Oct 22, 2011 – Pesticides and Longevity
QUESTION:
You have referenced a couple of times in your various answers of research showing that pesticides only last about 3 weeks or so. Can you provide the source of this research and where such can be found? Thanks.
ANSWER:
I do get that question a lot, and hopefully I always have had the presence of mind to preface each response with that magical phrase - "it all depends" - because there are no cut and dry answers to the question of how long a pesticide "lasts" once applied. There are too many variables, and these variables include (but are not limited to) - the strength of the concentration applied, the surface it is applied to, the pH of the water or the surface it is applied to, the amount of sunlight or other UV light that hits the treated surface and how long this occurs each day, the moisture level of the treated surface, the level of organic matter in the soil that is treated along with the level of microorganisms in that soil, the heat the sprayed surface is exposed to, and of course the specific pesticide molecule being discussed. Some are more prone to falling apart than others, such as the rapid degradation of pyrethrum once applied. But, even pyrethrum may have an extended life if it is fogged into a dry, dark, coolish wall void as compared with an application into a sunny and warm room.
We end up only be comparing expected lives of pesticides, and one other consideration is that even long after the active ingredient has disintegrated sufficiently that it no longer kills insects, it still may be detectable in the water or soil or on a surface. We speak in terms of the "half life" of chemicals, which is that length of time required for half of the molecules to break down into other things, thus leaving half of the molecules still there until another of those time periods has passed, at which time half of THOSE remaining molecules will be gone, etc. Even though the pesticide active ingredient is still, technically, present on that surface there no longer is enough of it to efficiently kill the intended pest.
For termiticides, if it still is done, we had the standard "ground board" tests done on several specific ground sites in the U.S. The intent was to be able to compare apples with apples so that the effective residual of each termiticide active ingredient could be determined and compared with other active ingredients. For these we heard from product manufacturers that after such-and-such a length of time the product was still effectively killing or repelling subterranean termites, and we heard numbers such as anywhere from 5 years to 20 years or longer. Those were probably the only standardized tests that have been done to determine just how long any particular pesticide molecule remained effective.
However, we do know that certain pesticide molecules are highly resistant to degradation, and these are the inorganic (mineral based) products such as diatomaceous earth, boric acid, and silica gel. We use these in dust products that last for many years, but this is because their effect on insects is much different from that of synthetic molecules and since they are mineral they simply do not break down, any more than we would expect a chunk of granite to stop being granite in a few years. Boric acid is used in baits, as it must be ingested by the bug to be toxic, and even long after the bait is no longer palatable it still would be toxic if ingested.
A few years ago at an educational conference one of our respected industry researchers spoke, and I don't even remember what the primary topic was, but the talk got around to how long insecticides lasted for insect control. His comment was that he would be surprised (speaking about synthetic pyrethroids) if ANY of them lasted more than two weeks. Again, depending on the circumstances this could be far shorter or far longer. We can come up with formulations such as microencapsulated products where the active ingredient is protected from all those environmental hazards, and now perhaps the effective residual is a month or more. One manufacturer claims their pyrethroid granules will last for 3 months, but maybe that granule is hidden under foliage on a relatively dry soil. Put that same granule out in the sun on hot concrete and maybe it's less than a week.
So, there really is no good source that is willing to commit to hard and fast numbers. The chlordane that may have provided 40 years of termite control under the slab of a home would probably disappear in just a few days when applied to a hot asphalt driveway. The permethrin sprayed under the eaves for spider control is going to last longer than the same material applied to alkaline soil in the sun along the base of the house.
Now, didn't I do a great job of dancing around a direct answer?
View past Ask Mr. Pest Control questions.
You have referenced a couple of times in your various answers of research showing that pesticides only last about 3 weeks or so. Can you provide the source of this research and where such can be found? Thanks.
ANSWER:
I do get that question a lot, and hopefully I always have had the presence of mind to preface each response with that magical phrase - "it all depends" - because there are no cut and dry answers to the question of how long a pesticide "lasts" once applied. There are too many variables, and these variables include (but are not limited to) - the strength of the concentration applied, the surface it is applied to, the pH of the water or the surface it is applied to, the amount of sunlight or other UV light that hits the treated surface and how long this occurs each day, the moisture level of the treated surface, the level of organic matter in the soil that is treated along with the level of microorganisms in that soil, the heat the sprayed surface is exposed to, and of course the specific pesticide molecule being discussed. Some are more prone to falling apart than others, such as the rapid degradation of pyrethrum once applied. But, even pyrethrum may have an extended life if it is fogged into a dry, dark, coolish wall void as compared with an application into a sunny and warm room.
We end up only be comparing expected lives of pesticides, and one other consideration is that even long after the active ingredient has disintegrated sufficiently that it no longer kills insects, it still may be detectable in the water or soil or on a surface. We speak in terms of the "half life" of chemicals, which is that length of time required for half of the molecules to break down into other things, thus leaving half of the molecules still there until another of those time periods has passed, at which time half of THOSE remaining molecules will be gone, etc. Even though the pesticide active ingredient is still, technically, present on that surface there no longer is enough of it to efficiently kill the intended pest.
For termiticides, if it still is done, we had the standard "ground board" tests done on several specific ground sites in the U.S. The intent was to be able to compare apples with apples so that the effective residual of each termiticide active ingredient could be determined and compared with other active ingredients. For these we heard from product manufacturers that after such-and-such a length of time the product was still effectively killing or repelling subterranean termites, and we heard numbers such as anywhere from 5 years to 20 years or longer. Those were probably the only standardized tests that have been done to determine just how long any particular pesticide molecule remained effective.
However, we do know that certain pesticide molecules are highly resistant to degradation, and these are the inorganic (mineral based) products such as diatomaceous earth, boric acid, and silica gel. We use these in dust products that last for many years, but this is because their effect on insects is much different from that of synthetic molecules and since they are mineral they simply do not break down, any more than we would expect a chunk of granite to stop being granite in a few years. Boric acid is used in baits, as it must be ingested by the bug to be toxic, and even long after the bait is no longer palatable it still would be toxic if ingested.
A few years ago at an educational conference one of our respected industry researchers spoke, and I don't even remember what the primary topic was, but the talk got around to how long insecticides lasted for insect control. His comment was that he would be surprised (speaking about synthetic pyrethroids) if ANY of them lasted more than two weeks. Again, depending on the circumstances this could be far shorter or far longer. We can come up with formulations such as microencapsulated products where the active ingredient is protected from all those environmental hazards, and now perhaps the effective residual is a month or more. One manufacturer claims their pyrethroid granules will last for 3 months, but maybe that granule is hidden under foliage on a relatively dry soil. Put that same granule out in the sun on hot concrete and maybe it's less than a week.
So, there really is no good source that is willing to commit to hard and fast numbers. The chlordane that may have provided 40 years of termite control under the slab of a home would probably disappear in just a few days when applied to a hot asphalt driveway. The permethrin sprayed under the eaves for spider control is going to last longer than the same material applied to alkaline soil in the sun along the base of the house.
Now, didn't I do a great job of dancing around a direct answer?
View past Ask Mr. Pest Control questions.
