Dahlias: Beauties Filled with Beasts

Dahlias are stunningly beautiful flowers, perfect to generate sales at farmers’ markets, to florists, and for events. They are perceived by many casual growers to be relatively easy plants to grow. They produce clusters of luscious blooms from early summer through late fall. What is not to love about dahlias as a cut flower?

When you first start growing them on a commercial scale you can be easily lured by thoughts of “Hey, this is easy!” the first year or two. When insects and disease discover your crop, your life will change and not necessarily for the better. Finding pests on your prized plants is a frustrating, disheartening experience and a wake-up call to some of the problems with dahlias. I don’t want to rain on anyone’s parade, but if you get into dahlias be ready to deal with issues that can crop up and reduce your profit margin.

We will leave the powdery mildews and root rots for a pathologist to write about, and concentrate on what entomologists are good at: telling you about insects and mites.

Thrips

The major insect that must be dealt with when growing dahlias is thrips. Several species feed on dahlias; two of the most common are western flower thrips (Frankliniella occidentalis), and flower thrips (F. tritici). All have a single mandible they use to slash the plant tissue, injuring the plant cells. The thrips then suck up the plant juices exuding from the wound. This feeding causes a stippling effect on the dahlia foliage and flower petals.

Thrips are slender creatures with tube–like body shapes. Females insert eggs into foliage or flower buds. The hatching nymphs will go through two nymphal stages before becoming a reproducing adult. Adult thrips have fringed wings and are fairly good fliers. Although thrips rarely kill dahlias, they can affect the appearance by causing stippled leaves and damaged flowers, resulting in leaf drop and stunted growth.

In 2016, Brian Kunkel, University of Delaware, and I evaluated several low-risk materials for greenhouse thrips control. See IPMFALL16 for charts showing which low-risk materials work the best. These newer materials are very effective, safe for the user, and have a low impact on beneficial organisms. The drawback is they are expensive compared to older classes of chemistry.

Biological control options are available. A native predator called Orius insidiosus, commonly called minute pirate bug, is found throughout the United States and in parts of Canada. It builds up in numbers on its own late in the season. You want to build a population up in June and July when thrips become active. To do so, purchase minute pirate bugs from a biological supplier.

The female minute pirate bug lays her eggs inside plant tissues, preferably that of heavy pollen bearers, so the young can feed on the pollen before searching for prey. After the eggs hatch into nymphs, growth time from egg to mature adult takes at least 20 days. It is possible for several generations to occur during a single growing season.

We have used ‘Purple Flash’ ornamental peppers in greenhouses as banker plants on which minute pirate bugs are released in order to increase their numbers. They fan out from these plants and feed on thrips and other pests in production areas.

Spider Mites

This past summer’s heat wave affected not only the United States and parts of Canada, but the entire globe, with worldwide temperatures 1.5 degrees warmer than normal. In Maryland, we had a record number of days (53) with temperatures above 90F. This weather makes perfect conditions for mites to flourish.

Spider mites are about the size of a tiny speck of sand, making them difficult to see with the naked eye, but you can locate them with a 10 X magnifying glass. Spider mites are often more apparent during dry, hot weather. They tend to build up on the undersides of foliage, making contact with miticides a real challenge. You will need good coverage of a fine mist spray to reach those on lower leaf surfaces.

The-low risk material to use for effective mite control is 0.5% horticultural oil, but making contact with the mites is critical for this material to work. If you catch mites just as the population is starting to build, miticide growth regulators are safe and effective. Hexygon and TetraSan are two mite growth regulators. They do not work on adult mites, but when applied to protonymphs and deutonymphs (young mites), prevent them from shedding their skin and going to the next life stage, so they die.

I have four or five more insects to tell you about, but I do not want to completely discourage you from ever growing dahlias again. I’ll see many of you at the Grand Rapids National Conference, and I will expand on this list of insects and mites that attack dahlias.

Photo 1 Thrips on dahlia flower
Photo 2 Thrips close-up under microscope
Photo 3 Mites, two spotted spider on dahlia flowers
Photo 4 Mite injury on dahlias
Photo 5 Mites, two spotted spider egg and adult on salvia

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]