Western Flower Thrips Management on Greenhouse-Grown Crops Greenhouse producers worldwide are familiar with the Eggs hatch in two to four days. Nymphs feed on both western flower thrips, Frankliniella occidentalis (Pergande), leaves and flowers. The first nymphal stage lasts one to one of the most destructive insect pests of greenhouse- two days; the second nymphal stage, two to four days. grown crops. Western flower thrips, the primary thrips Second instar nymphs are typically more active and tend species encountered by greenhouse producers, is extremely to feed more than first instar nymphs. The second instar polyphagous, feeding on a wide-variety of horticultural nymph eventually migrates to the plant base and enters crops grown in both commercial and research greenhouses. the growing medium to pupate. Western flower thrips also This insect pest has been included in greenhouse pest pupate in leaf debris, on the plant, and in the open flowers control brochures since 1949. It was not considered a of certain types of plants including chrysanthemum. There major insect pest of greenhouse-grown crops until the are actually two “pupal” stages: a prepupa (or propupa) and 1980s. This publication addresses biology and damage; pupa. Both stages commonly occur in growing medium or scouting; and cultural, physical, insecticidal, and biological soil underneath benches. management. The issues discussed should provide insight Growing medium or soil type and pH and pupation depth on the importance of dealing with western flower thrips may influence pupal survival. Pupation depth depends on holistically instead of solely relying on insecticides. growing medium or soil type. Pupae stages do not feed Biology and Feeding Damage and are tolerant or immune to most insecticides commonly Knowledge of biology and damage is important in applied to manage western flower thrips nymphs and understanding the challenges associated with developing adults. a sound pest management Adults emerge from the pupal stage after approximately program. Western flower thrips six days. Although adults have wings, they do not fly well, are small (approximately 2.0 mm but may be dispersed throughout a greenhouse via air in length) insects that possess currents created by horizontal airflow fans or wind entering piercing-sucking mouthparts from outside. Adults are attracted to certain flower (Figure 1). The life cycle consists colors (yellow, blue, and white), plant volatiles (E-ß- of an egg stage, two nymphal stages, two pupal stages, and an Figure 1 Figure 2. Western flower thrips life cycle adult (Figure 2). In general, the life cycle (egg to adult) takes two to three Adult weeks to complete. However, time from egg to adult depends on temperature, with the optimum range between Egg 26 and 29°C (79 and 84°F). Under these conditions, the Pupae life cycle may be completed in seven to 13 days. Females can live up to 45 days and lay (oviposit) between 150 and 300 eggs during a lifetime. Females primarily feed on flower pollen, which may contain nutrients such as First Instar carbohydrates, proteins, sterols, and vitamins that enhance development rate and reproductive ability. Western flower thrips may attack plants with elevated concentrations of nitrogen because of the abundance of amino acids and proteins, and female egg production is Prepupae Second Instar higher after feeding on plants containing abundant levels of amino acids. Females typically lay eggs underneath the epidermal layer of the leaf surface or in flower tissues, which protects them from exposure to contact insecticides. Kansas State University Agricultural Experiment Station and Cooperative Extension Service farnesene), and flowering plant types (chrysanthemum, instar nymphs gerbera, marigold, and rose). Western flower thrips exhibit acquire the virus, thigmotactic behavior, meaning the body needs to be in which is then constant contact with a surface, which is why they are transmitted by located in secluded habitats on plants. This also protects adults. Both them from exposure to contact insecticides. direct and indirect Western flower thrips have a haplo-diploid breeding damage may result system, which means females develop from fertilized in an economic loss to greenhouse eggs and males develop from unfertilized eggs. Unmated Figure 4 females can produce males (sons) parthenogenetically producers. (without mating) whereas females must be mated in order Distribution of to produce additional females (daughters). Female western infested plant flower thrips also may mate with their own offspring. The material is one sex ratio (females to males) is dependent on the population of the primary density, with males tending to be more prevalent at “low” means of long- population densities and females typically more abundant distance spread of at higher densities. Increasing population densities this pest. Western of western flower thrips in greenhouses enhances the flower thrips are probability of females encountering and mating with males difficult to manage immediately after emerging from the pupal stage. High in greenhouses Figure 5 population densities create an age structure consisting for a number of young, fecund females producing a predominance of of reasons including broad host range, high female daughters. But as adult females age, they tend to produce reproductive capacity, rapid life cycle (egg to adult), small more males. size (approximately 2.0 mm long), unusual feeding habits, Western flower thrips cause direct damage by feeding on preference for secluded habitats (unexpanded leaves and plant leaves and flowers. Western flower thrips possess unopened flower buds), and resistance to insecticides. As piercing-sucking mouthparts, but they do not feed such, the only way to effectively deal with western flower exclusively in the phloem sieve tubes. Instead, they tend to thrips in greenhouse production systems is by using a feed on the mesophyll and epidermal cells of leaf tissues holistic approach, implementing a variety of strategies using a single stylet in the mouth to puncture cells, and including scouting and cultural, physical, insecticidal, and then insert a set of paired stylets, which lacerate and biological management. damage cell tissues and function to imbibe cellular fluids. Scouting As a result, western flower thrips feed on a multitude of Scouting or monitoring is important to determine the food types within plants. Symptoms of feeding include numbers of thrips present in the greenhouse. Additionally, leaf scarring, distorted growth, sunken tissues on leaf scouting will detect seasonal trends in populations undersides (Figure 3), and deformed flowers (Figure 4). throughout the year and assess the effectiveness of Flowers and leaves have management strategies implemented. The main technique a characteristic “silvery” used to scout for western flower thrips adults is to place appearance due to the either blue or yellow sticky cards above the crop canopy influx of air after the (Figures 6 and 7). Cards are counted weekly and numbers removal of plant fluids of adults are recorded. Visual inspection such as looking (Figure 5). Black fecal into open flowers, and/or shaking open flowers over a deposits may be present white sheet of paper are additional methods that may be on leaf undersides. used to scout for nymphs and adults. Gently blowing into Damage to plant leaves open flowers will agitate western flower thrips and increase may also occur when movement making them easier to detect. But a relationship females, using their sharp Figure 3 between numbers of western flower thrips captured on ovipositor, insert eggs colored sticky cards and the abundance present in flowers into plant tissue. And has not been established. wounds created during feeding or oviposition may serve as entry sites for plant pathogens such as fungi. Greenhouse producers can establish action thresholds — the number of thrips detected either on colored sticky Western flower thrips also cause indirect damage by cards or visually — that warrant implementation of a pest vectoring the tospoviruses: tomato spotted wilt virus and management strategy. impatiens necrotic spot wilt virus. The first and second 2 In a two-year with the thrips possibly moving from weeds and/or greenhouse study, field-grown crops (e.g., corn and soybean) and vegetables an action threshold into greenhouses. It will not be effective if doors are of 20 western flower continuously left open or infested plant material is moved thrips adults per among greenhouses. blue sticky card Alternative cultural and/or physical management strategies per week in a cut that may be implemented include overhead irrigation or carnation (Dianthus misting, which has been shown to decrease the abundance caryophyllus) crop of western flower thrips populations, by creating an Figure 6 was established environment less favorable for development; use of to determine the ultraviolet (UV) absorbing plastic films, which appear to need for insecticide influence adult flight behavior by reducing the levels of applications. Western UV light entering greenhouses or aluminized reflective flower thrips numbers fabrics that may inhibit or repel adults from entering — based on blue greenhouses; mechanical brushing of plants, which has sticky card counts been demonstrated to reduce western flower thrips damage from December in greenhouse-grown vegetables; leaving greenhouses through March 1994 fallow (empty) for several months and heating for four to and 1995 — were five days at 30°C