Thrips 101

by Mark Nolen

Mark grow beautiful roses and is always willing to help others bloom and grow too!


Usually in late April or early May a phantom insect shows up in my rose garden. Usually the days are getting warmer and the irises are in bloom. A few years ago, spring came early and the unusually warm temperatures brought an unexpected invasion of this insidious garden pest. The unseen enemy ruined many of the rose blooms that I intended to take to our June Rose Show. If you don’t already know the insect that I am writing about it is the Western Flower Thrip. Since I have never seen just one, I will refer to them as thrips.

Thrips are tiny flying insects that are carried into your garden by the wind on warm sunny days. They are extremely small (about 1/8th on an inch in length) and they are rarely seen other than in the developing petals of a rose. Most often they are undetected until the rose bud fails to open. Frequently, the edges on the outer petals of the bloom are brown and the petal itself is malformed. The brown discolorations are caused by the insect chewing and sucking the juices of the petal for its nourishment.

If you peel apart the petals of a suspected bloom, you can see the tiny brownish insects scurrying for cover. If you cannot see them, you can tap an open bloom over a sheet of white paper, and they will become more visible. Thrips are rarely harmful to the overall health of the plant as they only affect the blooms. However, since the bloom is the essence of the rose, we all should have an interest in doing what we can to control the harmful effects of thrips.

Our efforts to control thrips are enhanced if we understand the life cycle of this insect. Ordinarily a thrip will live between 12 days and about 40 days depending on the weather. In warmer temperatures, the life cycle is shorter. The insects can over winter in weedy areas near your flower beds, and in the leaf litter and soil in your rose beds. In the spring as temperatures warm, the immature adults begin to feed reaching sexual maturity in a few days. The mature female will cut a slit in a developing bud and lay an egg. Thrips tend to favor lighter colors, but they can be found in all colors of roses. In about 3 days a nymph will emerge from the egg and will begin to feed on the tender petals of the unopened bud. The larval stage lasts about 5 days and this is when the most damage to the rose bloom occurs. The next stages is a non-feeding stage and the pupae generally fall to the ground emerging as adults after a few days.

The following pictures illustrate the damage that can be caused by thrips. The first picture shows the damage occurring when the rose bloom is in the bud stage. Notice the dark areas in the petals that is caused by the chewing and sucking of the thrip in the larval stage. At this stage most biological or chemical insecticides that have a contact mode of action will not work because the thrip is already inside of the petals of the bloom. Buds that are damaged by thrips have a tendency to never open because the malformed outer petals contain development of the bloom.

Infected Rose Bud

The second picture illustrates thrip damage on a fully open bloom. Notice the deformed and darkened petals edges, and the damaged bud. The best course of action at this stage is to dead head the bloom and discard. Do not place the infected bloom in your compost pile.

Infected Rose Bloom

So what are some good cultural practices that we can be doing to control thrips. Try not to plant roses close to weedy areas. Monitor you garden frequently and deadhead any blooms that are infested with thrips. By dead heading you are reducing the population of thrips in the garden. Always keep your rose beds clean especially in the fall as plant debris can harbor over wintering insects.

Some beneficial insects, such as Ladybugs and spiders will feed on thrips, however, the thrip would only be subject to predation when they are outsides the protection of the flower bud. Once the bloom is beginning to open predatory insects can get between the petals and eat the larvae.

Some biological controls can be used to control thrips. Generally, their mode of action is by contact which means the biological control must come in contact with the adult stage of the thrip.

BotaniGard ES is a product that will control and eradicate thrip populations. It is a highly effective biological insecticide containing Beauveria bassiana, an entomopathogenic fungus, which attacks many insects including thrips, mites, and aphids. It works on “CONTACT”. This product can be purchased on Amazon. The product has a shelf life of 18 months, so it could be used for two years if necessary and it is purchased in March of the first year of application.

I cannot emphasis that word contact enough. The big question then is, when should I apply the insecticide? The thrip is focused on one avenue of attack, the emerging buds on a rose as they develop in the spring. Around here the final pruning of roses ends around mid – April. New growth begins to emerge and buds begin to form by early May. The insecticide must be on the rose plant before the adult thrip emerges from the ground, so I would plan to have sprayed the insecticide by or near the beginning of May. The spray should be directed to the new buds as that is where the thrips will be. Repeat your spray regimen every 5 to 7 days as long as you see evidence of thrip damage.

Use of nematodes is another biological way to control thrips in your garden. There are specific types of nematodes that will attack a specific insect. Go to Arbico Organics to find the nematode that will attack the pupal stage of the thrip in the soil. The nematodes should be applied in the spring as soon as the soil begins to warm. It is doubtful if the nematodes will survive the winter in our area, but the use of the beneficial nematode for an entire summer will substantially reduce the thrip population as it will continually attack successive generations that are produced in a single year.

As a last resort chemical insecticides can be used to control the damage caused by thrips. I like to use the Bayer Advanced products as they are approved for home use. Bayer Advanced makes a 3 in 1 rose care product. Not only will this product fertilize and protect against fungus, it will kill many insects including thrips. It comes in granular form and it is applied to the ground every six weeks. Another Bayer Advanced product is the Complete Insect Killer which will kill thrips. This product is a liquid concentrate and must be sprayed. This product works on contact as well as systemically. Please read the direction carefully and apply both products in the correct formulations.

I have one last piece of advice with respect to controlling thrips; I find myself doing this in my garden all the time. When I see thrip damage on a bud that is trying to open, I will remove a few rows of petals on the bud. When doing this I will notice thrips scurrying around in the petals. I swipe the thrips with my finger and kill them.  Usually this will rid the bud of all thrips and it will develop normally. I hope this information will be of some value to you, and I wish all of you have happy thrip hunting this spring.

Mark Nolen