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Onion Pests -  Western Flower Thrips





Figure 7.7
Onion thrips feeding causes silvery streaks. 

Figure 7.8
Thrips are most commonly found in the neck of the onion.

Figure 7.11
Banded thrips adults are important predators of western flower thrips.

Western Flower Thrips (Frankliniella occidentalis)

DESCRIPTION:

Western flower thrips (WFT) are similar in appearance to onion thrips; however, adult females are slightly longer (0.08 inch or 2.0 mm), more yellow in color, and have 8-segmented antennae, red eyes, and longer setae (hairs) on the segment just behind the head (prothorax).

LIFE HISTORY:

WFT reproduce sexually; males and females are common. WFT populations typically increase in the late summer to early fall, especially on plants that have bolted and produced seed.

DAMAGE:

WFT injure onion plants similarly to onion thrips; however, their populations are typically 10 to 100 times lower, and so cause much less onion crop damage.

MANAGEMENT OF WESTERN FLOWER THRIPS

CULTURAL:

        • Remove or destroy volunteer onion plants and debris. Thrips can use these as overwintering hosts from which they can infest newly emerging onion plants.
        • Avoid planting onion adjacent to alfalfa fields when feasible, since alfalfa harbors overwintering thrips.
        • Plant younger fields upwind from older fields to avoid thrips infestation of less mature fields downwind.
        • Inspect transplants for thrips infestation and discard infested onions. Thrips from these transplants may be different strains than those that occur in Utah. Introducing different strains may increase insecticide resistance and transmission of iris yellow spot virus and other diseases.
        • Fertilize onions with adequate, but not excessive amounts of nitrogen. In Utah, it is recommended that no more than 200 lbs of nitrogen per acre be applied in multiple applications throughout the onion growth period. Moderate, consistent availability of nitrogen has been associated with a healthy onion crop and reduced onion thrips densities.
        • Mulch with straw or other materials. Mulch placed on the plant bed may reduce onion thrips populations and improve onion growth. Mulches suppress thrips populations by enhancing predator populations, creating barriers that prevent the resting stage larvae from accessing the soil, and lowering soil temperatures, slowing thrips development and population increase.
        • Use trap crops. Plant small strips or patches of an alternate crop (buckwheat, carrot, crucifer, cucurbits, and some flowers, such as phacelia, are highly attractive to onion thrips) within an onion field to attract thrips. These alternate crops can then be disked under or sprayed with an insecticide when thrips populations increase.
        • Use overhead sprinkler irrigation. Sprinklers can reduce thrips populations by physically washing thrips from plants and forming a crust on the soil surface, reducing thrips’ ability to seek shelter there.
        • Plant onion varieties that are more tolerant to thrips injury. Varieties with tolerance to thrips injury require fewer insecticide applications. Using less insecticide can result in lower control costs, slower development of resistance, and preservation of natural enemies. Onion varieties with an open neck growth and dark, glossy leaves are less attractive to thrips than varieties with tight necks and lighter green leaves. Studies conducted in Colorado showed relative susceptibilities of some onion varieties:
          • Highly Tolerant: 'White Keeper'
          • Moderately Tolerant: 'El Charro', 'Snow White', 'Vega', 'X201', 'Zapotec'
          • Susceptible: 'Blanco Duro', ‘Brown Beauty’, ‘Brown Beauty 20’, ‘Colorado 6’, ‘Sweet Perfection’, ‘Tango’, ‘Valdez’, ‘White Delight’
          • Highly Susceptible: 'Early Red Stockton’, ‘Mambo’, ‘Red Baron’, ‘Redman’

CHEMICAL:

The high frequency of insecticide use for managing onion thrips, as high as eight applications per season, has caused rapid development of resistance to several classes of insecticides, including organophosphates, synthetic pyrethroids, and carbamates. Because onion thrips reproduce without mixing genes with males, have a high reproduction potential, and short generation time, the likelihood of insecticide resistance is increased. Despite the ease of use and widespread accessibility of many insecticides, they are most effective when used in conjunction with other management practices as described above.

BIOLOGICAL:

Natural enemies of onion thrips include the banded thrips (Aeolothrips spp.) big-eyed bug, minute pirate bug, green lacewing larvae, and predaceous mites. These predators, however, are usually not abundant in onion fields until late in the summer when most thrips feeding damage is already done. Incorporating management practices that reduce the use of toxic insecticides and increase cultural practices will promote onion thrips predation.