Feb 29, 2012 – Sometimes They All Just Get Along
QUESTION:
How can an Insect Growth Regulator be mixed with a general pesticide and still be effective? Can you help me understand this? To me it seems sort of like mixing a repellent and non-repellent insecticide and rendering one or both of them useless.
ANSWER:
I think several factors are at work here, if I mix what I have heard over the years with what my perceptions are. First is the question of just how repellent the contact insecticides are, and I think most of these products can be repellent to some degree without being so repellent that insects refuse to rest on the dry deposits. The repellency also likely varies from insect to insect. Ants, for example, seem to have extremely sensitive little systems, and some of the early pyrethroid active ingredients appeared to be repellent enough that foraging ants just would not cross a treated surface. This has become less so with later "generations" of pyrethroids so that we now do get reasonably good success against ants. We also can mask the repellency of pyrethroids by encapsulating them, so microencapsulated formulations keep most of the active ingredient inside that microscopic capsule, releasing it slowly and allowing insects to contact it without being repelled.
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How can an Insect Growth Regulator be mixed with a general pesticide and still be effective? Can you help me understand this? To me it seems sort of like mixing a repellent and non-repellent insecticide and rendering one or both of them useless.
ANSWER:
I think several factors are at work here, if I mix what I have heard over the years with what my perceptions are. First is the question of just how repellent the contact insecticides are, and I think most of these products can be repellent to some degree without being so repellent that insects refuse to rest on the dry deposits. The repellency also likely varies from insect to insect. Ants, for example, seem to have extremely sensitive little systems, and some of the early pyrethroid active ingredients appeared to be repellent enough that foraging ants just would not cross a treated surface. This has become less so with later "generations" of pyrethroids so that we now do get reasonably good success against ants. We also can mask the repellency of pyrethroids by encapsulating them, so microencapsulated formulations keep most of the active ingredient inside that microscopic capsule, releasing it slowly and allowing insects to contact it without being repelled.
Other insects, probably most of the larger ones like roaches or crickets or earwigs, may have much less sensitivity to repellent products, so we can use them in crevices or other harborage of these insects and still expect the insect to happily rest in contact with the active ingredient. Much of the repellency is also going to be there at the start, particularly if the surface is still damp or very fresh, and perhaps it diminishes over time even while an effective residual still exists. This thought we can classify as my best guess.
Another factor, though, might be the stability of the IGR's compared with the more traditional active ingredients. Typically we might expect an effective residual from pyrethroids of a couple to a few weeks. Immediately upon being exposed to environmental factors of heat, light (UV), water, or alkaline surfaces many insecticide active ingredients begin to self destruct. The molecule that defines that active ingredient begins to break apart, and eventually there is little to no more of it left on that surface. This may be much faster outdoors than indoors, but in either case there will be a "half life" expectancy for any chemical molecule, and at some point in the near future there will no longer be enough of the original material left to effectively kill the insects. Without a doubt much of that repellency also disappears as the molecules change.
IGR's, however, are pretty darned stable. When methoprene first came into our market the manufacturer told us to expect a minimum of 6 MONTHS of effective residual indoors, and in some of their lab tests they even got a full year of effective residual. So, the IGR is going to be around long after the other non-IGR has gone away. Other IGR's like nylar will also probably have good, long residuals.
Methoprene (Precor) and Hydroprene (Gentrol) also have another interesting characteristic, if we are being told the truth by the manufacturers, and that is for the molecules to "flow" away from the point where they are applied and to redeposit onto other nearby surfaces. This is the reason the Gentrol Point Source tabs work - the hydroprene in the tab flows out of it and over surfaces around it to cover something like 9 square feet with the active ingredient. So, these IGRs will end up on surfaces where the repellent insecticides are not. I know the jury is still out on Gentrol for The Common Bed Bug, but if it works (and industry use tells us it does) this may be one of the reasons. Bed bugs may hide where they do not detect the repellent products but have no way to avoid a non-repellent IGR that flows into their hiding places.
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