Issue: May 10, 2003
Please give me advice on eradicating thrips. They appear annually on iris blossoms and then disappear. I have tried several things to no avail. I sprayed as the blossom stalks emerged and have kept the beds free of debris. Do thrips cause water spots in the iris blossoms? If not, what is causing this damage? It is not overhead watering as I show irises and take great care not to water from overhead.
Thrips may cause the "water spots". They feed with rasping mouthparts that scrape the flower petals and leaves of many plants, including irises. The result is areas of petal without color pigments appearing as water spots. There are other causes, as you alluded, such as overhead watering. Since you are aware that you have a thrips infestation, that infestation is the likely cause. You are familiar with thrips, but other readers may not know this pest, so I will describe it. A thrips (this is the correct spelling for singular and plural) is a small yellow, brown, or black insect, often not noticed by gardeners. They are less than 2 millimeters (about 8/100 inch) in length, so often are not noticed, but their damage is very noticeable. They are long and narrow, feed by rasping the plant cells and drinking the juices that escape through the wound. This is what causes the damage. There are many species of thrips, each having a wide host range and being a problem for many plants. Rose growers, like iris growers, have tremendous problems with thrips infestations. Even though they are small, a careful inspection of the flowers, especially light colored flowers, will reveal small, narrow insects crawling around on the petals (and sepals), especially into the folds of the flower where they are protected. As you have described, treatment is difficult. They are weak flyers but blow easily in the wind. They may build up populations in many plants, lawns, grain fields (which you have in the Roswell area), and in weeds (we have a lot of weeds this year). As their population increases or the first host plant matures (weeds are drying now), they migrate to other plants. Because they migrate into our home gardens from other areas, treatment of a single garden may not be effective, but we should take measures to protect our plants. Disposal of spent blossoms helps remove thrips. There are insecticides (organic and synthetic) that are labeled for thrips control, but there are natural predators that feed on thrips. We often see thrips damage early in the year. Damage declines as the natural predator population increases. Use of insecticides may prevent the build-up of the natural predators and may actually prolong the period of thrips activity. Enclosing special (show) plants in cages wrapped with spunbonded row cover fabric will help exclude the thrips, limiting damage to the plants until the thrips population drops.If you choose to use insecticides, there are spray applications and systemic materials labeled for thrips. Please read the label and follow the directions exactly. While you will be killing some thrips, others will migrate into your garden and do some damage before they are killed.back to top
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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.