How to Kill Thrips

Thrips are tiny sap-sucking insects around 1/16 inch in length, with long, narrow bodies and fringed wings. Unfortunately, due to their small size, you might not realize there's a problem until significant plant damage has occurred. These pests suck juices from leaves and petals, and can transmit viruses that further weaken the plant. Insecticides don’t always work because it's difficult to apply them to the protected areas where thrips feed. However, continuing treatment using the proper products can help you in your battle to control and kill thrips.

Symptoms of Thrips

Thrips target many evergreen and flowering plants, but one of their favorites is roses (Rosa spp., U.S. Department of Agriculture plant hardiness zones 2 through 11, depending on the variety). Symptoms of a thrips infestation on a rosebush include distorted buds that don't open properly and brown edges or brown and silver-white streaks on the petals. Rose thrips include Western flower thrips (Frankliniella occidentalis) and flower thrips (Frankliniella tritici), both of which are brown or yellowish in color. You can detect the insects by shaking or disturbing the plant. If it's infested with thrips, you'll see the tiny insects moving around.

It's best to get a handle on thrips before the infestation becomes too large. Since the thrips life cycle is short, usually 10 to 15 days, and they lay anywhere from 40 to 250 eggs, populations rise rapidly; thus, quick action is necessary.

Safety First: General Guidelines

• Wear protective clothing, such as a long-sleeved shirt and pants, gloves and safety glasses. Following application, remove your clothes and wash them, in addition to washing your hands with soap and water.
• Carefully read and follow the directions on the product label.
• Secure the products in a lockable area out of the reach of children and pets.

Insecticidal Sprays to Control Thrips

Bifenthrin

One insecticidal product that provides good control and is available in ready-to-use formulas is bifenthrin. Once applied, it starts working immediately, protecting plants from future infestations for up to three months.

1. Shake the bottle vigorously to mix the product before using. Attach the bottle to the end of a garden hose, making sure the activation switch on the bottle is in the “off” position before turning on the water.
2. Turn on the water, point the spray nozzle toward the infected plant and set the switch to “on.” Spray the product uniformly over the infected plant to cover all surfaces, as well as the surrounding soil. 
   
3. After the plant is treated, slide the switch to the “off” position and turn off the water. Relieve water pressure by switching the bottle to the “water” position and allowing the water to run from the spray nozzle until it drips. Switch the bottle back to the “off” position and disconnect it from the hose. 
   
4. Repeat the treatment every seven days for three to four weeks. 

Cyfluthrin

Because thrips build up resistance to insecticides, it’s necessary to change from one product to another for better control. According to Cornell University, you should switch to another insecticide three to four weeks after using the first. Cyfluthrin shows success with thrips control and continues working for up to one month, killing on contact. It comes in ready-to-use formulas, so no mixing is required. Don’t use this product on plants that are flowering, on edibles or where bees are foraging.

To apply ready-to-use cyfluthrin, shake the bottle to thoroughly mix the product. Then adjust the sprayer's nozzle to the correct spray pattern. Hold the bottle approximately 12 inches from the plant you're treating and spray the product evenly to cover all the plant's surfaces, including the undersides of its leaves. Spray the foliage until it's wet but not dripping and repeat the treatment every seven days for three to four weeks.

Treating the Soil

Annual soil applications of a systemic insecticide containing dinotefuran prevent thrips infestations for up to one year and may be used to treat ground or container plants. Water-soluble systemic insecticides are absorbed into a plant’s tissues, moving throughout its vascular system. Ingesting the treated sap kills the thrips. The benefit of these types of insecticides is that there's no spray drift to harm beneficial insects or pets. On the downside, because the product is water-soluble, it can easily pollute bodies of water, especially when it's used on sandy soils where runoff can occur or where groundwater is close to the soil's surface.

1. Water around the plant enough to moisten but not saturate the soil before applying dinotefuran granules. Rake any mulch from the area so the insecticide has better soil contact. 
   
2. Use 1/3 to 2/3 cup for each inch of the trunk’s diameter if applying to trees. If applying to shrubs, use 1/4 to 1/3 cup for each foot of height. 
   
3. Sprinkle the granules evenly over the soil within 18 inches of the trunk or base of the plant. 
   
4. Lightly water the treated soil to settle the dust and dissolve the granules.