Thrips are tiny insects that are difficult to see. They can devastate peonies, leaving unsightly leaves and buds that fail to open. Control includes a combination of cultural practices and insecticides.
Thrips leave spots as they suck juice from peony leaves. They also get inside buds and destroy the flowers before they have a chance to open. Infested leaves and buds eventually dry out and may drop off. They may also develop silvery speckling.
Remove infested buds and leaves and destroy them. A strong spray from a water hose knocks thrips off the plant and vigorous peonies usually recover from the damage. Avoid high-nitrogen fertilizers, which make the infestation worse. A reflective mulch helps early in the season, but once the plant leafs out to shade the mulch it is no longer effective.
Thrips are difficult to control with insecticides alone, but in conjunction with good cultural controls, insecticides will help reduce thrip populations. Choose an insecticide labeled for use against thrips and follow the label instructions carefully. For best results, timing is important. You may find that you have to wait until next spring for insecticides to have much of an impact on thrip populations.