MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.   

The ‘Q’ biotype of silverleaf whitefly (Bemisia tabaci) is originally from Europe. Silverleaf whitefly, previously referred to as the sweet potato whitefly, has been the dominant pest on poinsettia crops over the last 10 to 20 years, but is less of a problem in bedding plants where we still see plenty of greenhouse whitefly. The only way to distinguish these two species is when they are in the pupal stage. Silverleaf whitefly has pupae shaped like a “contact lens” as opposed to the “birthday cakes” of the greenhouse whitefly. Over the last 10 years, poinsettia growers in Michigan have told me on many occasions that an application of Marathon (imidacloprid) does not seem to last as long or work as well as it used to for whitefly control. In 2005, Tim Dennehy identified the Q biotype in Arizona. He characterizes resistant strains of the Q biotype known from Europe as being virtually immune to pyriproxyfen (Distance), having strikingly reduced susceptibility to buprofezin (Talus) and a reduced susceptibility to the neonicotinoid insecticides; imidacloprid (Marathon), acetamiprid (TriStar) and thiamethoxam (Flagship).

In November 2005, I asked Extension educators Tom Dudek, Dean Krauskopf, and Jeanne Himmelyn to collect a few whiteflies from some greenhouses in their region. They sent me samples of whiteflies from 10 different greenhouses. I sent the whiteflies to Frank Byrne in California for strain identification. Out of 10 greenhouses, six had the Q biotype silverleaf whitefly, three the 'B' biotype silverleaf whitefly, and one had greenhouse whitefly. This means that the Q biotype is likely to be found throughout the state in about half of our greenhouses.

What does it mean to have Q biotype whitefly?

Having Q biotype does not mean your whiteflies will be highly resistant to imidacloprid and the other nicotinoid insecticides. It is more likely that they will have a low level of resistance resulting in a moderate level of control instead of high level of control after applying Marathon or other nicotinoid insecticides. In Europe there have been documented cases of a very high level of resistance to imidacloprid. Having the Q biotype means that this is possible. Cross-resistance to other nicotinoid insecticides is expected, but the level of cross-resistance is not known. I have a graduate student, Mamy Rakotondravelo, working on this right now.

If I have Q biotype whitefly which insecticides will work the best?

If you look at the following list of products recommended for whitefly control, you will see that six of them are either nicotinoid insecticides or Q biotype has already been reported to be resistant to them. This leaves us with the eight products in bold print as the most promising insecticide products for control of Q biotype.

Treating for Q biotype

Growers can still use Marathon or one of the nicotinoid insecticides (Flagship, Safari, or Tristar) for treating Q biotype whiteflies. They may not work as well as they used to, and there is a chance that you will get a poor level of control. In any case, you should be prepared to begin a foliar spray program as soon as your scouts indicate that whiteflies are becoming a problem. Choose one of the products in bold print and spray once per week for two or three weeks. Repeat this later in the crop if necessary. For poinsettias, try to go into first color as clean as possible. For bedding plants, make sure plants are relatively free of whitefly before shipping. Even though Q biotype is not expected to be a problem when people plant bedding plants in their yard, we still don't want buyers to become alarmed seeing whitefly on their plants. The potential for this to happen is greater this year because of all the publicity about Q biotype whitefly. Some people have asked about rotating classes of insecticides to slow down the build-up of resistance. This is a good strategy if you are in continuous crop production year-round and carry-over your own whiteflies. In this case, use a single product that works for four weeks, and then switch to another effective product in a different class for the second four weeks. Follow that with a third product from a third class for four weeks. However, remember that most of you get the whiteflies coming in on plant material that you purchase. You could have been faithfully following a rotational program in your greenhouse for 10 years and still have Q biotype now, because it came from somewhere else. The presence of Q biotype in about half of our greenhouses, shows you how fast resistant types of insects spread throughout the greenhouse industry.

Can I have my whiteflies tested?

I am still looking for samples from different greenhouses. If you have not had a sample collected yet, and you have whiteflies, you can ask Tom, Dean, or Jeanne to collect a sample and get it to me. I will continue testing this winter and spring.