In the context of Bt crops literature, Resistance is defined as a heritable trait conferring a pest the hability to overcome the control by the Bt crop. Several authors agree in that "insecticide resistance be defined as an individual trait, which is an inher- ited ability of an insect to tolerate doses of a toxicant that would prove lethal to the majority of individuals in the normal population of the species". Here there seems to be an equivalency of both terms, resistance and tolerance. Part of the literature suggests that resistance involves change in the proportion of susceptibles in the population, while tolerance should be reserved to the status of the population prior to exposure to the Bt crop. Crava et al (2013) state that "We used the term tolerance in the sense defined by Finney
(1971), to refer to a quantitative measure of resistance that is normally distributed among individuals within a population". Other literature use both terms interchangably. Any suggestions about finding clear definitions of these terms?
Hi,
In my opinion they key between "resistance" and "tolerance", regardless of insecticide, is inheritable trait. If the insensitivity, no matter how small, is passed on to subsequent generations it is resistance. Unfortunately confusion has arisen as low level resistance has often been referred to as tolerance. Tolerance is simply the 'upper end' of the normal distribution of response to a toxicant.
Of course, the definition of resistance also keeps changing to add to the confusion!
Hi,
In my opinion they key between "resistance" and "tolerance", regardless of insecticide, is inheritable trait. If the insensitivity, no matter how small, is passed on to subsequent generations it is resistance. Unfortunately confusion has arisen as low level resistance has often been referred to as tolerance. Tolerance is simply the 'upper end' of the normal distribution of response to a toxicant.
Of course, the definition of resistance also keeps changing to add to the confusion!
Resistance attributed to exposure to Bt normally takes several years. Exposure to Thuricide for example 18 years gave rise to 30-40 % reduction in the efficacy to bagworms as well as substantial reduction in the binding affinity of the labelled toxins to the receptor sites.
This is a very interesting discussion and question. There are different ways various words are used in this context by different authors and researchers. It is therefore important to define exactly what you mean when using such terms. I am sure there will be many answers all slightly disagreeing and agreeing with one another to this question.
To me "resistance" works at two levels. The first is at the field level where a product or a trait no longer provides economic control of the prevailing population of pest via a heritable trait selected by the treatment. The second is at the organism level where an indiviual has a hertiable trait which is selected for by the treatment. So I can have a bug in my lab which is resistant to a certain toxin, if there are enough of them in the field that is resistance.
The Insecticide Resistance Action Committee (IRAC) have tried to standarize terms. Their deifination is useful... (http://www.irac-online.org/about/resistance/)
"Resistance may be defined as ‘a heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species’. Cross-resistance occurs when resistance to one insecticide confers resistance to another insecticide, even where the insect has not been exposed to the latter product. Clearly, because pest insect populations are usually large in size and they breed quickly, there is always a risk that insecticide resistance may evolve, especially when insecticides are misused or over-used".
I am not so sure "Tolerance " has such a formal definitation. So use it wisely and be clear what you mean. An insect pest can have a baseline sensitivty (before exposure to a control agent/toxin). There will be some variation and we may conclude that some with higher LD50 are more tolerant.
Another use of "tolerant" I have employed is in IPM. I researched predatory mite population which were initially afftected by certain fungicide sprays. However after years of use different vineyards had populations which were affected far less - they had adapted to the spray regimes. In this situation I used "tolerence" to describe such populations. If they'd been a pest which was no longer controlled it would be resistance but in this case it was a beneficial organism.
Enjoyable discussion - I look forward to other views. But in short when you use such terms be careful to describe what you mean.
My comments on this subject is based on field application of Bt for bagworm. Of course it differ with different biopesticides and different insects or target.
I am not so sure (speaking personally) about the latest IRAC definition. If a field population has a metabolic mechanism conferring a 10-fold insensitivity to an insecticide that still demonstrates field control, it is not considered resistance. Add a penetration mechanism that confers a further 5-fold insensitivity and suddenly it is resistance if there is field failure? Neither mechanism is considered resistance in its own right? Seems strange to me.....
Hi Graham - this is great discussion - thank you. My brain is going round and round! I agree with you, and I think we have two uses of the same (similar) words...the field failure is resistance (to product/use so it no longer works). And we have resistance mechanisms in resistant pests leading to field resistance (failure). Unless the mechanism/mecnaisms stop control then they are not resistance - but the organism can have detoxication/penetration resistance mechansims - which may or may not lead to field failure.
We may need some better definitations and some new words to distingish between field failures and the type of resistance mechanisms you are talking about. It is good to have debate - we all (should) know what we are talking about but we don't always use the same words for the same things. So as I said it is important to describe and define terms we use.
We've not managed to get a definition of "tolerance" which was the original question! This could take some time and the involvement of people far more learned than myself.
My impression regarding the use of a 'practical' definition of resistance involving repeated field failures is that it serves to prevent the elimination of actives from field programs based on lab data alone (where methodology can influence resistance factors for different actives and thresholds for declaring resistance vary). Various authors have listed different scales at which a change could be described as resistance, before inserting a practical definition to aid the IRM/IPM practitioner. To discuss evidence at each scale probably requires clarification and sufficient data to support a statement e.g. a resistance mechanism was identified, resistant phenotypes were present, this field population is resistant to X based on definition Y.
I'm not sure I technically agree with avoiding the use of resistance when discussing non-targets, where the mechanisms and processes are the same, though I can appreciate the communication value in the beneficial context.
Perhaps tolerance should not have a technical definition in the same way, so that the verb and the noun can still be used with care when discussing resistance. I don't feel it should be used as a stage in the development of resistance. As previously suggested, tolerance could be applied most usefully to response distributions, particularly within and between susceptible/baseline populations.
Mark, Simon, very interesting. But previously, with the definition as supplied by Eduardo, there was a single definition of resistance which related to phenotypes and mechanisms, which may or may not be exemplified in the field. If it was, then it was control failure. Now we seem to have a predicament as to what to call insensitivity that is sub field-control failure. If one does a lab bioassay and finds an insensitive population compared with a susceptible standard population, inevitably the LC50s ratio is referred to as the resistance factor. Presumably under the new definition this is not the case?
• Well. I think resistance (pest resistance to a pesticide) is a scientific term; it shows a relative toxicity (or other pesticidal action) in a given population in comparison with an unexposed population; resistance is measurable, you quantify it in the terms of folds for instance.
• However, tolerance is an abstract term. There not any unit/way of expression for the tolerance is available except a non-figurative way of more tolerance / tolerance less etc.
Words can be tricky. Definitions are conventions in order to organize comunication. Thus, I agree that one of the key points is defininig what is meant when using some term. But we cannot go around defining terms each time we use it; some conventions would be useful particularly for terms that are used with high frequency. What abouth the following informal synthesis I have just ellaborated from reading related literature?
Susceptibility is the the sensitivity of an organism to some substance (toxin in this discussion). Tolerance is the other side of the coin. Both are an attribute of an individual organism but organisms live in populations. So, susceptibility/tolerance become variables with a (probably) normal distribution, so that at different toxin concentrations different proportions of the population are affected (let’s say killed). One may need a “high” toxin concentration to kill the whole population. Then, resistance can be taken as is clearly defined by several authors. For example, “…We define resistance as a “genetically based decrease in susceptibility to a pesticide”…. The roots of this definition are in the book produced by the National Research Council of the National Academy of Sciences of the United States, in which Brent (1986) defines resistance as “any heritable decrease in sensitivity to a chemical within a pest population …” Source: Tabashnik et al (2014) J. Econ. Entomol. 107(2): 496-507. I have just read that these researchers suggest not to use the term “tolerant”, but still the term is out there …
Hi everybody and thanks for all the contributions. I may have some problem with configuration of my RG page because I am having troubles to read all the contributios in the right sequence. Anyway, My last comment was an attempt to grab and interpret what literature seems to offer more clearly. Dr. G. Moores' last comment was "You can select for resistance, you cannot select for tolerance". Dr. Moores: I am not sure whether you posted this statement in response to my comment. In such case, I don't quite see that we are in disagreement. It seems as if Farhanullah Khan and Grant A. Herron did interpret this that way. But again, Where is the opposition? Have you interpreted that I meant tolerance is heritable? At least from my comment, tolerance is not referred to that way. By the way, a comment to Mark J Miles: The only formal definition I found for tolerance is this, from Finney (1971): the highest concentration of a particular pesticide that an individual can withstand
without being killed. Cheers.
Dr.Eduardo Trumper
could I ask on the basis of
"The only formal definition I found for tolerance is this, from Finney (1971): the highest concentration of a particular pesticide that an individual can withstand"
That
we could say the tolerance is individual based expression while the resistance indicate a population's response to a toxicant .
Hi Eduardo, no I am not in disagreement at all with your last comment: it was an attempt to make a succinct answer to your original question.
Ok, Graham, I am sorry for the misunderstanding. Anyway, your statement made me think in terms of clear differences between both terms. Thank you.
Dr. Khan, regarding your question/comment about another difference being that tolerance refers to individual while resistance refers to population, all I can say is what I have read in the literature about insect resistance to Bt Crops. Dr. David Andow (2008) states that "resistance is caused by genes in the target insect that reduces susceptibility to a toxin, and is a trait of an individual ...". Also we have this definition: "Although the terms “field-evolved resistance” and “evolution of resistance” refer to populations, the word “resistance” can be used to indicate heritable, lower susceptibility of an individual relative to conspecific individuals" Tabashnik et al (2009).
Andow, D.A. 2008. The Risk of Resistance Evolution in Insects to Transgenic Insecticidal Crops. Collect. Biosaf. Rev., Vol. 4:142–99.
Tabashnik, B.; J. Van rensburg, and Y. Carriere. 2009. Field-evolved insect resistance to Bt crops: Definition, theory, and data. J. Econ. Entomol. 102(6): 2011-2025.
I agree with Mark resistance means that no matter what you use the strain is fighting against it. It depends on the genetic make up, where they may attack the chemical agent, several other factors also affect it like the concentration and the exposure period. were as for tolerance is a type of resistance where the strain adopts to the agent and no more respond to it, they develop resistance with time of exposure. they develop reisitance with time
Dr.Hala khyami-horani
If you mean: in tolerance, a gene is temporary become active in an individual under the effect of certain environment to bear it’s adverse effects and transmit to the next generation as dormant with the same tendency to become active in the same circumstances, I agree
It is very interesting and useful discussion. Difference between tolerance and resistance, is it for toxin or for the other insecticide, is in the genetic base. I agree with Dr. Graham Moores, we can't select for tolerance - it is in the limits of physiological norm of species. So, we can use diagnostic dose of toxin, to select resistant individuals. The LD 50 value changes not very fast, and in the population we can find one or two individuals survive under the diacnostic dose (two-fold of LD 95), wich are really resistant.
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