Yan Chen, Steven Arthurs and Dennis Ring
Chilli thrips, Scirtothrips dorsalis, is native to south Asia and has become a worldwide pest in countries having horticulture commodities. It attacks more than 100 plant species, including chili peppers, tea, strawberries, tomatoes and many other vegetable, fruit and ornamental crops. In the U.S., it was first reported in Hawaii in 1987 and then established in Florida in 2005. It is now widespread throughout the Southeast.
In Louisiana, chilli thrips damage has been reported in landscapes or in production nurseries in peppers and many ornamental plants, such as Knock Out roses, cleyera, Indian hawthorn, duranta, ligustrum, viburnum, camellia and bottle brush, and herbaceous plants such as begonia, coleus, snapdragon, zinnia, coreopsis and verbena.
Chilli thrips feeds on stem terminals, young leaves and developing flower buds and fruits with a rasping-sucking mouthpart. This type of feeding causes bronzed, curled and distorted leaves, which may look like herbicide burn or leaf rust (Photo 1). Severe infestation can defoliate or slow plant growth.
Detecting and managing chilli thrips is difficult because they are very small, only 0.016 to 0.024 inch in length. Compared with the more common western flower thrips, chilli thrips are about one-quarter of the size of western flower thrips, with a more bulged abdomen (Photo 2). All thrips inhabit secluded areas on plants, such as unopened flower and terminal buds, which reduces exposure to insecticide applications. They also superficially resemble some other thrips species that, if mistaken, may result in poor control because the insecticides selected may not be efficient against chilli thrips.
Since 2013, a team of horticulturists and entomologists from the LSU AgCenter and the University of Florida has worked on management options to keep the chilli thrips at bay in both production nurseries and landscapes. The team started with monitoring this pest in gardens and nurseries and evaluated critical questions such as at what pest level insecticides are needed to prevent the thrips from causing crop damage and what biorational pesticides that are “soft” on beneficial arthropods can be used to reduce the use of conventional insecticides.
Because chilli thrips prefers to feed on tender plant tissue, more infestation cases in landscapes, especially with roses, have been reported in mid-May and in September to early October. These are the time periods many landscape plants will have their new growth. In production nurseries, because plants are routinely pruned to promote branching and new growth, chilli thrips can be problematic throughout the growing season. Therefore, it is important for landscape managers or home gardeners to periodically check plants during the seasons when chilli thrips are active and for nursery growers to be familiar with early damaging symptoms of this pest.
Detecting pests and treating the plants before a high population builds up are key actions. Early injury symptoms – leaf curl and distortion – need to be monitored weekly. Tapping foliar terminals over a sheet of white paper will dislodge thrips that can be examined with a hand lens. In nurseries, thrips can be monitored with yellow or blue sticky cards situated next to the susceptible plants (Photo 3).
Because this thrips can overwinter as adults in leaf litter or weeds, cleaning up debris from infested plants and removing weeds are important activities to reduce the overwinter population. This is especially critical for nurseries that have had chilli thrips infestations in the past. At the beginning of a local outbreak, severely infested branches should be cut and bagged for disposal.
The research team also found that high nitrogen and phosphorus contents in plant leaves contribute to higher numbers of chilli thrips on Knock Out roses. Applying fertilizer lightly, such as a split application at the recommended rate, may avoid promoting chilli thrips reproduction.
The use of pyrethroids, organophosphates or other broad-spectrum insecticides is not recommended for controlling chilli thrips in landscape plants because of their potential effects on beneficial species, including minute pirate bugs (Orius spp.), lacewings and predatory mites or spiders that help prevent outbreaks of chilli thrips as well as other pests. However, natural enemies alone may not provide satisfactory control on plants preferred by chilli thrips.
AgCenter research showed that a rotation between spinosad and the biological control fungi Metarhizium brunneum (Met52) and Beauveria bassiana (BotaniGard 22WP), or insect growth regulator (azadirachtin, Molt-X), and horticultural oils (such as the ultra-fine oil or SuffOil-X) reduced chilli thrips populations by 88 percent to 95 percent. These products are considered “soft” on beneficial arthropods and are available to commercial landscape professionals. For home gardeners, insecticides containing spinosad, such as Conserve, can be rotated with ultra-fine oil or soapy water to treat infested plants during thrips active seasons.
For nursery growers, insecticides containing abamectin (Avid), acephate (Orthene), chlorfenapyr (Pylon, greenhouse only), flonicamid (Aria), imidacloprid (i.e., Marathon), spinetoram (XXpire), spiromesifen (JUDO) and spinosad (Conserve) can help control chilli thrips in production nurseries. Rotation among different classes and modes of actions is recommended to reduce the risks of developing insecticide resistance and the outbreaks of secondary pests.
Yan Chen is an associate professor at the Hammond Research Station in Hammond. Steven Arthurs is an assistant professor in the Entomology and nematology Department, University of Florida. Dennis ring is a professor and entomology extension specialist in the Department of Entomology.
Photo 1. Chilli thrips damage on Knock Out roses. Photo by Sheldon Johnson
Photo 2. Chilli thrips (lower left) is a tiny insect, which is about one-quarter the size of the western flower thrips. The latter is more commonly seen in garden plants and prefers pollen and nectar. Photo by Yan Chen
Photo 3. For production nurseries, sticky cards placed on sensitive hosts (such as cleyera) is an efficient way to detect this pest early and treat before damage occurs. Photo by Yan Chen