Dog days in the greenhouse: sweating our asters off!

A few thoughts on pest management in fall aster production.

Fall asters in a commercial greenhouse.
Fall asters in a commercial greenhouse. Photo by Jeremy Jubenville, MSU Extension

The dog days of summer are here! We typically use that term to describe the time around late July and early August when daytime temperatures are the hottest of the year. This year, early July in southwest Michigan was scorching hot with temperatures a full 10 degrees Fahrenheit above average (or more!) in most locations. While many of us were melting in the heat, the conditions were near optimal for thrips and spider mite development. As a result, many growers have been working hard to keep their plants clean and beautiful for the upcoming the fall season.

Fall aster

Managing thrips in fall aster production can be a huge challenge. Under ideal circumstances, we like to clean a greenhouse and then let it rest before filling it with a new crop. Life, however, rarely goes according to plan. Production space can be difficult to find at the beginning of May. In many cases, early crops of asters, mums and celosia are placed in areas where late spring products are waiting for their turn to be shipped. Even if the new crop started clean, any residual thrips population within the greenhouse is going find it and try to eat it. Because thrips damage to fall aster foliage is more prominent than in many other species, final product quality is often linked to the level of thrips control in the previous crop.

With conventional methods, manage thrips as you normally would for spring crops. However, warmer temperatures require heightened diligence. When the thrips population is increasing or already at a high level:

  • Spray at least three times over a two-week period (minimum every five days).
  • Make sure to rotate the Mode of Action after two or three applications.

Experience has shown us that if we meet both criteria, then we're more likely to get satisfactory results. Rotating the Mode of Action (MoA) reduces the likelihood of selecting for insecticide resistance and helps maintain the usefulness of a whole class of chemical products.

Spraying twice a week in July can be problematic. Wearing personal protective equipment (PPE) in hot and humid weather is uncomfortable (to say the least). Furthermore, spraying when the sun is high can increase the risk of phytotoxicity. Many growers either stay late or come back to the greenhouse around 7 or 8 p.m. to avoid this issue. Pesticide applications this late in the day will often leave the foliage wet all night, which opens a very wide door for disease problems. Treatments applied in the morning can help mitigate phytotoxicity and disease concerns, but then worker access to the area becomes limited to those using label-specified personal protective equipment for the duration of the restricted-entry interval (REI).

Ornamental plants that are produced outside the greenhouse tend to have fewer pest problems. It’s an interesting effect that’s due in no small part to natural predator and parasitoid populations (often referred to as “natural enemies”). Even though the vents are wide open and the sides are rolled up, natural enemies do not seem to readily enter greenhouses like our most problematic pests do. We see this all the time in chrysanthemum production.

Wouldn’t it be nice to get the same level of pest control on the inside of the greenhouse as we do on the outside so that we don’t have to spray so much in hot weather?

A well-designed biological control program for indoor production can simulate the effect of natural pest suppression in outdoor production. Controlling thrips populations with biological control agents (BCAs) can provide a seamless transition from spring to summer production programs. Low residual thrips populations at the start of fall aster production can translate into higher quality products and less time wearing personal protective equipment.

If you’re interested in learning more on how to successfully use biological control to manage greenhouse pests in spring floriculture crops, then you won’t want to miss the upcoming short course at Great Lakes Greenhouse in Kalamazoo, Michigan, on Aug. 21. This course features guest educator Graeme Murphy from bioLogical control solutions and is designed to serve as both a foundational learning experience for growers who are new to biocontrol as well as an avenue to help experienced growers refine their techniques. Full session descriptions and registration information can be found at the Biological Control Short Course website.

Conventional product suggestions for thrips management can be found in the Michigan State University Extension Greenhouse Insect Management sheet for 2018.

Questions about mode of action? Don’t hesitate to contact a member of the MSU Extension floriculture team. We’re happy to help!

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

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