The use of pesticides is, unfortunately, broadly misunderstood, and misuse can lead to bigger and worse problems.
You may have heard me advise the use of the “apply thoroughly, three times at one week intervals” regimen, but folks have a tendency to spray once and hope they’ve controlled the pest. The reason that can be a problem is multifold.
For one, “thorough” may be misapplied – I recommend spraying ALL exposed plant surfaces to dripping AND drenching the potting medium. In collections in which multiple plants are grown near each other, spray ALL your plants at every treatment.
Secondly, please understand that most pesticides – including systemics – only kill adults, leaving eggs and immature critters that can later mature and reproduce. The goal of repeated, thorough treatments, is to kill newly-maturing adults before thy can do so.
Third – and the core of this article – is that, just like every other living creature, pest populations carry a wide genetic distribution, so what affects one may not affect another. How many times have you gotten sick, but someone in the same household or workplace only gets a mild version or doesn’t get ill at all? The same is true of bugs’ reaction to toxins. Here’s the scenario:
- A population has genetic variation, so a few individuals may have traits that make them less susceptible to a particular pesticide.
- When pesticides are applied, the susceptible insects are killed, but the resistant ones survive.
- With repeated applications, the proportion of resistant individuals in the population increases over time, and the pesticide becomes less effective.
There are several mechanisms that can lead to pesticide resistance, including:
- Metabolic resistance, in which the insect produces more enzymes that can break down the pesticide, making it ineffective.
- Changes in target sites, where a mutation in the gene for a protein that the pesticide targets can change the protein’s structure. This modification makes the pesticide unable to bind effectively, rendering it harmless.
- Penetration resistance caused by the insect’s outer layer, or cuticle, becoming thicker, slowing the rate at which the pesticide can penetrate it.
- Excretion or sequestration occurs when the critter has developed the ability to excrete the absorbed toxin quickly or “sock it away” in a location that is less harmful.
Then, of course, it is also possible the bugs can learn to simply hide from the treatment – another good reason for doing “thorough” treatments.
One last point about “pesticide rotation”: rather than doing the first treatment with “pesticide A”, followed by “B”, then “C”, the better practice is to use a single pesticide for all three treatments targeting a single outbreak, then, if you see a pest resurgence in a month, repeat the entire process using a pesticide having a different mode of action (MOA). If nothing else, that may save you money by avoiding the purchase of another pesticide, and it certainly improves the safety and environmental impact over using several.