Happily, several years have passed since I’ve noticed thrips in my garden, but they are back!
Thrips are flying insects that are less than 1/20 of an inch long and eager to suck juices from plants. There are over 500 genera and 5,000 species of thrips, some of which are beneficial predators of mites, scales and even other thrips. Others feed on fungal spores or pollen, and some feed only on favored plants, such as avocados, citrus, blueberries, or various others.
My garden’s visitors almost certainly are Western Garden Thrips (Franklinia occidentalis), which attack a wide range of herbaceous ornamentals, vegetables, fruits and some shrubs and trees, including roses and stone fruits.
Thrips are poor flyers but can find juicy plants and infest all of a large shrub or a good-sized bed of a plant that they find tasty.
In my garden, they found a 200 sq. ft. bed of Bergenias (an unidentified cultivar of the popular species, Bergenia crassifolia). This plant from central Asia has numerous common names: Elephant Ears, Pig Squeak, Large Rockfoil, etc. It is an attractive groundcover plant, growing one-to-two feet high, with clusters of small, bell-shaped pink flowers on stems that rise above the foliage in early spring.
Bergenias are generally free of pests, at least until thrips discover them.
Thrips might make only limited damage to ornamental plants and could be ignored. When conditions are favorable, however, they can cause great cosmetic damage.
When thrips attacked my Bergenia bed, nearly all the leaves, each about four by six inches, became discolored and distorted, and their upper surfaces turned silvery gray, the result of losing their waxy surface. The leaves also were stippled with black specks of the thrips’ fecal matter.
The normal growth of Bergenias has new leaves emerging above older leaves that turn brown without dropping off. New leaves cover the old leaves, but removing the dead leaves improves the plant’s appearance. In this instance, the action of the thrips revealed the dead leaves, and the entire bed was devastated.
The damage was limited to the leaves. The stems and roots of the plants were alive and well, and able to produce new leaves. Our treatment was to remove all living and dead leaves, using garden clippers, and then spray the plants with Spinosad, an organic pesticide that kills insects both by direct contact and by ingestion.
I used a Spinosad-based product, Captain Jack’s Deadbug Brew, in a container designed conveniently as a hose-end sprayer. Despite its promising name, the spray doesn’t guarantee success: a follow-up spray might be needed to control the newest of up to eight generations of thrips for the year.
Thrips are members of the order Thysanoptera. The earliest fossils of thrips date to the Permian Period, 250 to 299 million years ago. This period was the last part of the Paleozoic Era, which ended with the largest mass extinction in Earth’s history, when nearly 90% of marine species and 70% of terrestrial species died out.
That thrips were among the survivors of that period suggests the difficulty of eliminating them from our gardens.
When the gardener suspects that thrips are present, they can be detected either by close examination of the underside of affected leaves, or by beating or shaking a branch or flower onto a sheet of paper. Yellow sticky traps also can be used to monitor their presence.
The most important way to encourage biological control of pest thrips is to conserve naturally occurring populations of beneficial insects, e.g., predatory thrips, minute pirate bugs, predaceous mites, and green lacewings . This can be done by controlling dust and avoiding the use of persistent pesticides in the garden.
For other control, see the University of California’s Integrated Pest Management information page on How to Manage Pests—Thrips.