Linda Benedict, Ring, Dennis R.
Thrips are insects belonging to the order Thysanoptera, meaning fringe-winged insects. One common name is thunderflies because large numbers migrate before thunderstorms. Thrips is a unique word because it is both singular and plural. These tiny insects are elongate, cylindrical and 1/25 to 1/8 of an inch in length. The nymphs are pale-yellow and highly active. The adults are usually black or yellow-brown and may have red, black or white markings. Thrips jump when disturbed. The adults may be winged or wingless. Wings when present are long, narrow, strap-like and fringed with hairs. There are about 264 species in the United States and Canada that feed on plants. Many other species are predaceous. Parthenogenesis (reproduction without mating) occurs in many species. Male thrips are usually smaller than the females.
The Chilli thrips, Scirtothrips dorsalis (Hood), is native to southern Asia and was first detected in the United States in 2005 on roses in Florida. In 2007, it was discovered on roses in Houston, and in 2009, it was found on Knock Out roses in New Orleans. This thrips feeds on more than 150 species of plants belonging to more than 40 plant families. Economic losses from this introduced pest have been estimated to be from $3 billion to $6 billion annually in the United States. Chilli thrips are predicted to expand their range to parts of the United States where temperatures do not reach or go below 25 degrees Fahrenheit for five consecutive days each year. Twelve to 16 generations per year are expected in Louisiana.
The life cycle from egg to adult is completed in 12 to 22 days depending on the plant species attacked and the temperature. Eggs are laid in the tissue of plants near floral structures, leaf veins and terminal plant parts. Larvae (nymphs) emerge from eggs in six to eight days when temperatures are optimal. They feed on tender young plant parts. The next stage is the prepupal stage, and it lasts up to one day. The thrips then pupates on the plant or in the soil near the base of the plant. This stage lasts two to three days. Females lay from 60 to 200 eggs during their life. Adults are pale and have dark wings. Young Chilli thrips are pale. These thrips feed on several plant tissues. Feeding causes bronzing, curled leaves, distorted leaves, leaf drop, dwarfed and stunted plants, and bud shed.
To aid in detecting thrips, place a sheet of white typing paper beneath the leaves or flowers and shake the plant. The thrips will fall onto the paper and can be more easily observed and identified than when on the plant. Also look for the small spots of varnish-like excrement on the leaves. Use a 10X to 15X hand lens. Sticky traps, both blue and yellow, can aid in monitoring thrips. Blue sticky traps appear somewhat more effective than yellow. Both of these traps can be purchased through online horticultural sources.
Management programs for this insect are in the process of being developed. Foliar applications of acephate, imidacloprid or spinosad have been effective. Pyrethroids have been less effective. The thrips are on new foliage, and this foliage must be treated. Conserving natural enemies and rotating classes of insecticides is recommended.
One of the main concerns in Louisiana about Chilli thrips is the threat to Knock Out roses, which have few pests attacking them. Chilli thrips may cause severe injury to these roses, which are prevalent in the state. Thus, sampling for the Chilli thrips on Knock Out roses is needed. Older rose varieties require frequent applications of insecticides, and this spraying will manage the Chilli thrips.
Dennis Ring, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.
(This article was published in the winter 2012 issue of Louisiana Agriculture magazine.)