MSU Extension Floriculture & Greenhouse Crop Production

As the chilly nights and falling leaves set the stage for All Hallows’ Eve, the scene inside many of our Michigan greenhouses is a strong signal that the December holiday season is just around the corner. In what were seemingly endless drifts of lovely green foliage only a few weeks ago, we now find hints of white, red and pink in clearly delineated masses of rapidly developing holiday cheer. Poinsettias are probably the most famous floral symbol of the Christmas season, but their beauty doesn't manifest overnight. It takes time, patience and a little bit of luck. In some seasons, we're very lucky: the weather is favorable, disease incidence is negligible and pest pressure is low. The 2018 season isn't one of those seasons.

This season, the struggle is real.

For decades, whiteflies have been one of the most—if not the most—troublesome pest for poinsettia growers throughout the United State and Canada. Before the 1980s, the whitefly species that gave growers the most trouble was the greenhouse whitefly (Trialeurodes vaporariorum). It flourishes on a wide range of plant species and can, at the right temperature, produce offspring at staggering rates. Around 1985, the sweet potato whitefly (Bemisia tabaci) began showing up in greenhouses and quickly displaced the greenhouse whitefly as the predominant whitefly species on many crops, including poinsettia.

This is where things get confusing.

Bemisia tabaci is native to North America and had never been a much of an agricultural problem, so what gives with the sudden invasion? It turns out that what we knew as Bemisia tabaci is not actually a single species, but a group of more than 24 closely-related "species" with distinct geographic origins (also known as "biotypes") that look identical but have key genetic differences with huge biological and ecological ramifications. Evil Doppelgängers! Two of these look-alikes have become serious threats to a wide range of agricultural crops in North America: biotype B and biotype Q.

Biotype B is the aforementioned "sweet potato whitefly" and has shown itself to be a spectacularly invasive species with the ability to rapidly colonize and establish large populations on a wide range of fruit, vegetable, field and ornamental crops. Growers across the nation discovered that many traditionally effective insecticides did not provide adequate control of biotype B and, as a result, the industry suffered unprecedented crop losses. Research ensued, new products eventually came on the market, and adequate control of whiteflies was more or less restored.

Biotype Q was first detected in Arizona and Florida between 2004 and 2005 and has since been reported in over 25 states. Although the diet breadth of both biotypes appears to be equally wide, biotype Q did not demonstrate the same invasive ability as biotype B and seemed content to stay within the confines of protected agriculture. Not to be outdone, however, biotype Q has proven to be less susceptible to a wide range of insecticides and seems to develop resistance to others at a faster rate than biotype B.

In 2016, samples of biotype Q were collected from field locations across 12 counties in Florida. With few viable control options and limited expected lifespan of currently effective products, the prospect of a Q-dominated whitefly invasion of outdoor production fields casts a long shadow over our fruit and vegetable industries.

The presence of whiteflies in a poinsettia crop is not an uncommon occurrence. In fact, finding whiteflies in poinsettias is pretty much a fact of life at this point. As with most crops, cutting suppliers are very good at managing pests and do their best to achieve 100 percent control, but anyone who has been in the business for even a little while knows this is an unrealistic expectation. One of the calling cards of an effective agricultural pest is its ability to hide from us until they've established themselves on the crop. This often involves cryptic life stages that are difficult or impossible for us to find when we're inspecting thousands of individual cuttings.

In the modern plant production industry, there are at least two things we can expect:

1. Cuttings and young plants will come in with something.
2. These hitch-hikers will be very difficult to detect.

Since it is possible to receive shipments that harbor both biotypes, our initial tactics can have a strong influence on whether we are ultimately successful in managing a pest that is extremely difficult to control. Recent research has shown that biotype B will outcompete biotype Q in the absence of insecticides.

However, if we apply pesticides at the beginning of the production process, we may find ourselves with a whitefly population that consists entirely of Qs and laughs at just about everything we throw at it. For example, I have recently come across a particularly defiant population that has shown reduced susceptibility to azadirachtin, buprofezin (Talus), cyfluthrin (decathlon), dinotefuran (Safari), imidacloprid (Mallet) and spiromesefin (Judo/Savate).

Based on efficacy trials and field reports, Dave Smitley from the Department of Entomology at Michigan State University recommends the following products for managing resistant whitefly populations on poinsettias:

The following recommendations come from a joint effort by the U.S. Department of Agriculture’s (USDA) Agricultural Research Service (ARS), the IR-4 Project and the Mid-Florida Research and Education Center (read the summary report here):

Product notes and application considerations

• At this point in the season, do not use foliar sprays if at all possible. The leaf canopy has developed to the point where achieving complete coverage with a product that depends on contact activity is not a realistic expectation. Systemic products are usually applied as a drench, but they often take one to two weeks to really start working. If you find yourself in a situation where you need to knock the adult population down in a hurry, consider using a foliar spray product with translaminar activity such as Rycar or TriStar.
• Safari and Flagship should be applied as a soil drench.
• TriStar can be applied as a foliar spray.
• I have not found any Bemisia populations this season that have been susceptible to imidacloprid (Mallet/Marathon). I do not recommend using it on poinsettias to control whiteflies.
• In similar fashion, growers do not seem to be getting positive results from insect growth regulators (IGRs) such as azadirachtin and buprofezin. Look to other products before trying an IGR on whiteflies in poinsettias.
• From my observations and grower reports, Mainspring and Rycar seem to be working well so far. Use Mainspring as a drench when the crop has an extensive leaf canopy.
• Some growers have used Kontos (spirotetramat) as both a foliar spray and as drench and have not seen evidence of phytotoxicity on their poinsettia crop. That being said, test any product on a small number of poinsettias before applying it to the whole crop. This is especially important now that the bracts are showing color.
• One other option to consider would be to use a pathogenic fungus such as Beauvaria bassiana (e.g., Botanigard WP) and Isaria fumosorosea (e.g., PFR-97, NoFly) through a cold fogger. In order to maintain a high spore count and increase the likelihood of canopy penetration, consider fogging every three to five days. Once again, test this option on a small number of plants before using it on the entire crop.

Whew, that's a lot of info! Let's wrap this up!

In the same way we see ghouls and goblins running around our neighborhood from house to house every Halloween, growers will almost assuredly find some creepy-crawly critters skulking about the greenhouse in search of their own form of sugary treats. If you find yourself struggling with whiteflies this year, don't hesitate to contact me or another member of the MSU Extension floriculture team. We're here to help!

Finally, the MSU Extension floriculture team is currently collecting whitefly samples from around the state for genetic analysis. Got whiteflies? Let me know! I'll be happy to come collect some from your greenhouse.

Product suggestions found in this article are based on a combination of efficacy trials and field observations from industry professionals. Reference to commercial products or trade names does not imply endorsement by Michigan State University Extension or bias against those not mentioned.

Thank you to Dave Smitley at the Michigan State University Department of Entomology for his review of this article.