Ask Us About Insects By Jim Bethke

Q Are there any foreseeable problems tank mixing an IGR with another class of insecticide?

A There is some debate about the pros and cons of tank mixing, but here are my thoughts on the subject. First, the main reason to tank mix is to impact two different stages of an insect pest at the same time to try to eliminate the entire population, or you may have two different insect pests that you are trying to kill. It really depends on the pest insect or insects. The more common pests (mites, thrips, whiteflies and leafminers, to name a few) tend to develop resistance if they are sprayed too often with the same chemical class and a portion of that population survives and reproduces.

Unfortunately, there are cases of insecticide resistance to IGRs, which means that tank mixing an IGR with another class of insecticide places two chemical classes on the same population, and if even a small portion of that population survives and reproduces, they will build resistance to both chemical classes. This type of resistance is called multiple resistance, and it's tough to combat. So if you're constantly applying the same insecticide, the chances of insecticide resistance are high. That's why I do not like the idea of tank mixing insecticides to try to control both the immature and adult stages with an IGR and another chemical class. I've seen enough examples of high levels of resistance because of increased frequency and off-label use of insecticides that using less is definitely an advantage.

In my opinion, if you attack just one stage diligently, you will take the population to extinction and will have solved the problem without overusing pesticides. The key is to monitor the pests closely so you know what stage to treat. For more information on insecticide resistance and the terms that are associated with it, visit the Insecticide Resistance Action Committee website,

Q Should I change chemical classes every two weeks?

A That's a general rule that many people like to use, but I don't think it's always a good rule. I think it depends on the insect you are concerned with, and my general rule would be to rotate chemical classes every insect generation. Therefore, if you have killed the majority of the pest with only a few surviving and reproducing, then changing chemical classes should kill this next generation. Sometimes, it may seem that we are providing boring information, but we try hard to educate clientele about each pest, such as how long they live or how many eggs they lay. This information can lead to accurate methods for control of each pest.

For example, aphids can go through an entire generation (reproductive female F0 generation to reproductive female F1 generation) in five days at warm temperatures, depending on the species. That's why it seems that they just appear in the crop. It also means that if you are waiting a month to change chemical classes, you have been treating up to six generations of the insect with the same active ingredient. On the other hand, it takes whiteflies about 25 days to go from F0 female to F1 female. Then it makes sense to change chemical classes every 25 to 30 days.

Jim Bethke

Jim Bethke is floriculture and nursery farm adviser with the University of California Cooperative Extension in San Diego County.

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