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AVRDC Crop Protection Guides |
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Onion Family
Insect Pests Onion
thrips |
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 Damage symptoms |
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Insect characteristics |
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Where to look |
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Technical information |
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Control |
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Damage symptoms Leaves are silvery or have tiny brownish marks or spots. They may be wilted or distorted. Outer leaves are brown at the tips. In cases of severe injury, leaves drop and bulbs are small and misshapen. |
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Insect characteristics Modified piercing-sucking mouthparts. Thrips are very small and silver-shaped, just barely visible to the naked eye. They are about the size of a flea. Immatures are either yellow or white. Older individuals are yellowish-brown and move quickly. They feed by sucking plant sap. |
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Where to look Thrips are difficult to find by those not familiar with them. Look for areas on the leaves where the damage is most obvious and then carefully focus on that area and look for signs of movement. They often congregate along the leaf veins. |
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Technical information Adults have narrow wings that are fringed with hairs, which are only visible under the microscope. They are distributed worldwide and can reproduce without males. Eggs are inserted within leaf tissues and after two larval instars, the insects drop to the soil to pupate. Definitive species identification should be made by trained taxonomists. Several generations can develop each season; hot and dry weather favors population buildup. Other hosts include tomato, pea, and beet. |
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Control Insecticides are commonly used to control thrips. Approximately 40 insecticides are currently recommended. In SE Asia, numerous combinations of chemicals are often used. Recommended insecticides include dichlorvos, nated, malathion, solithion, bromophos, phenthoate, cartap and methomyl. Thrips are reported to have developed resistance to DDT, endosulfan and dimethoate; these chemicals should not be used. Plowing and crop rotation are useful tools to combat thrips. Plow fields after harvesting to eliminate resting sites for the pest. Avoid planting onion crops in succession. Maintaining moisture in the soil reduces thrip damage. Moist soils develop fungi that kill thrip pupae in the soil. Sprinkler irrigation will seal pests in soil while pupating. Heavy rains drown thrips in plant crevices and soil. Mulching with straw may provide shelter for thrip predators, thereby reducing thrip populations. Silver color mulches are reported to control thrips, especially on shallot, during the seedling stage. With regard to host-plant resistance, white-bulbed varieties are less susceptible than red varieties. Glossy-leafed varieties are also less susceptible. Leaf structure makes a difference. Thrips prefer varieties whose young leaves tend to have a flat side that is closely pressed to the flat surface of the opposite leaf, thus providing many crevices into which the thrips larvae can hide for protection. In contrast, varieties with round leaves with wide angles between the two youngest leaves reduce the number of crevices for shelter. Sticky yellow and blue color traps can reduce thrip damage in the greenhouse. The organic chemicals anisaldehyde, benzaldehyde, cinnamaldehyde and salicyadelhyde can be used as attractants in traps placed in greenhouses. Trapping is not effective in field operations. |
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Last updated: July 2000. Information from: Field Guide: Insect Pests of Selected Vegetables in Tropical and Subtropical Asia. 1995. B.L. Parker, N.S. Talekar and M. Skinner. Publication 94-427. Pest control recommendations added. |
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