African violets (Saintpaulia ionantha) are popular flowering houseplants, prized for their ease of growth and long blooming period. However, these plants can be susceptible to various pests, including thrips. These tiny insects can cause significant damage if left unchecked. Here's a comprehensive guide to identifying and treating thrips on African violets.
Stippling as a result of thrips feeding.
Identifying Thrips
Thrips, belonging to the order Thysanoptera, are small, slender insects characterized by fringed wings. They are rarely seen because of their small size. Most adult thrips are elongate, slender, minute (less than 1/20 inch long), and have long fringes on the margins of both pairs of their long, narrow wings.
The young, called nymphs, are cream to pale green and only visible with magnification. Immatures (called larvae or nymphs) are oblong or slender and elongate and lack wings. Most thrips range in color from translucent white or yellowish to dark brown or black.
Thrips feed by puncturing the epidermal (outer) layer of host tissue and sucking out the cell contents, which results in stippling, discolored flecking, or silvering of the leaf surface.
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An adult, egg, and two larvae of Cuban laurel thrips.
Signs of Thrips Infestation
- Flowers or leaves may develop silvery streaks.
- Heavily infested leaves appear brownish or silvery.
- Growing points may become contorted.
- Some species of thrips leave sooty spots of black fecal matter on the leaves.
- Pollen spill is often an indication of the presence of thrips.
Thrips feeding is usually accompanied by black varnishlike flecks of frass (excrement). They feed by puncturing the surface of the plant parts with their single large mandible and slurping the plant juices that seep from the wound.
Identifying and Treating Thrips - #plants #plantcare #houseplants #garden #pests #planttips
Thrips feeding on plants can damage fruit, leaves, and shoots and very noticeably affect plants’ cosmetic appearance. Thrips feeding can stunt plant growth and cause damaged leaves to become papery and distorted, develop tiny pale spots (stippling), and drop prematurely.
Petals may exhibit “color break,” which is pale or dark discoloring of petal tissue that was killed by thrips feeding before buds opened. On some plants thrips can cause severe stunting to the early season flush of terminal growth.
Thrips Species Affecting African Violets
Several species of thrips will feed on and reproduce on violets. Some of the most annoying sucking pests of African Violets are the thrips.
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The species that attacks the flowers of African violets (formerly Saintpaulia ionantha; now Streptocarpus ionanthus) is the western flower thrips* (Frankliniella occidentalis).
Adult Western Flower Thrips.
Thrips Life Cycle
Thrips hatch from an egg and develop through two actively feeding larval stages and two nonfeeding stages, the prepupa and pupa, before becoming an adult.
Female thrips lay their eggs in tiny slits cut into the surface of leaves, flowers, and stems. Females of most plant-feeding species lay their elongate, cylindrical to kidney-shaped eggs on or into leaves, buds, or other locations where larvae feed. The eggs can be laid any time of year and hatch within a few days in warm, indoor conditions.
Fullgrown nymphs, in most species, drop off the plant to the soil where they burrow down and pupate. The pale prepupae and pupae of most species drop to the soil or leaf litter or lodge within plant crevices or galls.
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Thrips have several generations (up to about eight) a year.
Organic and Non-Toxic Control Methods
For those who prefer organic and non-toxic methods, several options are available to manage thrips infestations.
- Keep plants moist. Plants that are kept too dry are more likely to be damaged by thrips.
- Insecticidal soap. Insecticidal soaps, which are considered nontoxic to humans and pets, give good control. Be sure that flowers and growing tips are well covered with the spray.
- Use superior horticultural oil sprays. Highly refined oils sold as superior or horticultural oils will also control thrips. The oil suffocates the insects. These oils are highly refined and under proper conditions, can be applied to plants in foliage without damage. Follow label directions to avoid damage to some plants that may be sensitive. Superior oils are also considered nontoxic and are less likely to harm beneficial insects.
Contact insecticides that do not leave persistent residues can be effective for greenhouse thrips and other species that feed openly on plants. These products have low toxicity to people, pets, and pollinators and relatively little adverse impact on biological pest control because they do not leave toxic residues that would kill natural enemies migrating in after their application.
Contact insecticides include azadirachtin (AzaMax, Safer Brand BioNeem), insecticidal soaps (Safer), narrow-range oil (Bonide Horticultural Oil, Monterey Horticultural Oil), neem oil (Green Light Neem, Schultz Garden Safe Brand Neem Oil), and pyrethrins, which many products combine with piperonyl butoxide (Ace Flower & Vegetable Insect Spray, Garden Tech Worry Free Brand Concentrate).
Biological Control
Predatory thrips (Table 2), green lacewings, minute pirate bugs, mites, and certain parasitic wasps help to control plant-feeding thrips. Where thrips are a problem, learn whether that pest has specific natural enemies important in its control.
Euseius predatory mite attacking thrips.
For example, a minute pirate bug, Macrotracheliella nigra, and green lacewing larvae are important predators of Cuban laurel thrips. Euseius species mites are important predators of citrus thrips.
With greenhouse thrips in Southern California up to 50% of its eggs are killed by a tiny wasp, Megaphragma mymaripenne. After feeding inside during its larval stage then pupating, the emerging adult parasite leaves a relatively large round hole in the tiny thrips egg.
Chemical Control Methods
Many insecticides registered for use indoors are available.
- Use chemical insecticides. Sprays containing pyrethrins, a plant-derived insecticide, are effective and more benign than other chemical pesticides.
Before using a pesticide, learn more about the biology of your pest species and the characteristics of available products by reading the label and consulting the Active Ingredients Database in the online version of this Pest Note at www.ipm.ucanr.edu. Often you will learn chemical control cannot be effective until the next season, when new plant growth develops.
Certain products are available only by hiring a professional applicator.
Preventative Measures
Prevention is key to keeping your African violets healthy and thrips-free.
- To limit future problems, inspect plants regularly. With regular inspection, pest problems can be caught when just beginning and control is easier.
- It is also recommended to isolate newly acquired plants for 2-3 weeks to limit introducing pests indoors.
- Avoid planting susceptible plants next to these areas, and control nearby weeds that are alternate hosts of pest thrips.
- Grow plants that are well-adapted to conditions at that site. For example, plants adapted to grow in full sun can be stressed when planted in shady conditions and may be more susceptible to thrips damage.
- Provide appropriate cultural care to keep plants vigorous and increase their tolerance to thrips damage.
- Keep plants well irrigated, and avoid excessive applications of nitrogen fertilizer, which may promote higher populations of thrips.
- Old, spent flowers can harbor thrips, so their removal and disposal is sometimes recommended.
Inspect new plants carefully, including the bottom of the pot, for mealybug eggs. Proper humidity can make a difference in the health of your plants. Weekly examinations are usually frequent enough to detect pests before populations increase to damaging levels.
One of the most important factors in maintaining a pest-free culture is to take care and introduce only pest-free plants into your culture. Any practice which would reduce plant stress and promote good vigorous growth will enhance pest control.
Additional Tips
- Light blue cards coated with a thin film of oil suspended within two feet above indoor plants can be used as indicator traps.
- Investigate the availability of resistant cultivars. For example, western flower thrips more often damages fragrant, light-colored, or white roses.
- Rose cultivars, with sepals that remain tightly wrapped around the bud until just before blooms open, have fewer thrips problems.
Table 1: Common Insecticides for Thrips Control
| Active Ingredient | Example Product | Comments |
|---|---|---|
| Azadirachtin | AzaMax, Safer Brand BioNeem | A botanical-based synthetic. Use product within 10 days of breaking seal. Do not apply to stressed plants or new transplants until roots are well established. |
| Narrow-range oil | Bonide Horticultural Oil, Monterey Horticultural Oil | An oil and contact insecticide. Do not spray stressed plants. Target pest must be completely covered with spray. Check label for plants that can be treated. Do not use with sulfur products; check label for tank mix restrictions. |
| Neem oil | Green Light Neem, Schultz Garden Safe Brand Neem Oil | A botanical with unknown mode of action. Do not spray stressed plants. Check label for list of plants that can be treated. May injure flowers. Target pest must be completely covered with spray. Best for thrips on foliage. |
| Insecticidal soap | Safer | An insecticidal soap. Must contact insect, so thorough coverage is important. Do not make more than three sequential applications. Test for phytotoxicity. Do not spray new transplants or newly rooted cuttings. Do not add adjuvants. |
| Beauveria bassiana | Various brands | An insect pathogenic fungus. Apply every 7 days if warranted. Do not tank mix with most fungicides and wait 48 hours after application to apply a fungicide. |
| Spinosad | Entrust Naturalyte, Entrust SC | A botanical and insect growth regulator (IGR). Must contact insect. Repeated applications as necessary. Label permits low-volume application. Do not exceed 22.5 oz/acre per application. |
By following these guidelines, you can effectively manage thrips infestations and keep your African violets thriving.
