Tiny thrips are well-known in tomato, onion and tobacco fields, especially by Georgia growers. It’s like a hate-hate relationship. At least, that is what the grower may think because thrips can cause so much damage when it comes to disease devastation. But first, let’s look closer at what thrips do and how they operate.
Rajagopalbabu Srinivasan, an entomologist with the University of Georgia, says thrips transmit tomato spotted wilt virus (TSWV). There are more than 7,000 different species of thrips, he adds, but only nine of them transmit TSWV. And they transmit the virus quickly, according to Srinivasan, even before insecticides can control them. Spread early in the growing season, the virus can eventually affect yield and losses for growers.
“Once thrips acquire the virus, it remains infected for the rest of its life,” he says. “Also, the virus multiplies inside of thrips.”
Srinivasan says thrips overwinter on weeds, and those weeds can serve as hosts for TSWV. He adds that volunteer peanuts in a field are an excellent host for TSWV.
For this article, the western flower thrips (Frankliniella occidentalis) and tobacco thrips (Frankliniella fusca) are of great concern to produce growers. On tomatoes, growers are well aware of damaging western flower thrips and tobacco thrips because they can transmit TSWV, said David Riley, an entomologist in the Department of Entomology and the coordinator of the Master of Plant Protection and Pest Management masters degree program (MPPPM) at the University of Georgia on the Tifton, Georgia, campus. Various weather conditions can increase the disease pressure, he says. For instance, warm and dry conditions during the springtime may cause thrips to arrive earlier than usual and reproduce faster. Warm and dry conditions may even cause TSWV to intensify. On the other hand, rain may prevent immature thrips from maturing.
In Vidalia onions, he says tobacco thrips and onion thrips are the main species that can cause direct feeding damage. In turn, that feeding reduces bulb size in warm and dry springs, which acts like a breeding ground for thrips populations.
Growers can easily spot thrips
Riley says growers can easily identify thrips in tomatoes by inspecting flowers and in onions by directly inspecting the plant, “but yellow sticky traps can also help to detect when thrips are first moving into the field from surrounding weeds.”
According to entomologists at Cornell University, the TSWV virus is picked up by the larval stage, yet adult thrips usually transmit the disease. Larvae acquire TSWV within 15 minutes; however, they cannot transmit the virus right away. Depending on the species of thrips, larvae may transmit the virus within a 3- to 10-day period and continue doing that for 22 to 30 days or for the rest of their lives. However, the Cornell University entomologists indicate that adult thrips don’t transmit TSWV to their progeny.
TSWV can cause severe symptoms in susceptible tomato varieties, Riley said, especially the heirloom ones.
Early control essential but tricky
Early control is important with thrips but can be difficult to achieve. For instance, western flower thrips are highly tolerant to insecticides, according to entomologists at the University of Georgia. A large part of that reason is because these small insects like to hide within plant blossoms where it is hard for growers to accurately apply insecticides. Additionally, some insecticides can even cancel out natural predators of thrips, leaving the insects with plenty of options and time to spread disease. When they have this time and arrive in large numbers, they can blemish fruit, and that can hurt sales in the marketplace whether that is on the farm, at a farmers market or marketed directly to the grocer. Great numbers of tobacco thrips may stunt plants and delay maturity.
Riley says insecticides are not always the answer for control because they may not act fast enough to reduce thrips from transmitting the virus. Of course, if the insecticides can disrupt thrips’ feeding, then control is possible.
Controlling TSWV will require several management practices. “In onions, we recommend using spinetoram or cyantraniliprole when thrips reach five thrips per plant,” Riley says. “In tomato, you have to use preventative treatments of neonics or cyantraniliprole before and just after transplant to reduce virus transmission, but the main recommendation in tomato is to use TSWV-resistant tomato varieties because the virus is the main threat to fruit production.”
However, according to the University of Georgia, these resistant varieties seem to have little effect on thrips numbers, and the resistance may break down strains of the virus. For the latter reason, entomologists recommend that growers strongly look closely at different and perhaps newer varieties coming on the market in the following year.
In addition, University of Georgia entomologists suggest various controls including applying reflective mulch, systemic-acquired resistance, insecticidal sprays and weed management.
Riley indicates that since thrips are attracted to weed hosts, thrips can arrive each year, so growers will need to practice strong management control practices yearly.
Rocky Womack has written about agriculture and business for more than 25 years and currently serves as a contributing writer and correspondent for agriculture and business magazines, domestically and internationally.
COVER AND PHOTOS BY DAVID RILEY, UNIVERSITY OF GEORGIA.