By Frank Giles
Chilli thrips have been a thorn in blueberry growers’ side for several years, but the pest is now growing an appetite for strawberries. With a host range of more than 200 plants, chilli thrips has plenty of options to sustain populations and spread to other areas.
In strawberries, the pest can strike early in the season and spread quickly. A single female can lay about 40 eggs in her lifespan. Eggs are laid individually, hatch in five to eight days and are too small to be seen with the naked eye. As populations build, various life stages will be present in the field.
The pest attacks both the strawberry foliage and fruit. Heavy feeding on leaves causes necrosis of leaf veins and petioles. Feeding on fruit causes bronzing and cracking.
Sriyanka Lahiri, an assistant professor of entomology and nematology with the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS), has been studying the pest and its impact on strawberries. Research and field observations show the pest can reduce yields by up to 80% if not managed properly. Management is difficult because the pest can build up resistance to chemical controls.
Hot Spots and Hosts
Field scouting is a critical component in the management of chilli thrips. Scouting primarily relies on looking for the leaf and fruit symptoms the pest causes. Leaf samples can also be collected to count the pests, but enough leaf samples should be collected across the field to generate a representative number of the pest.
Lahiri also says it is important to know where chilli thrips are residing and what other host plants are attractive to the pest. She recently published a study that showed the movement of the pest.
The research showed that chilli thrips tend to cluster in hot spots along the margins of fields. The study noted: “Such hot spots occurred mainly along the borders of strawberry fields that were adjacent to natural areas containing potential alternative hosts for thrips and were consistent in their appearance at similar locations within the fields during both years of the study. While we observed the ingress of chilli thrips into field interiors over the course of each season, in general, populations remained much higher on plot borders than interiors.”
Alternative Hosts
The UF/IFAS team conducting the study also looked at alternative hosts that chilli thrips prefer. That’s important because many strawberry fields are small and are surrounded by wooded areas where potential alternative hosts can be abundant. Some of those host plants include common species like laurel oak, water oak, ragweed, sweet gum, grape and laurel cherry. The pest will migrate into these hosts during the off-season and summer months.
Another important observation from the study was that the chilli thrips hot spots in strawberry field borders were observed more frequently in areas adjacent to wooded areas when compared to residential areas or along main roadways.
Lahiri suggests this knowledge could help growers develop more targeted applications of chemicals to manage the pest.
“Our findings are important to growers as they can now save money and time by having to spray a lower volume of insecticides in smaller portions of their field,” Lahiri says. “They can protect the beneficial insects in and around their field by doing this, which in turn will assist with maintaining more healthy strawberry plants.”
Specifically, growers should spray no closer than about 330 feet of their field border. They should leave the rest of the field either untreated or manage it by using biological control agents, botanicals and flowering plants. Hot spots and the presence of the pest migrating further into the interior of the field should be confirmed by scouting.
Control Measures
Growers mostly manage chilli thrips with pesticides. Coverage can be difficult because the female lays eggs inside leaf tissue where they are protected from sprays. In addition, adults stay concealed in leaf curls or under the calyxes, which makes coverage difficult to achieve. It also is recommended that weeds are managed in the field and along field edges since they can be a refuge for the pest.
Spinetoram, cyantraniliprole and acetamiprid are among the materials that can suppress the pest. The UF/IFAS Florida Vegetable Production Handbook (see is.gd/strawberrypests online) has more information on chemical control options.
Given the difficulty managing the pest with chemical controls, research has looked at biological options as well. Minute pirate bugs and foliar-dwelling entomopathogenic nematodes [Thripinema spp. (Tylenchida: Allantonematidae)] have been found to be effective under controlled conditions and can be integrated into field programs.