Onion thrips (Thrips tabaci) and western flower thrips (Frankliniella occidentalis) are a major problem in many protected crops, especially in cucumber. Other damaging species which are sometimes found in glasshouses are the rose (cereal) thrips (Thrips fuscipennis) and Echinothrips americanus.
Thrips go through six stages, namely egg, two larval stages, prepupal and pupal stage and finally the adult stage. The adult thrips are found in flowers and on leaves where they deposit their eggs. On sweet pepper leaves the egg laying sites are easily recognized as wart-like deformations; in cucumber and other crops these deformations are not visible. The larvae feed on all above ground parts of the plant and are extremely mobile. Pupation takes place on the ground, except in the case of Echinothrips americanus. This species pupates on the underside of the leaf.
Thrips cause damage to the plant by piercing and sucking out cells on the leaf surface. This causes silver-grey spots on the leaves, with darkgreen spots (excretions) and reduces the production of the plant. At high infection levels leaves may even wither.
The most important virus transmitted by thrips is tomato spotted wilt virus (TSWV). Frankliniella occidentalis is the major vector of this virus that causes a lot of damage in the Mediteranean countries.