Some insects (e.g. some aphid species) are transported passively by winds over long distances. They could have been exposed to insecticides in the region of origin, and not in the receiving region.

Thank you so much for answer, but the pattern I observed is quite different like the places having more exposure to insecticide are suceptible as compared to those having more exposure, if you have any idea or reference for such observations.

Maybe the gene(s) or biochemical mechanism which is responsable for insecticide resitance has normally another or additional function? How did you measure the insecticide resistance? How many species / which species did you scan for resistance in one area?

For your answer insecticide usage history needs to be delineated. BTW which species and insecticide are you talking about?

Please, what species are you talking about? If biodiversity in place with high resistance is of high level, it is possible that there is individuals with different degree of resistance (or may be tolerance?).

My work is on chemical, biochemical and molecular aspects of insecticide resistance in Spodoptera litura.

Ma'am according to Metcalf (1989), there are migrant populations that convey genetic traits which basically provide tolerance to insecticides but as such resistance has not been discussed there. Since the work is on Spodoptera which has a history of migration and multi-migration. So, it is possible that there would be some genetic development. That genetic development might be showing tolerance but not exactly resistance (since the tool or method for resistance used by you is not defined here) for the Spodoptera strain of those areas.

For further reading:

1) Metcalf, R. L. 1989. Insect resistance to insecticides. Pest Management Science 26:333–358.

2) Drake, V. A. 1985. Radar observations of moths migrating in a nocturnal low-level jet. Ecological Entomology 10:259–266.

3) Stefanescu, C., et al. 2012. Multi-generational long-distance migration of insects: studying the painted lady butterfly in the Western Palaearctic. Ecography 35:001–014.

Your question is more of generalized rather than technically detailed. it would've been better if you give a highlight of your observation. Anyways, genetic factor is one of the most common mechanisms of insecticide resistance among pest populations. The presence of individuals with resistant alleles at higher frequency in a given population could result in the dominance of chemical-tolerant individuals within that population. This may happen naturally even if the population is less exposed to a particular insecticide. However, you should carefully take your measurement to make sure if your population of interest hadn't had a history of migration. Previous exposure to a pesticide could teach the insects how to cope with that particular chemical and when they migrate to another area, they could still manage to tolerate considerable amount of exposure to the same or related group of chemicals used against them. The other possible reason might be avoidance. Some insect pests have a highly developed sensory organs that enable them to avoid insecticide-treated areas. Some populations within the same species could also have better experience regarding their environment than the their counterparts. Hence, insects could develop tolerance to a particular chemical application just by merely avoiding treated areas.

@Mohammed A. Mohammed: Thanks for the valuable information, can you please share some references so that I could study them in detail.

and thanks abhishek for references, also I can not share detailed information since work is in progress

Non-insecticidal compounds such as pollutants can exert selective pressure on insects, also conferring them resistance to insecticides.

Resistance can be due to physical or metabolic changes in a species. Physical like changes in gut pH that decrease in solubility and therefore rate of adsorption of an insecticide is reduced.the change in pH might be possible with polyphagous species and you said the area is rich with biodiversity. There are many possible physical changes that might happen and can be related with feed like change in the amount of fat stored as most insecticides are fat soluble and therefore accumulate in fatty tissues and are ineffective. I will attach reference later.

Hi Rashmi, resistance may occur through spontaneous mutations. One important form of resistance is metabolic resistance, where the insecticide is metabolised into a less harmful metabolite. This is well known for pyrethroids metabolised by CYP enzymes. Organophosphates are metabolically activated, so it really depends on which insecticides you are investigating. Another possibility is the migration of resistant animals to uncontaminated area, so the mobility (migration behaviour) of your test species is another important factor to consider. Hope this helps. Cheers, Lennart

Thank you so much for reply Mr. Weltje, actually I am studying all the possible reasons for my results, and I am not able to find any metabolic detoxification enzymes correlation paper ( how the activity of one enzyme may suppress or enhance other enzyme activity) if you have any please provide reference.

Due to use of insecticide in agriculture.

I'm not quite sure I understand what you are looking for. An enzyme you can look for in the literature is monooxygenase