Whether you grow vegetables or ornamentals in the greenhouse, Western flower thrips (Frankliniella occidentalis) are problematic. These pests cause visible damage to a wide range of crops. They also transmit viruses, vectoring diseases including significant ones which can plague greenhouse crops: impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV).
Thrips can overwinter in the greenhouse, causing a dramatic increase in population during the next growing season. They are also often introduced in new plant material. Western flower thrips are now found across the nation and are a common greenhouse pest in other parts of the world. These pests, native to the western areas of the U.S., are pests of outdoor crops too.
It’s already known that ultraviolet-C (UV-C) light waves can kill bacteria, viruses and fungi. They can kill some arthropods, including aphids, whiteflies and thrips. Irradiating plants without causing lasting damage, while significantly impacting the pest population, is the next step is effectively using UV-C as an IPM tool for Western flower thrips control.
Control with Ultraviolet Light
Researchers in the Entomology Research Lab at the University of Vermont have been studying the effectiveness of UV-C to manage Western flower thrips in greenhouse settings. For their experiments, they’ve used marigold and calibrachoa plants to determine whether UV-C light can impact that thrips populations without significantly damaging the plants.
Richard Benford and Somaiyeh Ghasemzadeh shared their research data during a recent Rodale Institute webinar, which can be viewed at rodaleinstitute.org/education/webinars/archive.
Benford explained that researchers developed and used three different systems for exposing the plants to UV-C light in varying doses. Using mercury vapor lamps, the UV-C was delivered using each system after determining the dosing which would not cause significant harm to the plants. The timing of the UV-C dose, in conjunction with the energy level of the UV-C light wave, determines the dose administered.
“Studies have proven that you can effectively suppress Western flower thrips at different stages of growth with UV-C dosing that has no or minimal lasting effect on plants,” he said. “The idea is to try to get the light to the underside of leaves.”
In the first system, the UV-C light came from below the plant, with the lamps angled to reach the underside of the leaves, which is where Western flower thrips hide and primarily live and reproduce. The second system utilized larger lamps and added reflective coatings in order to treat more parts of the plant. This second system, known as the “dome,” could be scaled up and automated for use as new flats are brought into the greenhouse, effectively irradiating the plants and preventing infestation, perhaps as plants pass by on a conveyor belt, Benford said.
The third system is a “comb” system in which UV-C lamps are arranged in a panel and are “intended to irradiate plants which are already in a greenhouse,” with panels passing down the greenhouse rows and irradiating the plant, including the undersides of leaves.
Decreasing Pest Populations
Western flower thrips are “the most prevalent species attacking greenhouse crops” in the U.S., as well as in other countries, Ghasemzadeh said, and they are “a serious pest to agricultural crops and ornamental plants.”
Understanding the lifecycle of the pest, as well as determining when it is most vulnerable to UV-C irradiation, was an important component of the research. The Western flower thrips has an adult stage, an egg stage, two larval and two pupal stages.
During the pupal stages, the pest hides in the container substrate or plant debris. Researchers have not studied the effect of UV-C on this stage of the lifecycle. All other stages occur primarily on the underside of plant leaves, and the effects of UV-C treatments vary depending on the stage of development. All stages of the pest will be present simultaneously once a population is established.
Both adults and larvae suck on plant cells in the leaves, flowers and fruit. They cause visible damage to plants and also vector viruses. Adults are highly mobile but tend to hide on the underside of leaves. They are less susceptible to being eliminated with UV-C. However, the UV-C does impact their ability to successfully reproduce.
By reducing adult fecundity, UV-C causes a “cascading effect of the thrips population,” Ghasemzadeh said, “by slowing the population growth over time.”
Eggs are laid by adults and inserted into plant tissue, with 90% being on the underside of leaves. Both the eggs and the two larval stages, also found on the leaf undersides, are susceptible to UV-C. When UV-C can be directly targeted to the underside of leaves, where the most susceptible developmental stages – which are also not actively mobile as are the adults – are found, control of the Western flower thrips, without plant damage, can occur.
Targeting the eggs and larvae “prevents infestation from building up and spreading across the crops,” Ghasemzadeh said.
The challenge researchers now need to overcome is how to best target the underside of the leaves with the UV-C. That is the key to effectively utilizing this method as a pest management tool.
“We need to deliver the UV-C light precisely where the thrips are active and laying,” Ghasemzadeh said.
Utilizing UV-C as an alternative to pesticides can help greenhouse growers protect their crops in an environmentally-friendly manner.
