WSU ONION ALERTS
THRIPS: Thrips populations are beginning to build up in onion fields throughout the region. In fields with larger onion plants (5-6 leaves), we are seeing a lot of juvenile thrips emerging. We recommend that you monitor onions for thrips often (minimum of once per week). A good strategy is to stop at ten sites in the field and count the thrips on each of five plants. You should find most of them in the neck between the newest leaves. The treatment threshold for thrips depends on the size of the onion plant and the cultivar. Small plants can only tolerate a few thrips without seeing economic injury. Larger plants can tolerate more thrips, but the damage accumulates over the season. Thrips feeding injury can significantly decrease bulb size and yield. Onion thrips also spread the Iris Yellow Spot Virus.
RAPID POPULATION BUILDUP: Thrips have an extremely high reproductive potential. A single female can produce 50-200 offspring (most of these are other females). Males are not needed for reproduction and the ratio of males to females in thrips populations is usually about 1:1000. Development time for one generation ranges from 10 to 30 days, depending on the temperature, and there can be up to 5 generations in a season in the Columbia Basin. Thrips populations have exponential growth – one female thrips and her offspring can become a population of millions in just a couple of months. Let’s do the math (using conservative numbers)… one female thrips lays eggs that develop into 50 more female adult thrips in 15 days > they each lay eggs that develop into 2,500 more female adult thrips by 30 days > they each lay eggs that develop into 125,000 adult female thrips by 45 days > they each lay eggs that develop into 6,250,000 female thrips by 60 days.
THRIPS MANAGEMENT: There are several cultural management tactics that influence thrips populations in onions, but in this report we are focusing on chemical controls. In the Columbia Basin, chemical controls for thrips are usually initiated at the 3- to 4-leaf growth stage. Due to the rapid development of thrips and the presence of protected, non-feeding stages of the pest, several insecticide applications may be needed to achieve adequate control. Tim Waters has accumulated a lot of information in his years of studying insecticides for thrips management and makes the following suggestions:
When making spray applications, it is best to use large volumes of water and high pressure to help the insecticide reach the base of the onion leaves. Be sure to use surfactants or “wetter-spreaders”. Buffer spray solutions (if needed to acidify alkaline spray water) when recommended to do so on the product label. Be cautious when applying insecticides with other products that contain a strong sticker because stickers can reduce insecticide efficacy.
- Radiant (spinetoram) (IRAC 5) – A good early and late season choice. Foliar application or chemigation.
- Lannate (methomyl) (IRAC 1A) – Best used later in the season. Foliar, overhead chemigation or drip chemigation.
- Movento (spirotetremat) (IRAC 23) – A good early season choice. It is slow-acting.
- Agri-Mek (abamectin) (IRAC 6) – A good early or late season choice.
- Exirel (cyazypyr) (IRAC 28) – Good activity to be used early or late season. Foliar application, overheard or chemigation.
- Minecto Pro (abamectin+cyazypyr) (IRAC 6+28) – May be more effective than the two active ingredients alone. A good choice for high thrips pressure. Probably not a good fit for chemigation.
- AzaDirect – This product can provide added benefit when used as a tank mix or in rotation with other insecticides.
- Pyrethroids (Warrior, Mustang, etc.) (IRAC 3A) – Not effective in most areas. Many thrips populations are resistant to pyrethroids. Pyrethroid applications have been known to flare or increase thrips numbers following application.
INSECTICIDE RESISTANCE MANAGEMENT: Rapid development of resistance to insecticides has been a problem in controlling thrips. To prolong effectiveness of insecticides, do not apply insecticides of the same chemical family (same mode of action, same IRAC code) more than two times per growing season, and rotate insecticide modes of action between applications. Pay attention to IRAC (Insecticide Resistance Action Committee) codes. They can be found on the product label. Organic growers do not have as many insecticide options and must be very careful to limit exposure of thrips to the same modes of action repeatedly; that scenario is exactly what drives resistance in thrips populations.
More information on onion downy mildew can be found in the PNW Disease Handbook and there are several good images of disease symptoms on the PNW Veg Extension Group photo gallery. Most of the information presented above was sourced from the writings and presentations of Dr. Lindsey du Toit, WSU Plant Pathologist.
SOURCE: Washington State University Extension
