Myths about insecticide resistance

I wasn't going to touch the topic of insecticide resistance, because I don't have much background in this field. But since insecticide treatment remains one of the most important control measures, and many people choose to do treatments by themselves, I decided to cover it. Since insecticide resistance is the biggest issue that we are facing right now (at least in my opinion), I am going to focus on it for a while. However, the goal is not just to collect information, but to understand it and use it to our advantage. Knowledge is the best weapon we can have against this evil creature.

Although I've done extensive research and tried to digest the material before writing, mistakes could be present, so please don't hesitate to correct me.

Enough has been said on why and how to protect your bed. The principle is the same for how to protect yourself during the day, so I won't go through all the details. After I moved to my new place, I bought a table with four round metal legs and without retractable keyboard, and caulked my chair as soon as I bought it. Since my infestation had started, I have been resting my feet on a stool while I am at my computer, with both the chair and the stool protected by Vaseline. For upholstered furniture, my only suggestion is to not use them until the bedbugs are gone. You may or may not need to do all these, depending on the severity of your infestation. I just don't want to take any chance. If you are able to successfully protect yourself for a few months, you may start to relax a bit, since nymphs can only survive for a few months without feeding, and adults are easier to detect. Hopefully you will also stop wondering if the little dots on your body are nymph bites or phantom bites as I did.

Now you have set up your defense, it's time to launch your assault, and you will need some weapons. Naturally you will think of insecticides. As we all know, insecticides can cause side effects to our health and environment, but like many others, I can honestly say that if I could choose between the suffering from these side effects and the suffering from bedbugs, I'd choose the former. My real concern is insecticide resistance.

There are some myths about insecticide resistance.

1. Insecticides create resistant insects
Insecticide applications don't create individual resistant insects. Instead, due to genetic variation, there are almost always a few rare individuals in the population that are naturally resistant to the insecticide applied. These individuals usually exist before the insecticide is even applied. Insecticide applications simply remove the susceptible individuals. The resistant individuals survive, mate and pass the resistance trait to their offspring. Eventually the whole population will be resistant to the insecticide. This is basically an ecological selection process, with the insecticide being the selection pressure (more on this next time).

2. Insects only develop resistance to chemical insecticides.
Insects can develop and have developed resistance to virtually all classes of insecticides. Metabolic resistance is the most common. Insects possess this mechanism are able to detoxify the insecticides with their enzyme systems. Resistance to biological insecticides such as growth regulators has also been frequently reported. Resistance to inorganic insecticides was discovered before the development of organic insecticides such as DDT. Diatomaceous Earth belongs to the group of inorganic insecticides. A study has found that Tribolium Castaneum had developed resistance to DE. I don't have access to the full content of the article, but according to the abstract, behavioral resistance was obviously involved. Insects possessing this type of mechanism are able to avoid the insecticides by changing their behavior. Behavioral resistance has been reported for a few classes of insecticides including pyrethroids. However, behavioral resistance is generally less threatening since the insects simply avoid the insecticide instead of detoxifying it, hence the insecticide can still serve the protective purpose. For example, if DE is dusted around the bed, the bedbugs will have to make a decision between crossing it and starving, and either way, they will die. There might have been another mechanism involved though, with which the resistant insects were able to lose water at a slower rate than the susceptible ones. Even though the study was on Tribolium castaneum, the finding is worrisome since DE is one of the main weapons that we are using against bedbugs.

Pyrethroid insecticides were once thought almost perfect since they were broad spectrum, low cost, low odor, quick acting, relatively low toxic, and relatively non-persistent in the environment. It was even believed that since they provided such quick knockdown effects, insects would not be able to develop resistance to them. However, at least three resistance mechanisms have been discovered in many insect species. The first one is target site resistance. Pyrethroids kill insects by holding the sodium channels of the nerve sheath open. But resistant insects are able to reduce their effectiveness (lowered sensitivity) due to mutations in sodium channel genes. The second one is metabolic resistance (detoxification with enzymes), and the third is behavioral mechanism, as mentioned above.

Beside insecticides, insects can also develop resistance to other environmental stresses such as heat, cold, starvation, desiccation, and so on. We already know that some bedbugs can live for 18 months without feeding, and can withstand extreme temperatures. The thermal death point for bedbugs is 45^oC, but don't assume that this will never change in the future. Could "isolating the bed" cause bedbugs to develop further starvation resistance? The answer is yes and no. If you are able to successfully protect yourself, then this won't happen. Because firstly resistance is a comparative term, even resistant individuals can only tolerate stress caused by starvation (or heat, cold, chemical etc) to a certain degree, and will eventually die without food. Secondly, without blood meals, bedbugs will not be able to reproduce, and hence will not be able to pass the resistance trait to the offspring. However, if you aren't able to protect yourself successfully, selection process may start provided all these conditions are met: a few resistant individuals are present, the starvation period is long enough to kill the majority of the susceptible bedbugs while not long enough to kill the resistant ones, the resistant bedbugs are able to feed to reproduce and the offspring are able to find blood meals for growth. The chance for this to happen is slim nowadays, but the process must have repeated numerous times in the past for bedbugs to be able to survive for such long period of time without feeding today. This also shows how dangerous it is to completely rely on one single control measure, regardless of whether it's starvation, chemical or anything else. Resistance to starvation is likely associated with slow metabolic rate.

3. Resistance takes long time to develop, so we don't need to worry about it now.
Resistance can develop very fast if insecticides are not used properly. The insecticide properties of DDT were first recognized in 1939. Resistance was first reported in houseflies in Sweden in 1946, only two years after its introduction there. Due to cross resistance, resistance can develop very fast or even exist before the insecticide is used. Cross resistance occurs when an insect population that has developed resistance to one insecticide exhibits resistance to other insecticides in the same chemical classes (also termed class resistance) or insecticides in the different class but have the same mode of action. For instance, cross resistance between DDT and Permethrin has already been discovered in some mosquito species. Although they are not in the same chemical class, they both target the sodium channels.

4. The more insects we kill, the better control we achieve
In terms of resistance control, this is not true. There are two reasons for this. First, only susceptible insects get killed and the resistant ones survive, therefore, the higher the mortality is, the higher the selection pressure is exerted on the insect population, and the quicker the selection process is carried out. If the insecticide kills all susceptible individuals, then the whole population will be resistant, rendering the insecticide useless. Second, susceptible insects are needed to dilute the resistance alleles (more on this next time).

5. The insects are resistant to the insecticide, so we should abandon it.
There are three reasons why this may not be true.
First, resistance could be localized. This is particularly true with crawling insects such as bedbugs. Although they are good hitchhikers and a few individuals might migrate to the neighbors, the population is generally confined to one residence. If a population in one residence appears to be resistant to an insecticide, it could be due to one of two reasons. First, repeatedly failed treatments caused the population to develop resistance. Second, the few bedbugs that started the infestation (from a hotel or a neighbor) were resistant to the insecticide. In either case, it would not provide good indication that resistance has spread to the whole neighborhood, although it will eventually. So after you read the University of Kentucky report (see my previous post), don't jump to the conclusion that all bedbugs are resistant to pyrethroids. If you read the first paragraph carefully, you will notice that the number of sampled sites was only four for Kentucky and Ohio, or two per state, too small to provide any meaningful indication of how widespread resistance was.

Second, the insecticide might still provide effective control even though the effectiveness has declined. For instance, although mosquitoes have been able to quickly develop resistance to DDT, DDT is still being use in Africa to fight malaria. However, the focus has been shifted from eradication to control. Since DDT irritates mosquitoes, indoor spray can effectively reduce malaria mortality by keeping mosquitoes out. After all, the ultimate goal is to reduce mortality rate rather than to kill the vector.

Third, individuals that possess the resistance gene usually lack other advantages that the susceptible individuals have. This is termed fitness cost. For instance, fitness cost could make resistant individuals more vulnerable to natural enemies. Therefore, in the absence of selection pressure such as an insecticide application, the frequency of resistance may drop, although this is not always guaranteed and the rate may vary. After the ban on DDT in Sri Lanka, DDT resistance has constantly reverted to susceptibility, but at a very slow rate. However, if no fitness cost is associated with resistance, then there is no reason for reversion to occur, hence resistance becomes stable and the insects will never lose the trait.

So now we know that insecticides have limitations. When used indiscriminately, they could cause significant consequences, such as shortened useful lifetime of the insecticides and resistant insect populations. Currently there are seventeen insect species that are resistant to all classes of insecticides, hopefully we won't create some super bedbugs that can never be eliminated by insecticides. On the other hand, if used properly, insecticide application remains an important control measure. The key, however, is to not rely heavily on one single control measure, but to take an integrated approach and combine all weapons available.