Dec 22, 2011 – Mix What You Need
QUESTION:
Some labels warn not to mix more than enough pesticide for a day's application. I've wondered though, how long can a mixture (conc + water) really hold without affecting its strength? For example, if I mixed a tank of Suspend SC on Monday afternoon around 3 pm, and then used it the following morning at 10am, did the chemical break down any considerable amount? I know some labels bear that warning just to discourage making too much mixture (some technicians might mix it and not have any use for it the next day, for example).
ANSWER:
Good question, and you hit on two of the most important reasons for mixing only as much as is needed on a job, so in my rambling style let's talk about these. The first is the problem with leaving pesticide in the tank after finishing a job. First, that tank, whether it is a hand tank or a power sprayer tank, now becomes a "storage" container when you transport it from one place to another, and since all pesticide storage containers must be labeled with the contents you must place a Service Container Label on that tank. This addresses the concern of knowing what is in that tank if an accident and a spill were to occur.
Second, you do not necessarily know when you leave Job A what your pesticide need will be on Job B, so if you leave some Suspend in the tank and your next job requires (as the BEST product for that pest on that next job) some other kind of insecticide, what are you going to do with that remaining solution of Suspend? You cannot just dump it on the ground or down a sink, and you should not just add that other insecticide into the tank on top of the Suspend, so starting each job with a fresh and emptied tank is very important. In addition, you cannot really accurately know the true volume of the solution left in that tank, so even if you used Suspend on the second job how will you know how much Suspend Concentrate to add to the tank to bring it up to the legal label rate. A third (or are we on our fourth by now?) consideration is that of forgetting what is still left in the tank. "Shoot, did I use Suspend on the last job yesterday or was it Termidor?" Back in the old days of stinky insecticides it was common to do the "sniff test" to determine what was in the tank, but even that was pretty bad and not to be counted on.
As far as how quickly pesticides degrade once mixed in water, this varies considerably. Back in the Olden Days of organophosphates and carbamates the insecticides degraded very quickly, and the diazinon or malathion left over on Friday could be quite badly compromised by Monday morning. Some products, like malathion, could lose half or more of their potency overnight. This was even more of a problem if the water used in that solution had a high pH, as alkalinity just tore up OP's and Carbamates. The synthetic pyrethroids are much less affected by these environmental conditions of pH, heat, water (hydrolysis), and UV light, but they still are going to degrade faster once mixed with water than they do sitting in their original container. If I were to offer an opinion on that Suspend left overnight I would suggest that it probably has most of its potency still remaining. Exactly how much has disintegrated is hard to say, but using it within the next 24 hours (all those earlier concerns aside) should be okay.
Many of the insecticide concentrates in liquid form today even may use water as the diluent in the concentrate, with the active ingredient stabilized in some manner to withstand the water. This is a wonderful evolution for our products, and eliminates the use of petroleum oils as diluents and harsh solvents like xylene for getting the active ingredient into solution. This is why most of our current products are essentially odorless.
Another consideration to keep in mind is that leaving a concentrate mixed with water for an extended period of time could lead to some physical changes that are a problem. We used to have that with one WP product that would turn the mixture BROWN over a few days in water, and when applied indoors it left a nice brown stain on things. Other products may settle out of suspension badly and be difficult to get back into suspension, leaving you with a solution in that tank that is not evenly dispersed. For some formulations, such as wettable powders or microencapsulated products, the material could form a sludge on the bottom of the tank that takes serious agitation to put back into suspension, leading to problems when you apply it the first time. For example, in a hand sprayer with a thickened goo at the bottom, the first time the sprayer is pressurized and the trigger pulled that goo is pushed up into the syphon tube in the tank, really fouling up the hose and spray valve innerds.
So, plenty of reasons for doing it the right way. Even if it is "likely" that the active ingredient is still near the original concentration in the mixture you do not know this for sure, and would not know how to mix more to return it to the proper level. Otherwise, leaving leftover material is asking for some trouble in the other ways.
View past Ask Mr. Pest Control questions.
Some labels warn not to mix more than enough pesticide for a day's application. I've wondered though, how long can a mixture (conc + water) really hold without affecting its strength? For example, if I mixed a tank of Suspend SC on Monday afternoon around 3 pm, and then used it the following morning at 10am, did the chemical break down any considerable amount? I know some labels bear that warning just to discourage making too much mixture (some technicians might mix it and not have any use for it the next day, for example).
ANSWER:
Good question, and you hit on two of the most important reasons for mixing only as much as is needed on a job, so in my rambling style let's talk about these. The first is the problem with leaving pesticide in the tank after finishing a job. First, that tank, whether it is a hand tank or a power sprayer tank, now becomes a "storage" container when you transport it from one place to another, and since all pesticide storage containers must be labeled with the contents you must place a Service Container Label on that tank. This addresses the concern of knowing what is in that tank if an accident and a spill were to occur.
Second, you do not necessarily know when you leave Job A what your pesticide need will be on Job B, so if you leave some Suspend in the tank and your next job requires (as the BEST product for that pest on that next job) some other kind of insecticide, what are you going to do with that remaining solution of Suspend? You cannot just dump it on the ground or down a sink, and you should not just add that other insecticide into the tank on top of the Suspend, so starting each job with a fresh and emptied tank is very important. In addition, you cannot really accurately know the true volume of the solution left in that tank, so even if you used Suspend on the second job how will you know how much Suspend Concentrate to add to the tank to bring it up to the legal label rate. A third (or are we on our fourth by now?) consideration is that of forgetting what is still left in the tank. "Shoot, did I use Suspend on the last job yesterday or was it Termidor?" Back in the old days of stinky insecticides it was common to do the "sniff test" to determine what was in the tank, but even that was pretty bad and not to be counted on.
As far as how quickly pesticides degrade once mixed in water, this varies considerably. Back in the Olden Days of organophosphates and carbamates the insecticides degraded very quickly, and the diazinon or malathion left over on Friday could be quite badly compromised by Monday morning. Some products, like malathion, could lose half or more of their potency overnight. This was even more of a problem if the water used in that solution had a high pH, as alkalinity just tore up OP's and Carbamates. The synthetic pyrethroids are much less affected by these environmental conditions of pH, heat, water (hydrolysis), and UV light, but they still are going to degrade faster once mixed with water than they do sitting in their original container. If I were to offer an opinion on that Suspend left overnight I would suggest that it probably has most of its potency still remaining. Exactly how much has disintegrated is hard to say, but using it within the next 24 hours (all those earlier concerns aside) should be okay.
Many of the insecticide concentrates in liquid form today even may use water as the diluent in the concentrate, with the active ingredient stabilized in some manner to withstand the water. This is a wonderful evolution for our products, and eliminates the use of petroleum oils as diluents and harsh solvents like xylene for getting the active ingredient into solution. This is why most of our current products are essentially odorless.
Another consideration to keep in mind is that leaving a concentrate mixed with water for an extended period of time could lead to some physical changes that are a problem. We used to have that with one WP product that would turn the mixture BROWN over a few days in water, and when applied indoors it left a nice brown stain on things. Other products may settle out of suspension badly and be difficult to get back into suspension, leaving you with a solution in that tank that is not evenly dispersed. For some formulations, such as wettable powders or microencapsulated products, the material could form a sludge on the bottom of the tank that takes serious agitation to put back into suspension, leading to problems when you apply it the first time. For example, in a hand sprayer with a thickened goo at the bottom, the first time the sprayer is pressurized and the trigger pulled that goo is pushed up into the syphon tube in the tank, really fouling up the hose and spray valve innerds.
So, plenty of reasons for doing it the right way. Even if it is "likely" that the active ingredient is still near the original concentration in the mixture you do not know this for sure, and would not know how to mix more to return it to the proper level. Otherwise, leaving leftover material is asking for some trouble in the other ways.
View past Ask Mr. Pest Control questions.
