Although thrips damage to leaves is unsightly, thrips activity does not usually warrant the use of insecticide sprays. For instance, while thrips damage on citrus or avocado fruit may look unpleasant, it does not harm trees or affect the internal fruit quality. When damage is noticed on ripening fruit or distorted terminals, the thrips that caused the injury are often gone. It’s not until later when tissue grows and expands that injury caused earlier becomes apparent. While viruses vectored by thrips may cause plant loss, insecticide sprays are not recommended to prevent viruses because thrips are not killed fast enough to prevent the transfer of the virus to new plants. Prevention of thrips infestations is the only way to prevent infection by thrips-vectored viruses.
No pesticide provides complete control of thrips. In comparison with other insects, most thrips are difficult to control effectively with insecticides. Reasons include thrips’ tiny size, great mobility, hidden feeding behavior, and protected egg and pupal stages. Improper timing of application, failure to treat the proper plant parts, and inadequate spray coverage are also common mistakes and can be more important in influencing the effectiveness of treatment than choosing which pesticide to apply. Before using a pesticide, learn more about your specific plant situation and the biology of your pest species. Often you will learn chemical control cannot be effective until the next season when new plant growth develops. If insecticides are used, they will only be partially effective and must be combined with appropriate cultural practices and conservation of natural enemies. Greenhouse thrips is an exception; because it is sluggish and feeds in groups on exposed plant parts, thoroughly applying most any insecticide will kill this species.
Narrow-range oil (Sunspray, Volck), azadirachtin (Safer BioNeem), neem oil (Green Light Garden Safe), pyrethrins combined with piperonyl butoxide (Garden Safe Brand Multi-purpose Garden Insect Killer, Spectracide Garden Insect Killer), and (at least for greenhouse thrips) insecticidal soaps (Safer), can be somewhat effective for temporary reduction of thrips populations if applied when thrips are present and damage first appears. These materials have the benefit of allowing at least a portion of the natural enemy populations to survive because they do not leave toxic residues. Sprays must be applied to thoroughly cover susceptible plant tissue, such as new leaf growth and buds. On plants with a history of severe, unacceptable damage, begin treatment early when thrips or their damage is first observed. Repeat applications (usually 5 to 10 days apart, depending on temperature) are usually required because these insecticides only kill newly hatched thrips and recently emerged adults.
Other insecticides for thrips include spinosad (Conserve, Green Light Lawn & Garden Spray Spinosad, Monterey Garden Insect Spray) and (available only to licensed pesticide applicators) abamectin (Avid). These materials are derived from beneficial microbes and have low to moderate adverse impact on natural enemies. Abamectin and spinosad should be applied no more than once or twice a year, and can be more effective against thrips than the previously listed insecticides. The beneficial fungus Beauveria bassiana (BotaniGard) can be applied to commercial landscapes but is not available for use in home gardens or residential landscapes.
With most thrips species, eggs are protected within plant tissue and prepupae and pupae are in the soil and will not be killed. No pesticide treatment will restore the appearance of injured tissue; plants will remain damaged until leaves drop, injury is pruned off, or new unblemished fruit is produced.
For ornamental nonfood plants, a licensed pesticide applicator can use the systemic organophosphate insecticide acephate (Orthene), but acephate can be highly toxic to natural enemies and it commonly causes spider mites to become abundant and damage plants within a few weeks after its application. Another systemic insecticide, imidacloprid (Bayer Advanced Garden Tree & Shrub Insect Control, Merit) provides some suppression of foliage-feeding thrips only, but it is also toxic to some natural enemies of thrips. Avoid the use of organophosphate insecticides (e.g., malathion), carbamates (carbaryl), or pyrethroids (e.g., cyfluthrin, fluvalinate, and permethrin) because all these materials are highly toxic to natural enemies, can cause dramatic increases in spider mite populations, and are not particularly effective against most thrips.