There are many different kinds of thrips; some are beneficial (can you believe it?) others are not, and this post is only on the destructive kinds, that feast on rose flowers. These same thrips hurt many other plants such as impatiens, petunias, members of the cucumber family, peppers, and fruits such as grape and strawberry. These are nasty guys and considering their size (1/8-3/16″ long) they do a lot of damage, especially to rose flowers, making them unsuitable for showing.
What does thrip damage look like? Buds that fail to open and/or buds that open to deformed flowers with brown edges on the petals. The thrips use their rasping mouth parts to to dig into the bud where they feed on the tissues and juices. The juices cause the petals to stick together, ball up and fail to open. Sometimes they also enter the leaves causing them to crinkle. If you are still not sure that you have thrips, place a piece of white copy paper underneath the flower you think has thrips and shake the flower. If small tan to black specks that move fall onto the paper they are probably thrips. Every year the thrips attack the same bushes in my rose garden, usually the light colored ones, while leaving others untouched.
So, what do you do? If you want to show your roses then you have to spray with a commercial insecticide. The spray will not reverse the damage to the roses that have already succumbed to the thrips, but hopefully it will stop the thrips from doing further damage. On the other hand, if you do not show your roses and can tolerate some damage for a short time, ignore the thrips and don’t spray. As long as only the flowers are hurt, the life of the bush is not threatened. You will lose blooms but avoid pesticides that may cause harm to unintended victims. Perhaps we all need to tolerate some imperfections in order to stop damaging environment. The thrips will disappear during the summer when temperatures warm up, their damage probably does not threaten the life of your plant, and you can enjoy the flowers of the flushes during the summer on reblooming roses, so perhaps you can adjust to life with thrips. Yes, I admit, the thrips will return in the fall.
Selecting varieties with sepals that keep the bud covered until they open, good cultural practices, and removing flowers that are infected will all help control this pest. A systemic spray such as Orthene will give good control, preventing or reducing the problem before it gets to be a major problem. Natural predators of thrips such as lady beetles and lacewings are not usually able to handle to job.
If you have a severe infestation of thrips and you can’t stand the damage they inflict, go the insecticide route. Top mist your plants with a good insecticide every 3-4 days.
Here’s a point to ponder: the same thrips that destroy your roses prey on spider mites, another pest of roses.