This has been a bonanza year for thrips. The population has exploded this spring and summer, causing a lot of damage to local flowers. However, these insects are hard to see and their presence may go unnoticed. They are very small, less than 1/16 inch long, tan in color and tend to hide in partially-opened blossoms. Thrips are mainly spread by the wind. They have multiple generations per year, and produce hundreds of eggs per female.
Thrips are tiny insects which can damage a host of agricultural and greenhouse crops as well as ornamentals. There are many varieties which specialize in different crops, but Western Flower Thrips are the ones which may feed on most all flowering plants. They are especially attracted to roses and petunias-they consider petunias as their candy treat. Indeed, this thrips variety likes petunias so much, that in greenhouses where the presence of thrips may be devastating, petunia plants are used as a bait to monitor for their presence.
Thrips have a unique method of feeding. They scrape off, or rasp, the bud, blossom and leaf tissues, producing discolored spots and streaks of various sizes. These spots and streaks on colored petunia blossoms are often white and are characteristic of thrips. With heavy infestations the plants leaves may also be involved. The leaves develop silver or bronze color and can become twisted and deformed.
On roses, thrips feed on unopened blossoms, causing the edge of the petals to be discolored, stick together and fail to open. They will feed on any colored blossom, but seem to prefer yellow.
Once identified, control is difficult. This stems from the facts that thrips are protected from sprays by hiding in buds of plants and after spraying, and new thrips are easily blown in from surrounding areas. Another factor to consider is that there are many "good" insects which feed on thrips, and play a big role in helping to keep them under control. Some insecticides kill the good insects as well as thrips.
If insecticides are used, ones containing spinosad, neem oil, pyrethrin, or insecticidal soap are effective and are the least harmful to good insects. More than one treatment will be needed, perhaps as often as every 7 to 10 days, but always according to the labeled instructions. Insecticides should be rotated to avoid resistance.
Because of the difficulty with chemical control, the best choice may be to simply take good care of your petunias with proper water and fertilizer and wait until this unusually heavy thrips population is controlled by Mother Nature.