How to Control Thrips on Roses

Healthy roses (Rosa spp.), hardy in U.S. Department of Agriculture plant hardiness zones 2 through 11, depending on the variety, often resist major thrips damage. Occasionally, however, the populations of these pests can become severe enough to cause major damage, or the thrips may spread viral diseases when they pierce stems, leaves and petals to feed. Common pest varieties include flower thrips (Frankliniella tritici) and western flower thrips (Frankliniella occidentalis). Promptly identify thrips and control them with a combination of cultural and chemical methods as necessary.

Most adult thrips measure 1/16 inch long, and their yellow-brown coloring and feathery wings are visible only through a magnifying glass. The larvae, responsible for most of the damage on roses, are even smaller and lack wings. Blowing lightly on a rose causes the thrips to move, which makes them easier to see. Thrips-affected rose plants may develop scars or deformed tissue growth, or their flowers may have silvery streaks or fail to bloom. Not all thrips are harmful. Predatory thrips, such as Franklinothrips orizabensis, feed on harmful thrips and don't require treatment. The larvae of these beneficial thrips usually are bright orange, and the adults are black with white patches. Control thrips only if they damage rose bushes.

A clean garden bed around rose bushes minimizes thrips. Keep your rose bed well-weeded, and mow the grass in surrounding landscapes regularly. Prune out infested rose blooms and stems along with spent flowers, which may harbor thrips. Wipe the shears you use with an alcohol-soaked rag after every cut so you don't spread the pests or diseases, and destroy the removed plant material after pruning. Rinsing roses' foliage with water during hot, dry weather also is helpful. Keeping the dust off the roses encourages predatory insects, including predatory wasps and mites, to move into the rose bushes and control pests. Avoiding heavy nitrogen fertilizer applications also minimizes thrips populations.

A fast-acting insecticide, such as insecticidal soap, that doesn't remain active on roses for long periods works best because it destroys thrips but doesn't put undue pressure on beneficial insects. Apply insecticidal soap before rose buds open; otherwise, the thrips will spread into the petals and become too difficult to control with an insecticide. Mix 2 ? tablespoons of an insecticidal soap concentrate with 1 gallon of water in a spray bottle, or follow the insecticidal soap label's recommended rate. The solution shouldn't be applied to plants when the temperature is above 90 degrees Fahrenheit or when they are in direct sunlight. Coat the affected rose bushes completely with the spray, drenching the foliage and flower buds. The soap may require two or three reapplications at four- to seven-day intervals to control the thrips completely.

Other insecticides aren't usually necessary for thrips control in a home garden, and they can harm beneficial insects that otherwise would control the thrips naturally. When rose bushes' thrips infestation and damage is severe enough to warrant insecticide usage, spinosad can be used. Although other insecticides can control thrips, they cause too much damage to beneficial insects so are best avoided. Because spinosad application amounts vary among brands, follow the directions on your spinosad product's label. One product directs users to mix 4 tablespoons of spinosad with 1 gallon of water. Thoroughly drench thrips-affected rose bushes with the solution before the buds open.