Thrips
Onion Thrips
Description
Adult: Adults are about 1.5 mm long. Their yellow and brown bodies are elongated with two pairs of fringed (hairy) wings. They have beak-like mouthparts, gray eyes, and seven-segmented antennae.
Larva: Early larvae, instars I and II (0.5-1.0 mm in length), are active feeding stages. Larvae are white to pale yellow, have an elongated and slender body, and resemble adults but without wings. Antennae are short and eyes are dark in color. Early larvae feed on new leaves in the center of the onion neck. Late larvae, instars III and IV (1.0-1.2 mm long), are inactive, nonfeeding stages. They are pale yellow to brown with a stout body. Antennae are bent to the head and wing buds are visible. They are found in the soil, at the base of the onion plant neck, and underneath bulb scales.
Egg: White to yellow, kidney-bean-shaped, and microscopic, eggs develop within leaf tissue with one end near the leaf surface.
Life History
Onion thrips is the dominant thrips species in onion fields. They overwinter as adults and become active in the spring, dispersing into onion fields. In Utah, females reproduce asexually (parthenogenesis) and insert eggs individually into leaves. Females will lay eggs for about 3 weeks. A complete generation requires 3 to 4 weeks during the summer months, and five to eight generations may occur each year. Thrips populations increase rapidly under hot, arid conditions, leading to economic crop losses.
Western Flower Thrips
Description
Western flower thrips (WFT) are similar in appearance to onion thrips; however, adult females are slightly longer (2.0 mm), more yellow, and have eight-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
Western flower thrips and onion thrips are the two most common vectors of tomato spotted wilt virus (TSWV) in solanaceous crops. See the TSWV in the “Disease Management” section. In addition to virus transmission, thrips will feed on leaves, developing Solanaceous Crops buds, flowers, and fruits, and if populations are high, can cause economic loss. Typical symptoms are “rasping” and stippling injury on leaves, and stunted buds, flowers, and fruits. Thrips feeding on the surface of well-developed fruits can cause scarring. An abundance of dark tar spots of thrips frass can contaminate fruits.
Monitoring
Use yellow or blue sticky traps in susceptible fields from seedling through flowering to determine the magnitude of the thrips population.
Management
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. Alfalfa harbors overwintering thrips.
- Plant younger fields upwind from older fields. 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. We recommend applying no more than 200 pounds of nitrogen per acre. Moderate, consistent nitrogen availability 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, rapidly developing resistance to several insecticide classes, 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 onion thrips enemies 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.