Look out for chilli thrips this fall
age is apparent on a Knock Out rose plant.
--Steven Arthurs Photo, University of Florida Institute of Food and Agricultural Sciences
HAMMOND — As cooler fall temperatures arrive, LSU AgCenter horticulturists are observing more cases of chilli thrips damage in nursery and landscape plants.
This thrips, Scirtothrips dorsalis Hood, is a recent invader in the Southeast, said AgCenter horticulturist Yan Chen. It was first detected in Hawaii in 1987 and became established in Louisiana 2008.
Most damage is seen on roses, including the Knock Out variety and most shrub or hybrid tea roses, Chen said. In addition, they have been found to attack shrubs such as ligustrum, cleyera, Indian hawthorn, duranta, viburnum and camellia and herbaceous plants such as begonia, coleus, snapdragon, zinnia, coreopsis and verbena.
This thrips feeds on young leaves, stem terminals and developing flowers buds and fruits with rasping-sucking mouthpart, she said. Feeding causes bronzed, curled and distorted leaves, which may look like herbicide burn or leaf rust. A severe infestation will defoliate or stunt the plant.
Finding chilli thrips is difficult because both young and adult thrips are very small, only 0.016 to 0.024 inch long and superficially resemble some other thrips species.
“However, we have observed that chilli thrips have low and high developmental thresholds of about 51 and 91 degrees,” Chen said. The most damaging populations are present during two peak times of the year – in mid-March to late May and late August to mid-October – when temperatures are favorable for them.
A team of scientists from the LSU AgCenter and the University of Florida Institute of Food and Agricultural Sciences have developed integrated pest management recommendations to better manage chilli thrips.
Detecting pests and treating the plants before a high population builds up is the key, Chen said. Early injury symptoms, leaf curl and distortion need to be monitored weekly during peak seasons. Tapping leaf terminals over a sheet of white paper will dislodge thrips so they can be examined with a hand lens.
In nurseries, thrips can be monitored with sticky cards placed near susceptible plants.
Insecticides containing abamectin, acephate, chlorfenapyr, flonicamid, imidacloprid, spinetoram, spiromesifen and spinosad can help control chilli thrips. Rotation among different classes and modes of actions is recommended to reduce the risks of insecticide resistance in chilli thrips and outbreaks of secondary pests, Chen said.
Experts don’t recommend using pyrethroids, organophosphates or other broad-spectrum insecticides in the landscape because of their negative effects on beneficial species. These include minute pirate bugs, lacewings and some predatory mites that help prevent outbreaks of thrips and other pests.
“However, natural enemies alone may not provide satisfactory control on plants such as roses that are preferred by thrips,” Chen said.
Greenhouses and nurseries have available many products that aren’t labeled for homeowner use, she said. They include a bioinsecticide Met52 EC, a biopesticide BotaniGard 22WP or ES and a botanical insecticide azadirachtin. Used in rotation with spinosad, these products and horticultural oils reduced chilli thrips populations by 88 percent to 95 percent.
These products are considered low-impact and “soft” on natural enemies and beneficial arthropods, she said.
“We also found that high nitrogen and phosphorus contents in plant leaves contribute to higher number of chilli thrips on Knock Out roses,” Chen said. She recommends applying fertilizer as split-applications at lower rates to avoid promoting chilli thrips populations.
In the landscape, it is important to reduce weeds that harbor thrips, she said. And at the beginning of a local outbreak, severely infested branches should be removed and bagged for disposal.
More information is available from Chen at LSU AgCenter Hammond Research Station by telephone at (985) 543-4125 or by email at email@example.com.
--By RICK BOGREN