When we talk about “thrip damage” in southeast Wisconsin, most individuals are referring to the brown discoloration seen on lighter colored rose petals. Move the petals around and you may also see tiny insects darting among the petals. Most likely, you are seeing the adult Western Flower Thrip, a member of the order Thysanoptera, that contains over 4,500 species around the world. Adult thrips are usually less than 1/8 of an inch long, yellowish, with fringed wings. In researching this article, I found that the Western Flower Thrip damage goes beyond the bloom and includes leaves and buds. That is when I realized that the thrip damage in my garden was more extensive than I thought.
The Western Flower Thrip development cycle can take 7 – 44 days depending on temperature. The warmer the temperature, the shorter the cycle. The cycle begins in spring, with the female thrip using a saw-like ovipositor to insert eggs into soft plant tissue, including leaves, buds, and petals. This is when the female causes the most damage, seen as scaring, dark spots and scabs. Each female Western flower thrip can lay between 150 – 300 eggs in her lifetime. After hatching, the larvae will feed on the plant fluids, causing damage to flower buds, and creating leaf deformation and silvering of leaves. I have observed this in my garden. The larvae molt twice and drop into the soil. It is in the soil that they pupate and emerge as adults with fringed wings. They can live for 30 – 45 days, feeding primarily on pollen. Male thrips do not lay eggs, but they feed aggressively on rose buds and petals. The male Western Flower Thrip may even aggressively compete with other thrips for territory on the flowers.
Identification and Control of Western Flower Thrips
Thrips over winter in weedy areas near your flower beds, and in the leaf litter and soil of your rose beds. Getting an early start on controlling thrips is important.
Symptoms to look for:
- Malformed Buds: The outer petals of the bud brown and tighten, causing the flower to become distorted or preventing it from opening
- Damaged Petals: On light colored or fragrant roses, you will notice brown streaking, brown edges, or scorched-looking blemishes on the petals
- Foliage Damage: Leaves may curl or develop pale, silvery flecks or stippling. You may also spot tiny, black specks of waste on the petals or leaves.Using sticky traps may help with early detection. I have not done this but will going forward.
Using sticky traps may help with early detection. I have not done this but will going forward.
How to treat thrips:
Since thrips reproduce quickly and hide in the flowers and leaves, effective control requires complementary solutions to target the various life stages. Solutions generally require contact with the thrip and the thrip is somewhat protected when it is within the protection of a bud.
- Pruning – immediately cut off and dispose of infested or deformed blooms. Keep the ground around your rose bush clean and free of plant litter.
- Beneficial Insects – Encourage natural predators such as lady bugs, lacewings, and minute pirate bugs. Predatory insects and mites will control the larvae and pupae, while nematodes will target the pupae in the soil.
- Organic sprays – Spinosad or neem oil applied when pollinators are not present
- Systemic Insecticides – For heavy infestations, systemic insecticides such as imidacloprid can be applied to the soil. Keep in mind that thrips are known to frequently develop resistance to insecticides and many of these products may injure the beneficial insects you are trying to recruit to assist in thrip control.
Controlling thrips throughout the summer can be challenging. It starts in early spring as the soil warms up and new tissue appears on the roses. Implementing a strategy to impact each stage of the thrips life cycle should lead to the best result. Focusing on using chemicals only may not be successful as thrips often become resistant. Sounds like a great opportunity to learn more about beneficials!
Greater Milwaukee Rose Society 