MSU Extension Floriculture & Greenhouse Crop Production

Author’s note: Be sure to also check out the southwest Michigan greenhouse crop status and culture notes for February.

Heads up!

It’s unusual to find the Florida fern caterpillar in Michigan, especially in February, but that’s what several southwest Michigan growers have found in their Boston ferns (see photo). As a group, caterpillars are barely an afterthought in spring floriculture crops. It’s rare to find them and even when we do, they aren’t particularly difficult to control in a greenhouse setting. In areas of the U.S. where they are a frequent agricultural pest, however, Florida fern caterpillars have developed resistance to some insecticides. We won’t go into the biology here, but if you’re interested, check out the Florida fern caterpillar article that Tom Dudek wrote a few years ago. If you have fern baskets hanging in the greenhouse and haven’t examined them in a while, it might be worth the time to send someone up there to inspect the crop.

Managing a Florida fern caterpillar infestation

We've noticed throughout the years that systemic insecticides can be a little hit-or-miss when it comes to foliar feeders. Drench applications of neonicotinoids, in particular, do not seem to work very well on caterpillars. One explanation is that they feed primarily on non-vascular tissue where concentrations are likely to be lower. As such, nearly every recommended management product is either a contact insecticide or one that leaves a stable residue of the active ingredient on the plant. This can be problematic for two reasons:

• Boston ferns have a complex leaf structure with lots of little hiding places.
• Fern baskets are usually hanging up over our heads.

In other words, it’s going to take some time to get the job done.

Dave Smitley from the Michigan State University Department of Entomology suggests the following products: Adept, Bt, Pedestal, Orthene, Mesurol or Duraguard ME. Crop protection products with translaminar activity should provide better results against foliar feeders than those that do not penetrate leaf tissue. You’ll notice that the following table, organized by translocation activity, includes one systemic insecticide. Mainspring (cyantraniliprole) is labeled for use on caterpillars and can be applied through irrigation systems. For growers who dread the thought of spraying all their hanging baskets, cyantraniliprole may be an elegant solution to their dilemma. As always, test a new product on small number of plants before applying it to the entire crop.

Thrips, mites and aphids update

The number of thrips I've been counting on yellow sticky traps are higher than they were at this point last year. February is not a time that I usually see a lot of thrips damage, but it's an important moment for detecting population trends. While the development time for individual thrips is temperature-dependent, their populations increase at an exponential rate. This means that a few extra thrips on a plant in early February can become a few million thrips by the end of April. Growers that employ a consistent scouting program can detect population trends and take care of any hotspots before they get out of control. Smitley includes a few monitoring tips in his annual greenhouse pest management recommendations.

We have found broad mites in Torenia, Thunbergia, Lamium, Hedera, SunPatiens and numerous varieties of Begonia (Rieger, Dragonwing and other hybrids). It’s also good practice to monitor other common hosts such as New Guinea impatiens, Scaevola and Verbena. While it is possible to view them with a hand lens, broad mites can be very difficult to find when population numbers are low. Growers that are consistently finding broad mites in specific crops may want to consider preventative measures such as spray applications or cutting dips.

Twospotted spider mites have been found in Cordyline, Mandevilla and Hedera ivy. These are very common host plants and should be inspected on a regular basis. Like thrips, spider mite infestations are much easier to control when the populations are small. They feed by piercing individual plant cells and sucking out the juices, so the wounds appear as yellowish-white speckles as though someone rubbed the leaf with sandpaper. Twospotted spider mites prefer the underside of lower canopy leaves and can be seen with the naked eye, although a hand lens makes it much easier.

Aphids aren't usually a problem until April, but we did find an infestation in calibrachoa plugs this week. It never hurts to inspect incoming plant material.

Taking a little extra time to monitor and manage infestations in January and February can be tremendously helpful in April and May when the temperatures are higher and everyone is busy. Conventional management product suggestions for all pests mentioned in this article can be found in the MSU Extension “Greenhouse Pest Management with Insecticides” tip sheet.

Thanks to Mark Crossley of West Michigan IPM for sharing his time and expertise, and to Dr. Dave Smitley at the Michigan State University Department of Entomology for his review of this article.