This month’s field scouting guide concentrates on thrips, including western flower thrips (WFT), which are contained in the Thysanoptera order.
We reached out to pathologists to learn how to spot and treat this destructive pest. This month, our contributors are Carla Burkle, Pennsylvania State Extension, and John C. Palumbo, University of Arizona.
The Basics on Thrips
Scientific name: There are many species of thrips in order Thysanoptera, including the Frankliniella occidentalis Pergande, or western flower thrips (WFT).
Geographical range: Species of thrips are all over the U.S., wherever commercial crops are grown.
Crops affected: Thrips impact many commercially grown ornamental and food crops. Onions, tomatoes, peppers, eggplant, cucumbers, melons, watermelons, peas, lettuce, spinach, and cabbage have all been preyed on by thrips populations.
Burkle: Thrips are a common problem in Pennsylvania, particularly onion thrips, WFT, and pear thrips. All life stages may be damaging as eggs are commonly laid inside plant tissue leaving a scar, but generally the larval and adult life stages are going to be the most damaging due to plant feeding behavior and the risk of transmitting tospoviruses to the plant.
The economic impact will depend on the species of thrips, the crop, and the density of thrips present at a vulnerable stage of the crop’s growth. Yield loss may be 50% or more in severe infestations. Economic impact also is affected if a thrips infestation transmits a tospovirus to the crop. If yield is not severely affected, there may still be a significant impact from management costs.
Palumbo: WFT is a major pest in winter produce crops grown in Yuma, AZ, Imperial Valley, CA, and Coachella Valley, CA, and of leafy vegetables, particularly lettuce and spinach. WFT has a great potential for causing cosmetic damage to head lettuce throughout the growing season, and particularly at harvest if not managed properly. Romaine and leaf lettuce types are at even higher risk, where thrips damage to harvestable leaves can result in excessive trimming and reduced plant weights due to both adult and immature feeding.
Adults and nymphs feed on both upper and lower leaf surfaces, in the leaf folds, and in protected inner leaves. Adults and larvae both cause feeding damage and contamination in lettuce and spinach. In rare cases, oviposition of eggs in developing watermelon fruit can cause some aesthetic problems. Feeding on the mid-ribs will cause scarring, leaf discoloration (bronzing), and distortion.
High populations of WFT can result in a significant number of cosmetic blemishes on marketable leaf portions. Baby spinach crops are susceptible to thrips damage on the young terminal growth because as leaves expand and elongate they become scarred and distorted. Such damage might turn into larger necrotic lesions in postharvest storage and transit.
The presence of live thrips can contaminate the harvested product. Thrips also can vector some plant viruses, including tomato spotted wilt virus. To date, no thrips-transmitted viruses have been found in economic levels on leafy vegetables in the desert.
Burkle: Resistance to several classes of pesticides is increasingly common among numerous pest thrips species. Integrated pest management practices may reduce the need for pesticides. Sanitation, exclusion, cultural controls, encouraging biological controls and natural enemies, and making effective pesticide applications will contribute to effective control.
Thrips management should not rely solely on pesticides. Foliar sprays of organophosphates (e.g., malathion), carbamates (e.g., carbaryl), and pyrethroids (e.g., bifenthrin and cyfluthrin) are not recommended since resistance is reported, they are highly toxic to beneficial insects and pollinators, and they may contribute to rebound outbreaks of other pests such as spider mites.
Labeled chemistries will vary depending on the crop, but in general, systemic pesticides applied with a penetrating surfactant at high enough spray volumes to provide good coverage are recommended.
Palumbo: Cultural management has only a limited impact on WFT populations, and natural enemies have not been shown to effectively control WFT in leafy vegetables, thus control with insecticides is often the only viable management alternative to prevent economic damage.
Crop Protection Considerations: Traditional Chemistries
Researchers Carla Burkle and John C. Palumbo advise growers to consider the following products.
Burkle: Contact insecticides such as azadirachtin, insecticidal soaps, horticultural oils, and neem oil may have some effectiveness against thrips if population densities are low. These products must contact the thrips insects to have any effect, so coverage must extend into protected plant parts such as flower and leaf buds where thrips may congregate.
Burkle: Monitor for thrips with yellow sticky cards or by gently shaking foliage and flowers onto white paper and tapping the contents into a vial of 70% rubbing alcohol. Even high populations detected through monitoring may not equate to severe crop damage, and control action may not be needed.
Thrips have many natural enemies, such as predatory thrips, green lacewing larvae, minute pirate bugs, predatory mites, and parasitic wasps. These natural enemies may be host-specific, so the presence of one does not necessarily mean they will provide adequate thrips control. Banker plants, strip harvesting, selecting reduced risk pesticides, and reducing the number of well-timed pesticidal sprays support the proliferation of natural enemy populations.
Avoid planting susceptible crops near alternative weedy thrips hosts. White or silver reflective mulches may be effective at protecting young, valuable crops. Reflective mulches are most effective at repelling certain insects when the crop canopy covers less than 50% of the soil in the row.
Palumbo: Direct observation of whole plants is the most accurate method of sampling for WFT. This involves careful examination of plant parts for the presence of WFT and feeding scars. Care should be taken to examine folds in leaf tissue near the base of the leaves for immatures. Experience and research has demonstrated that if three to five WFT are found on a small plant, there is probably 10 times as many hidden within folds in the leaves or that had dispersed from the plant.