Master Rosarian, Jacksonville Rose Society
Roses & You, July 2020
In 2009 through 2011 I successfully prevented major chilli damage in my garden for three years by applying granular Merit (imidacloprid) in late March. However, in 2012 and 2013 I lost many more bushes to chilli thrips. The adult chilli thrips is approximately 1⁄2 the length of the western flower thrip. Chilli thrips favor new growth. Deformed, crinkly new shoots and leaves as well as brown or gray scars, especially on the bot- tom sides of mature foliage are key evidence. Severe infestations will also result in horrifying ugly bloom damage.
To visually examine these thrips, tap an infested sprig of new growth on a piece of white paper, or swish it in alcohol. Any small specks like pepper on the paper or in the alcohol could be chilli thrips; confirm by looking with a strong magnifying glass or jeweler’s loupe. The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) also lists several systemic chemicals that are effective against chilli thrips. After going to the home page of the chilli thrips site, click on the “Management” link to see the list of chemicals.
An untreated chilli thrips infestation kills most varieties of rose bushes, or weakens them so much that they die from black spot and heat stress or are permanently stunted. University of Florida research found that on susceptible rose varieties, cultural and biological control measures could not control chilli thrips; therefore, to control them requires spraying and/or drenching with a systemic insecticide rated for thrips.
Dr. Vivek Kumar, a thrips researcher who worked under contract for the U.S. Department of Agriculture and Florida IFAS, believes that drenching the soil around roses with imidacloprid or cyantraniliprole is effective and would be less damaging to thrips’ predators and pollinators than foliar sprays.
In my own gardens, certain varieties seem less susceptible. ‘Napoleon’, ‘Smith’s Parish’, Rosa rugosa alba, ‘Mrs B. R. Cant’, and most of my Hybrid Musks show little chilli thrips when others must be sprayed or die. My Kordes hybrid teas ‘Beverly’ and ‘Wedding Bells’ also seem resistant. Left unsprayed, chilli thrips killed my ‘Knock Out’ roses within three years. These obser- vations that rose varieties vary in susceptibility to chilli thrips were confirmed by the small UF/IFAS research project for which our society provided supplemental funding in 2016.
Systemic insecticides, such as Merit (imidacloprid), Conserve (spinosad), or Orthene (acephate), provided effective control in UF/IFAS trials. These three chemi- cals have different Modes of Action (MOAs). Never use a systemic pesticide with the same MOA more than twice in a row.
You may have to spray as often as every two weeks, rotating chemicals by MOA. For even better control, as UF/IFAS scientists determined, drench your rose beds with a systemic in addition to spraying. Drench- ing my rose beds with Flagship (thiamethoxam) pro- vided good control in 2014. The manufacturer claims eight weeks of control after a drench and says the chemical is gentle to beneficials.
Rosarians in Houston, TX, have been able to control chilli thrips with SuffOil-X (a spray oil emulsion insec- ticide), but Florida researchers found it to be only marginally effective. If you spray for chilli thrips, avoid the middle of the day. Around midday, the adults fly in swarms and spraying would miss many of them. Spray very early or very late to minimize danger to pollinators. If chilli thrips are causing unacceptable damage to your roses, you will not be able to eradicate them— only suppress and control them.
Chilli thrips are polyphagous. They eat many, many different plants. If you see unusual scarring, stunting, or damage on other plants, check the website for the list of 100+ common Florida plants that suffer from them. So if they are on your roses, they are also established on other plants in your landscape. Just before I sprayed my new St. Augustine roses for the first time to control chilli thrips, I had pruned the ligustrum hedge along the west side of the house. To my dismay, the emerging new growth on the ligustrum showed chilli thrips damage almost immediately. Chilli thrips are opportunistic feeders. University of Florida scientists have identified over 100 economically significant plant species that they will eat. See the list at http://mrec.ifas.ufl.edu/ lso/thripslinks.htm#HOSTS.
Although it has not been confirmed by scientific research, another valuable observation from Geoff Coolidge of Cool Roses is that he suspects hot nights are more important than hot days to increasing the reproduction rates of chilli thrips. Their reproduction and damage may also be slowed by spraying with a water wand as you would for spider mites, but remember that the greatest concentrations of chilli thrips will be near the top of the plant while spider mites proliferate from the ground up.
For more information about chilli thrips, review the information on the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) chilli thrips web site at http://mrec.ifas.ufl.edu/lso/thripslinks.htm. Or watch the 2015 CLUE webinar by Dr. Stephen Arthurs, a UF/IFAS entomologist “Sustainable Roses: Chilli Thrip Research”, at https://gardeningsolutions. ifas.ufl.edu/mastergardener/volunteers/education/webin ars2016.html. Scroll down to the subheading: “January 2015 CLUE Webinar — Sustainable Roses: Chili Thrips Research (Part 4 of 4-part Series).” There you find links to the webinar video, a pdf of the slides, and the UF/IFAS EDIS publication about chilli thrips.
These sites have lots of pictures of the critters as well as pictures of their damage on roses and other plants. The pictures can be helpful because these thrips are so small that they can’t be identified by the naked eye. You have to learn to recognize them from the damage on your plants. Dr. Arthurs’ webinar has slides that show the most effective chemicals and which are least harmful to beneficials. The webinar is 49 minutes long, but the pdf and EDIS pamphlet cover the basics.