Onion thrips

Onion thrips are a pest of onions, garlic and other related plants as prefered hosts. 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.

Symptoms
Thrips 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).

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.

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.

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.