How to keep your garden plants free of thrips and other pests
Q: How do I know if my plants have thrips? My oranges have strange scars on the rind, but I can’t see any bugs.
Thrips are tiny (around 1/20 inch long) slender insects that cause mostly cosmetic damage to leaves, fruit, and flowers. Except in cases of very severe infestation, they rarely cause enough damage to kill a plant, but the cosmetic damage can be enough to devalue crops that require visual perfection. More significantly, they are a vector insect for many viral diseases. There are multiple species of thrips, each named after the specific plants they infest. The Western flower thrip is a significant crop pest that feeds on a wide variety of food and ornamental plants (not just flowers).
Noticeable signs of thrip damage include silvery or bronze speckling of leaves, leaf distortion, fruit or flower bud scarring accompanied by the presence of black frass (tiny insect poop). If you want confirmation of a thrip infestation, remove a suspect branch, leaves, or flower and dip in rubbing alcohol. Pour the alcohol through a white paper towel or coffee filter and the thrips should be visible.
Cultural control should be the first step in dealing with thrips. Diligent weed removal will eliminate their preferred hiding places. If a neighboring property is overgrown with weeds, try to relocate the affected plants away from the property line. Remove leaf litter from the garden since that, too, is a favorite shelter. Grow plant varieties that are well-adapted to your climate and try to reduce plant stress by regular watering and fertilizing. Look for varieties that are resistant to thrips. Reflective mulch, while not particularly attractive, disorients and repels pest insects effectively. Row covers can be useful for low-growing crops and have the added benefit of keeping other pests (such as grasshoppers) away.
Thrips have many natural enemies, including minute pirate bugs, lacewings, parasitic wasps, and predatory mites. Speaking of predators, the six-spotted thrip is a predator of spider mites. (Not all thrips are bad!)
Due to their size and ability to hide, pesticide effectiveness can be a hit-or-miss proposition. In a greenhouse setting, where natural enemies may not be plentiful, pesticide application may be the only available control. In an outdoor setting, non-chemical solutions should be tried first. Thrips develop pesticide resistance rather quickly, so spraying may not even be effective.
Los Angeles County
mglosangeleshelpline@ucdavis.edu; 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/
Orange County
ucceocmghotline@ucanr.edu; http://mgorange.ucanr.edu/
Riverside County
anrmgriverside@ucanr.edu; 951-955-0170; https://ucanr.edu/sites/RiversideMG/