Figure 1. Thrips larvae (note lack of wings) on the underside of a plant leaf.
Photo: John Obermeyer
I’m hearing that thrips are throwing many of us for a loop this season! Some of you may know that thrips do not overwinter here in Indiana, so each spring season they migrate up our way on winds blowing up from the south. Sometimes these southern winds occur earlier or later each spring, depending on weather conditions, and I am guessing that thrips hitched a ride up on southern winds a bit earlier this season than last – but this is just speculation on my part! Additionally, dry conditions and above-average temperatures can cause thrips populations to grow quickly to damaging levels.
As with all insects, there are many different kinds of thrips and each has preferences for the kinds of plants and fruits they attack. In strawberries, it is eastern flower thrips that is most commonly a problem, while in tree fruit it is commonly western flower thrips. Regardless of the species, thrips can damage blossoms early in the season, as well as newly developed and mature fruits. Thrips feed by dragging their sharp, waffle cone-like mouthparts across the surface of flower petals and fruit, rasping the surface cells and causing them to ooze, and it is this liquid that they feed on.
Signs of thrips themselves can be hard to detect if you don’t look closely and scout regularly, because they are small – about 1/16th of an inch long! To make sure you don’t miss thrips activity, keep a hand lens in your pocket and scout regularly (at least once a week) by shaking or tapping blossoms over a white paper plate/bowl where the orange-yellow-colored nymphs (Figure 1) and adults will contrast nicely against the white background. You can also dip blossoms into a container of isopropyl alcohol and thrips that are present will fall off into the liquid. Yellow sticky cards may also be used to monitor thrips activity. Symptoms of thrips are more obvious: blossoms that are fed on by thrips may be misshapen, and injury on small fruits may present as dull or bronzed-color fruits (note: mite injury may look similar), while on tree fruits injury may appear as silvering or silver patches on ripe fruit.
Scouting will help you determine the need for action, but it is important to monitor several blossoms and fruits at several different places in your fruit planting. If you’ve had serious thrips populations in the past, you can start in those same areas when you begin monitoring the next season. In most cases, it’s recommended that you sample 50 blossoms from 5-10 areas of your fruit planting to look for thrips, and sample 50 small fruit berries from each of 5-10 areas, or 10 tree fruits from each of 5 areas in your fruit planting for thrips. This may seem like a lot I know, but it is critical because thrips may not be present uniformly throughout your fruit planting and you don’t want to miss them if they are there. The action threshold differs among fruit crops, but in general the threshold is low, with insecticide applications being recommended when as few as 2-5 thrips are detected per group of scouted blossoms/fruits.
Last, remember that thrips can feed on many different crop and non-crop plants, so weed management and the timing of mowing weeds/weedy patches around your planting is important to consider. Mowing weedy edges or flower patches while fruit plants are blooming or fruits are developing may result in the thrips moving from those host plants right over to your fruit planting, doh! It may eventually be necessary for you to apply insecticides to help reduce thrips populations in your fruit planting, and if you do, use insecticide products that are labelled for thrips specifically (other insecticides may not be effective against thrips), and try to avoid broad-spectrum insecticides, which also kill important natural enemies of thrips, like minute pirate bugs.