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Larvae and adults feed on plant tissues and produce small silver patches on the upper side of leaf blades, an effect known as 'silvering'. The same patches can appear on petals where the pigment has been removed. On the underside of the leaves, the thrips and their larvae sit together in groups alongside their black dung spots. Leaves of affected plants yellow, wither, deform or shrivel. Feeding during bud or flower development later results in scarred, stunted or deformed flowers or fruits respectively and loss of yield.
Thrips are 1-2 mm long, yellow, black or fasciated insects. Some varieties have two pairs of wings, others do not have wings at all.They hibernate in plant residues or in the soil or on alternative hosts plants. They are also vectors for a broad range of viral diseases. Dry and warm weather conditions favor population growth. Humidity reduces it.
Some biological control measures have been developed for specific thrips. Predatory mites that feed on larvae or pupae are commercially available. Against varieties that attack the leaves and not the flower, try neem oil, especially on the undersides of the leaves. Spinosad application is generally more effective against thrips than any of the chemical or other bioclogical formulations. It lasts 1 week or more and moves short distances into sprayed tissue. It can however be toxic to certain natural enemies (e.g., predatory mites, syrphid fly larvae) and bees. Do not apply spinosad to plants that are flowering. In case of flower thrips infestation, some predatory mites or green lacewing larvae could be used. A combination of garlic extracts with some insecticides also seem to work well. Use of highly reflective UV mulch (metalized mulch) has been recommended.
Always consider an integrated approach with preventive measures together with biological treatments if available. Due to the high reproductive rates and their life cycles, thrips have developed resistance to different classes of pesticides. Effective contact insecticides include azadirachtin, insecticidal soaps, narrow-range oils and pyrethrins, which many products combine with piperonyl butoxide.