistance that may be encountered in greenhouses.
Additional factors that may influence the rate at which reversion occurs are relative fitness differences between resistant and susceptible populations of pests and initial gene frequencies (or proportions) of resistant and susceptible individuals. The fitness costs associated with maintaining resistant genes may be high and thus impact the rate of reversion.
MONOGENIC OR POLYGENIC RESISTANCE EXPLAINED
■ Another factor that may impact the reversion rate is whether resistance is monogenic or polygenic.
|The importance of rotating pesticides with different modes of action.
Furthermore, depending on the level of exposure (length of time), insect/mite species (or strain), and type of pesticide applied, the rate of reversion may not result in pest populations becoming completely susceptible.
One of the operational factors that greenhouse producers can implement to avoid resistance is applying pesticides with different modes of action in a rotation program. Nonetheless, it is difficult to determine the optimal use sequence and point at which to change pesticides.
Also, in order for rotation programs to be successful, there has to be no cross-resistance (resistance to pesticides with similar modes of action) and no resistance present to any of the pesticides before initiating the rotation program.
THE IMPORTANCE OF ROTATING PESTICIDES
■ Therefore, the concept of rotating pesticides with different modes of action, as a resistance management strategy to prevent the development of resistance, will only work if arthropod pests have not already developed resistance to the designated pesticides.
Furthermore, this strategy assumes that individuals resistant to one pesticide have a lower fitness (based on survival, behaviour and reproduction) than susceptible individuals, so the frequency or proportion of these individuals in the population decreases during intervals between applications of the same pesticide. As such, it is possible that a decrease in resistance due to an absence of selection pressure may occur too slowly to be practical.
In conclusion, the question of whether resistant insect and mite pests can revert to being susceptible again to pesticides depends on a number of factors including the intensity of selection pressure (based on the frequency of applications), immigration of susceptibles, and stability of resistance.
Also, implementing rotation programs that involve using pesticides with different modes of action will help to minimize the frequency of resistant individuals in a pest population.
Dr. Raymond A. Cloyd is a professor and extension specialist, ornamental entomology/IPM, Department of Entomology, at Kansas State University.