Western flower thrips
Western flower thrips, Frankliniella occidentalis, is a worldwide pest with a wide range of host plants, the main ones being vegetable crops such as sweet peppers, strawberries, melons, cucumbers, eggplant, beans and tomatoes, as well as field crops, flowers, fruit trees, citrus and many ornamental plants.
The thrips cause direct and indirect damage to its host plants by scratching their tissues and sucking sap. The direct damage takes the form of reducing the chlorophyll content and yellowing of foliage, accompanied sometimes by dehydration and defoliation. The damage to the flowers appears as light spots on the petals, necrosis, blackening of the margins of the petals and discolouration of the flowers. Various disease vectors, such as Botrytis and Alternaria, may penetrate via the feeding sites. On the fruit, there are a number of typical signs of damage: silvering in sweet pepper, bronzing in strawberry, malformation in cucumber, and small perforations in tomato. In addition, there is a decrease in yield as a result of the falling off of fruit. The indirect damage is caused by the transfer of a harmful viral disease, Tomato Spotted Wilt Virus (TSWV).
Morphology and biology
The Western flower thrips is a small insect. Its body is narrow, up to 1.2 mm in length, and yellowish in colour. The male is smaller and lighter in colour than the female. The wings of the thrips are feathered, and in the resting state, lay on the back forming a dark line along the back where they meet. At the tip of the abdomen of the adult female, protrudes the ovipositor. In the male, two small orange spots are visible.
In addition to the egg, the thrips has two larval stages, pre-pupa, “pupa” (the latter two do not feed and dwell in the soil) and adult.
The developmental period from egg to egg takes approximately 15 days at a temperature of 25°C. During winter, the thrips develops chiefly in protected crops, but it reaches its peak populations in the spring and autumn. The female lays kidney-shaped eggs into the leaves, petioles and soft stem parts. Females will develop from fertilized eggs, whereas unfertilized eggs will yield males. Fertilized females will produce about two-thirds female offspring and one-third males. In cucumber, the female will lay approximately 3 eggs per day at 25°C. Under optimal conditions the population can double itself within 4 days.
The thrips is mainly found in the middle and upper parts of the plant. In general it “hides” inside flowers, where it is nourished by pollen, its favourite food.