Onion Thrips

Information

Common Name:

English: Onion thrips, thrips, thunderflies

Spanish: Trips de la cebolla, trips

Scientific Name:

Thrips tabaci (Lindeman) (Thysanoptera: Thripidae)

Geographic Distribution:

World wide. It is an important pest of onions, other onion relatives and several crops in most parts of the world. Thrips can colonizes crops from sea level up to 2000 meters above sea level.

Recognition and Diagnosis:

Immature

Size and Form: Very small. From 0.5 to 1.2 mm. The form of the thrips body is elongated, elliptical and slender. Their eyes have darker coloration and are easy to see. Immature thrips have short antennas. The difference between immatures and adults is that immatures do not have wings, so they can not fly.
Color: White to pale yellow.
Location: The majority of immature thrips are found between the young leaf blades at the top of the plant. To observe them you need to separate the leaves from the neck.
Behavior: Immatures prefer to feed on the youngest leaves th leaf. If disturbed, they move quickly to find new refuge at the base of the leaves.

Pupae

Size and Form: Very small. Thrips pupae appear as an intermediate form between the immature and the adult. They have short antennae and the wing buds are visible but short and not functional.
Color: Pale yellow to brown
Location: In the base of the plant neck or in the soil.
Behavior: At this stage thrips do not feed.

Adult

Size and Form: Up to 2 mm. Adults have fully developed wings. The wings are very different from other insects. They have a single longitudinal vein in which there are several hairs connected perpendicular to the vein. The wing appears as fringe with hairs. When at rest, the wings are folded along the back of the insect.
Color: Pale yellow to dark brown
Location: The same as immatures. You can find them also in the flowers.
Behavior: Adults are more mobile than immatures and pupae because they can fly. They are attracted to yellow and white colors. They often will fly to one's clothes or land on exposed skin.

Eggs

Size: Eggs are microscopic and almost impossible to see.
Color: White or yellow.
Location: Eggs are inserted one by one by the females in the plant tissue. Only one end of the egg will be near the surface of the tissue to allow the immature to emerge. Adults prefer to lay their eggs in leaf, cotyledon, or flower tissues.

Host:

Onions, garlic and other related plants are the prefered hosts of thrips. They can be a problem in several other crops such as cabbage, cotton, celery, tomatoes, beans, cucumber and pineapple. You can find thrips in almost any cultivated and weedy plants.

Biology and Ecology:

Thrips can complete the life cycle in 14 to 30 days. When temperatures are over 30°C the life cycle can be shortened to 10 or 11 days. The adults may live up to 20 days. Thrips do not need to mate for reproduction. Females that do not mate will produce only female progeny. Each female can produce up to 80 eggs. In some places of the world the entire population of thrips is compoused of females. This reproductive aspect is very important because from a single thrips a population can be generated in very short time.

Damage and Importance:

Thrips are the most damaging insect pest of onions in the tropics. Thrips have a very peculiar feeding behavior. They start the feeding by piercing and rasping the leaf surface with their mouth parts to release the liquids from the plant cells. In this process, thrips release substances that help predigest the onion plant tissue. Later, with their mouth they suck up the plant content.

Thrips prefer to feed on the young plant tissue on the newest emerged leaves. When the leaf grows, the previous damage produced by the thrips enlarges, leaving empty spaces in the surface of the leaf. The appearance of the damage is silvery patches or streaks on the leaves that shine in the sun. When damage is severe, these small patches can occupy most of the surface of the leaf and the plant cannot adequately photosynthesize. The plant loses more water than normal through the damaged tissues and plant pathogens penetrate the injured plant easily.

In severe attacks the whole plant can turn white or silver and leaves can wither. In injured plants the bulbs may mature faster and the size become reduced. In some tropical countries up to 66% of the onion crop may be lost to thrips damage. Thrips may also serve as vectors of some viruses and other plant diseases, especially the fungus, purple blotch (Alternaria porri).

Control

Sampling and economic thresholds:

Thrips are easily detected by visual inspection of the onion plant. Adults can be monitored by the use of yellow or white sticky traps.

Where to sample:

Commercial monitoring of thrips is done by inspecting individual plants in the field. On the selected plants, inspection should concentrated on the newest leaves. For each plant the number of thrips and the amount of tissue damage should be recorded.
In North America, large onion growers sample up to a total of 50 plants per field with 5 plants per inspection site. Special consideration should be taken to monitoring the crop borders. In general, during the early part of the season there are more thrips in the borders than in the center of the crop because thrips build up in numbers outside the onion crop before they migrate to it. Sometimes control measures should be taken just in the crop borders.

Economic threshold:
There are several recommendations for the economic threshold for onions. You should remember that the economic threshold should be validated for each geographical region given the thrips incidence, the cultivar used, environmental conditions, and the price of the control measures and the onions in the market.
In New York State (U.S.A.), the threshold is 3 thrips per green leaf. In Honduras for small onions producers the recommended economic threshold is 20% of the plants infested with thrips (in this case it only is required to count the number of plants with thrips, not the actual number of thrips.)

Control Strategies:

Biological control:

There are several natural enemies that help in the control of thrips. Unfortunately none of them alone can reduce the thrips populations to a low, non-economical density. Also, the intensive use of pesticides in this crop limits natural enemies activity. There is a need for more study on the role of natural enemies in the onion system.

IPM practices:
Planting season: In most cases thrips are not a problem in the rainy season because the rain washes the tiny insects from the plant. At the end of the hot dry season, thrips populations are at their maximum. In some places it is better not to plant under these conditions because thrips control is almost impossible.; If the only crop in the dry season is onions there should be an onion free period (2-3 weeks) before each planting to interrupt the thrips cycle by removing host plants.
Irrigation: Irrigation of the onions is very important to control thrips. In some places, such as Australia, farmers use overhead irrigation to simulate rainfall and control the thrips. Even more important is to maintain a good water supply to the plants during the whole season. If the onion plant is under water stress the thrips damage may be magnified because the plant is losing large amounts of water from the damaged tissue. Also proper fertilization may help to reduce the impact of the thrips in the plant.
Plot location: Thrips are not good flyers, but they move long distances on the wind. Younger plots should be planted upwind of older plots, relative to prevailing winds, to make it harder for the thrips to find the new plantings.
Seedlings: Direct seeding of onions prolongs the growing season in the field and the susceptibility to thrips infestation. If the crop is going to be transplanted, the seed-beds should be distant from the old plantings and new plots to be planted. It is very important that onion seedlings are clean of thrips before transplanting to the field.
Remove unharvested plant parts: Volunteer onion plants are an important source of infestation for thrips. Remove or destroy all the unharvested plants from the plots.
Plant Resistance: Onion cultivars that have a more open growth characteristic (leaves separated from base rather than tightly bundled) are less attractive to thrips because they provide less refuge for them. Some of the reported cultivars with some degree of resistance to thrips are: White Persian, Grano, Sweet Spanish, Crystal Wax, Yellow Bermuda, Spanish White.
Inter-cropping: Mixed cropping of carrots and onions also may reduce thrips population.

Chemical control:
Pesticide recommendations: Because of severe pesticide resistance problems with DBM around the globe, it is very important to use pesticides as little as possible in an IPM program. You need to contact the local authorities for the specific pesticides to use in your country. It is recommended to make a pesticide screening study to find the best pesticides to use in rotation between the different pesticide families. Remember, you must use only pesticides that are legal to use for this pest and crop in your country.
Spray techniques: The secret of thrips chemical control in onions is the placement of the pesticide. It is necessary for the product to reach inside the plant base of the leaves where the majority of the thrips are located. Using high pressure and high water volume in the application enables this to occur.

References

Andaloro, J. T. and A. M. Shelton. 1983. Insects of Onions and Cabbage. Onion Thrips, N.Y. State Agric. Exp. Stn. Geneva Publ. 750.75, 2.

Andrews K. L. 1984. El manejo Integrado de Plagas Invertebradas en los Cultivos Agronomicos, Horticolas y Frutales en la Escuela Agricola Panamericana. Zamorano Press.

Hoffmann M. P., C. H. Petzoldt and A. C. Frodsham. 1996. Integrated Pest Management for Onions. New York State IPM Program Publication No. 119.

Soni S. k. and P. R. Ellis. 1990. Insect Pest. In: Rabinowitch H. D. and J. L. Brewster (editors) Onion And Allied Crops. Volume II. Agronomy, Biotic interactions, pathology, and Crop protection. CRC Florida.

Credits

Technical content: Alfredo Rueda and Anthony M. Shelton
Production: Ben Shelton, Cathy Weeden, and Linda McCandless
Pictures: From the Archives of the Integrated Pest Management Program at the New York State Agricultural Experiment Station in Geneva.

Contact Form:

For more information regarding this publication, contact Alfredo Rueda (aar4@cornell.edu). You may also contact one of our collaborators in your geographic region that may have more specific technical information on the topic.

Your contributions are important to us

In most cases, farmers and practitioners are the real innovators in pest management. If you have made an improvement or a new system to control this pest we will be happy to publish it here. Please send a description of your discovery to Alfredo Rueda (aar4@cornell.edu).

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