Thrips tabaci (the onion thrips),
Frankliniella occidentialis (the western flower thrips) and numerous other species
Thrips damage results from the piercing and rasping action of the cone-shaped mouth parts of the insect. At first, tiny, dark green spots appear on the leaf. These spots become white or silver with time and if widespread, can impart a silvery streaked appearance to the leaves, which will appear as a bright sheen in direct sunlight. Severely affected leaf tissues wither and collapse when plants are water stressed. Thrips are most commonly found between the newest growing leaves or in seed heads.
Conditions for Disease Development
Thrips over-winter in bulbs, as larvae or pupae in leaf litter or in the soil and on alternate hosts. The life cycle of these insects from egg to adult can be completed within two weeks. Thrips damage is greatest after periods of hot, dry weather. Cool, rainy weather reduces thrips populations and thrips damage.
Good crop management and sanitation generally keep thrips damage to a minimum. Healthy leaf tissue will endure thrips feeding better than stressed tissue. Thrips tabaci has a wide host range including numerous weed species; thus, weed control in and around an onion crop may reduce thrips levels. Also, cultivation and plowing to eliminate debris near the soil surface will reduce thrips populations. Insecticide control is feasible, however, several applications are usually required and resistance to insecticides has been reported.