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Western Flower Thrips

May 11, 2012

Growers commonly observe western flower thrips (Frankiniella occidentalis) feeding on their annuals, perennials, potted crops, and woody ornamentals throughout the year. They can be problematic in crops grown inside greenhouse structures and at outside facilities.  However, they are generally more prevalent during the warm spring and summer months of the growing season.

Western flower thrips cause injury to crops and transmit plant viruses which can reduce the appearance and quality characteristics of plants and may render them unsalable in many circumstances.  Thrips are problematic for growers since they can infest a wide range of plant species, their populations can increase very quickly, and they are often difficult to control.

Characteristics

Thrips are very tiny and can be difficult to see unless examining the plants closely. They are small, slender straw yellow to brown insects measuring only 1/8-inch long (1-2 millimeters). The immature thrips or nymphs look very similar to the fringed-winged adults, but they are slightly more slender and do not have wings. Plant injury from feeding is often the first indication that thrips are present.

Western flower thrips feed by piercing plant cells with their mouthparts and sucking out the sap, which causes the plant cells to collapse and the plant to appear.  The upper surfaces of damaged leaves often appear scarred, deformed, and distorted with silvery-white trails.  The youngest, most tender growth often looks distorted or irregularly shaped.  If flowers are present, they often appear deformed, flecked, mottled, or streaked.

In addition to the feeding injury they cause, western flower thrips are capable of vectoring tospoviruses including two destructive viral pathogens: impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV).

Detecting Thrips

Thrips are generally distributed evenly throughout a crop rather than in isolated areas.  When scouting, observe the upper leaf surfaces of the plant paying particular attention to the terminal growth or flowering parts, if applicable.  Look for stippling, feeding scars, and distorted growth.  When flowers are present, look for small brown spots on newly developing buds or with more mature blooms, the injury often appears as dark spots on the top of the flower petals or the flowers do not open normally (distorted).

Adult thrips can easily be caught on yellow or blue sticky cards.  Observing the number of thrips on sticky cards can provide an indication of the number of thrips present and may be useful when determining if a control strategy is necessary or if past control measures were effective.  

Controlling Thrips

Controlling thrips can be rather challenging, due to their small size and their ability to hide in tight inaccessible parts of the plant where direct contact of insecticides can often be impossible.  Adding to the difficulty is their high reproductive capacity, rapid life cycle (7 to 13 days from egg to adult at average greenhouse temperatures), and resistance to several insecticides.  

Western flower thrips can be controlled preventatively using biological strategies.  Products containing the entomopathogenic (beneficial) fungus Beauveria bassiana or Paecifmomyces fumosoroseus provide acceptable control when they are used routinely.  Predatory mites (Amblyseius cucumeris, A. swirskii), beneficial nematodes (Steinernema feltiae), and the predatory bug Orius insidiosus have also provided growers with effective control.

There are many insecticides labeled for controlling thrips; the most effective products are those containing the active ingredients abamectin, acephate, chlorfenapyr, chlorpyrifos, cyfluthrin, horticultural oil, insecticidal soap, methiocarb, pyrethrins, and spinosad.  

Due to their tendency to hide in cracks and crevices of leaves, flowers, and the growing points of the plant, it is usually necessary to apply effective insecticides repeatedly to achieve satisfactory control of western flower thrips.  None of the commercially available products will control thrips populations with a single application.  Several applications should occur at 5-7 day intervals to significantly reduce thrips populations to acceptable levels.  Remember to always rotate chemical families to reduce the likelihood of the thrips building resistance to any one class of chemicals.  

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

The mention of specific active ingredients does not constitute an endorsement or recommendation of, or discrimination against similar products not mentioned. ALWAYS READ PRODUCT LABELS AND USE THEM AS DIRECTED ON THE LABEL.

 

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