Temecula Valley Rose Society

Rose Care Corner, May 2006

By Frank Brines-ARS Consulting Rosarian

It's that time of year when we see all the critters in the world coming to visit our rose gardens, just in time to ruin our hopes of having that rose specimen taking high honors in the rose show. One critter which attacks both the leaves and the blooms of roses. Thrips are tiny insects with piercing mouth parts that feed on plants for sap, juice, and water. They can damage most types of garden plants but they are particularly nasty to rose blooms. Their biting and sucking can also transmit fungal and virus diseases. They reproduce rapidly, so it is important to action when you first see their damage in order to keep the populations in check.

What's their damage look like? Leaves may turn brown and curl and even they may even fall off. Blooms can become spotted and scarred. The thrips themselves range in size from about 1/25 to 1/8 of an inch long. Their color can range from clear to white to yellow to brown to black — generally a color similar to the plant they are mostly feeding on. Thrips crawl, jump, walk, and fly from plant to plant. They are so light that they can move across your garden on wind currents.

Where do you find them? They flourish feed, nest, and reproduce wherever there is water or high moisture. With our warm temperatures, they can remain active year round, but their populations explode as we begin watering and plants put on rapid growth. They lay their eggs on plant tissue and the newly hatched young immediately begin feeding on any part of the plant that provides sap and vital fluids containing nutrition. After a week or so, these larvae pass through two more stages that eat non-stop. They stop eating in their third stage and may develop wings, fly off, or crawl; however they get around, they're looking for leaf litter and mulch where they can pupate. They emerge to lay eggs. This life cycle can take as little as two weeks, so few can become a few thousand very rapidly.

So, when you see their damage, treat as soon as you can and repeat the treatment until they are under control. A common treatment that poses little risk is insecticidal soap. It quickly kills off thriving thrips and won't hurt plants but it doesn't leave much residue to keep on working. Once it dries, it's pretty much inactive, so you have to repeat treatments as often as needed (like twice a week during the growing seasons) and more often if you've got a big population.

If the thrips are just too persistent, you may want something a stronger which will provide more residues. Most of what is available is broad spectrum, killing a wide range of insects. While I try to stay away from heavy use of such insecticides, sometimes you just have to use them. One is permethrin concentrate. You can find it in well-stocked garden centers or on the Internet. It is a true insecticide which kills just about any insect in and around the garden (and that includes beneficials). It works well for whiteflies and aphids too. The residue lasts 1-2 weeks. It presents little danger to mammals (like us).




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