Thrips

Frankliniella occidentalis

The western flower thrips (Frankliniella occidentalis) is a tiny but highly destructive pest in agriculture, known for damaging a wide range of ornamental and food crops. Despite their size—adults are less than 2 mm long—their impact can lead to significant crop loss, especially due to their role as vectors of plant viruses. Understanding their lifecycle and how environmental conditions affect them is key to effective management.

Thrips eggs are opaque, kidney-shaped (reniform), and about 200 µm long. Females insert them into the epidermis and mesophyll layers of host plants. Eggs may be laid in leaves, flowers, or fruit, depending on the species and plant part availability.

There are two larval stages, both spindle-shaped and creamy-white to yellow in color. Though mobile, larvae prefer hiding within flowers, developing leaves, or under the fruit calyx. Differentiation between larval stages—and even sexes—is possible by examining the pattern and number of abdominal setae.

Thrips have two non-feeding pupal stages: the propupa and the pupa. Although both can move slightly, they are generally inactive unless disturbed. Pupation may occur on the plant or in the soil. Morphological features such as wing bud development and antennae position help distinguish between stages.

Adult thrips are slender with narrow, fringed wings. Females vary in color from yellow to brown to nearly black, while males are smaller and pale yellow. Seasonal and geographical variation affects coloration, with darker forms found in cooler climates. Using a 20x hand lens, you can identify long setae on the pronotum, typical of F. occidentalis.

Symptoms and Crop Damage

Damage varies by crop. On red or dark-colored flowers (e.g., roses, gerberas), feeding results in visible white streaks. In lighter-colored flowers, symptoms may go unnoticed despite heavy infestation. Severe damage includes:

• Bud and fruit deformation

• Silvering and necrotic spots on leaves

• Pimpling on petals (from egg-laying)

• Scarring and splitting of fruit skins

In fruits like tomatoes, apples, and grapes, this damage not only reduces visual appeal but can also invite fungal infections.

Virus Transmission

The most critical threat posed by F. occidentalis is its role in spreading plant viruses—especially tospoviruses such as:

• Tomato spotted wilt virus (TSWV)

• Impatiens necrotic spot virus (INSV)

• Groundnut ringspot virus (GRSV)

• Chrysanthemum stem necrosis virus (CSNV)

• Tomato chlorotic spot virus (TCSV)

Larvae acquire these viruses early and transmit them later as adults. Infected plants may show ring spots, mottling, wilting, or total crop failure.

Environmental Influence

Thrips development is highly temperature-sensitive:

• Optimal temperature range: 68–98°F (20–37°C)

• Below 50°F (10°C): They survive but do not develop.

• High temperature fluctuations negatively affect development and reproduction more than humidity changes.

• Humidity fluctuations also impact development and offspring sex ratios.

Integrated Pest Management (IPM) Implications

Controlling environmental conditions—particularly maintaining stable temperatures and humidity—can help suppress thrips populations in greenhouses. Fluctuating conditions stress their lifecycle, making it a viable control strategy alongside biological or chemical methods.