By Heidi Wollaeger
Western flower thrips are one of the most challenging insect pests of ornamental plants grown in greenhouses. The feeding damage from thrips may result in leaf drop, leaf yellowing, stippling, streaking and leaf distortion. Also, western flower thrips are known to transmit several viruses including Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV). Therefore, according to MSU Extension, controlling thrips is extremely important to maintain the plant quality of ornamental plants. It can be especially challenging during the summer months because of the high reproductive capacity of western flower thrips at warm temperatures.
Western flower thrips. Photo: Frank Peairs, Colorado State University
In western Michigan, the 10 day forecast is calling for temperatures in the mid- to upper 80s. Thus, greenhouse growers who are using biological control for western flower thrips might see some of their predators become less effective than they were in the spring production season when temperatures are cooler. For example, Neoseiulus cucumeris is a predatory mite that is commonly used for western flower thrips control in greenhouses. However, this predatory mite is not active at temperatures above 75 F. Therefore, growers continuing their biological control program into the summer will need to switch to the more heat-tolerant predatory mite, Amblyseius swirskii. A. swirskii is active at these warmer temperatures.
Also, if greenhouse growers are applying the beneficial nematode, Steinernema feltiae, for thrips control they will need to apply them later in the evening as they require median temperatures between 50 and 80 F. Growers should also water the crop before and after the drench application of the nematodes to increase efficacy.
Growers using Orius spp., a minute pirate bug, should continue to do so throughout the summer months, especially if they are distributing them through the banker plant systems. They are effective at temperatures over 60 F. Orius spp. is particularly effective as a supplemental strategy to control thrips during the late spring and summer months when the predatory mites are unable to keep up with the populations of western flower thrips