Western flower thrips (Frankliniella occidentalis) are among the most damaging greenhouse pests especially on strawberries. Adult thrips are slender insects about 1 mm long. Thrips feed by piercing the cells and sucking out their contents, leading to silver/gray patches. Thrips especially feed on growing points and flower buds, leading to distortion of these tissues as they develop. Greenhouse strawberry crops are sensitive to thrips damage which can cause severe plant damage and unmarketable fruit. This article will introduce the issue of thrips in strawberries, describe common symptomology, present an initial framework for integrated pest management and conclude with some additional resources.
In the United States, leafy greens, microgreens and other fast turnaround crops have historically been popular among controlled environment agriculture (CEA) growers. Recently, CEA companies are beginning to consider strawberries as another crop because it is also a highly perishable and higher value crop.
Unlike leafy green and herb production that often take less than two months to complete a full growing cycle, strawberry plants are frequently grown for a longer duration of time. Consequently, the longer a plant is placed in a CEA operation, the risk of a pest infestation generally increases.
Regardless of the pest, a comprehensive pest management strategy must be considered before starting an indoor strawberry operation. Preventing a pest infestation is easier than combating one that has already occurred. CEA growers should be trained to accurately identify insects, symptoms and scout/document all pest populations on a weekly basis to ensure pests are closely monitored and managed below an economically injurious level. Quick interventions are needed if thrips are identified because they can rapidly multiply, especially when a steady source of pollen is available from the flowers.
Symptoms of thrips: Western flower thrips themselves can be spotted with the naked eye if you look closely. Adults are long/slender insects about 1mm long, with color varying from yellow to dark brown (females) or pale yellow (males) (Fig. 1). Even easier to spot than thrips themselves are the damage they do to strawberry flowers and fruits. When thrips feed on flower petals, they can cause distorted and slightly discolored petals (Figs. 2 and 3). The flower petals may appear slightly discolored, and thrips can be quickly spotted crawling around the flower when agitated by blowing air directly onto the flower from one’s mouth. The fruits appear seedy and have dull bronze color (rather than shiny red color). Severely damaged fruit typically will not have the red glossy color (Figs. 4 and 5). Overall, thrips feeding damage can result in a large number of unmarketable fruits.Solutions:
It is imperative for CEA strawberry operations to incorporate a proactive and comprehensive integrated pest management (IPM) strategy. An effective IPM program typically includes a combination of biological, chemical, physical and cultural control methods. An effective IPM strategy depends on numerous factors and a strategy that works in one CEA operation may not work as effectively in a different CEA operation. Therefore, consulting with an entomologist at your local cooperative extension office that has expertise with CEA pest management is advised because they can recommend an IPM strategy that is tailored to your specific circumstances. With that said, here are a few strategies to help you begin your IPM plan.
Some general considerations to keep in mind when planning an IPM strategy for thrips.Physical – Mechanical Control:
Insect exclusion screens can reduce the number of thrips that enter a CEA operation. Blue or yellow sticky cards may be used to monitor thrips populations.Organically accepted chemical sprays:
Because strawberries are an edible crop, many common pesticides used for thrips in ornamental crops cannot be used. For strawberries, a combination of azadirachtin (ex. Neemix) and Beauveria bassiana (ex. Botanigard) may be applied every five to six days to help keep thrip populations under control. Always check the product label to determine if a chemical spray can be used for your crop in your state. Be sure to note compatibility of the product with any biological controls you plan to use and pay attention to the preharvest interval (PHI), which is the minimum time between when a pesticide is applied and the crop can be harvested.
Biological control: Several biological predators which target western flower thrips are available, including the predatory mites: Amblyseius swirskii, Amblyseius cucumeris and Amblydromalus limonicus. These predatory mites should be proactively deployed to prevent thrip infestations from occurring in the first place. These products can come in the form of sachets or bran. The sachets are a slow-release, longer-lasting form that can be placed in each pot, and the bran form can be scattered to distribute the predators it contains more uniformly across a crop. Many biological control companies have technical support that can recommend a specific protocol and product that is tailored to one’s CEA operation.
Cultural control: Inspect all new plant material entering a facility and be sure it is pest free before introducing it. Wearing personal protection equipment such as a clean lab coat or Tyvek suit is one tactic that may be used to reduce the risk of carrying in an invasive pest from outside the CEA operation. Additionally, planning your workflow for the day, such as never visiting a clean greenhouse after visiting a thrips-ridden greenhouse, is another strategy to reduce the risk of carrying in an invasive pest.