General situation, thrips population development

Temperatures and precipitation this spring are close to historic norms, and the thrips population predictions remain similar to last update. We're currently at the peak of 1st-generation adults; they'll be laying their eggs over the next few weeks. Currently 2^nd-generation egg hatch and adults are predicted to peak at 4/25 and 5/11, respectively, and 3^rd generation adults predicted to peak in early June.

General situation

Winter rainfall accumulations and average temperatures in the Sacramento Valley this year are looking similar to the long-term averages. The drying wind and higher temperatures last week mean our first tomatoes are going into the ground this week.

Immediate outlook

Thrips are likely 1st-generation juveniles (1^st-gen egg hatch peaked 2/23, and adults are predicted to peak 3/29). Currently 2^nd-generation egg hatch and adults are predicted to peak at 4/22 and 5/8, and 3^rd generation adults predicted to peak in early June. Thrips control is most effective in the 2^nd and 3^rd generations.

TSWV isn't transmitted to the eggs, and juveniles must acquire it while feeding on infected plant tissue. Infected adults then transmit the virus to new host plants during feeding. This generation of juveniles can acquire it by feeding on infected host winter weeds, winter crops, or cover crops. Many winter weeds can host the virus, including include prickly lettuce, little mallow (cheeseweed), and sow thistle. Fava bean is a common cover crop plant which can also host the virus. Controlling these weeds can help reduce thrips' ability to acquire and transmit the virus to newly planted tomatoes. This is especially important in old TSWV hotspots.

General situation Thanks to a wetter-than-normal February we're slightly above normal for our average winter rainfall accumulation in Yolo County, with more predicted for next week. The saturating conditions we've seen in some fields in the last few weeks will reduce the chance of survival of overwintering thrips pupae in the soil. However, this year's overwinter survival rate is likely to be greater than it was in 2023, which was significantly wetter and colder.

Resistance-breaking TSWV strains are now widespread in south Sacramento Valley processing tomato fields. A newly discovered strain (“CPN”) was identified in 2023, and appeared to be the dominant resistance-breaking strain in Colusa and Sutter counties. Yolo and Solano county fields contained a mix of the CPN and the Fresno (“YPT”) resistance-breaking strains. So while TSWV-resistant genetics still likely have value, they may not be as reliable as they have been in the past as a sole line of defense.

Immediate outlook  We're currently in 1^st generation egg hatch (peak 2/23). Generation 1 adults are predicted to peak at the end of March (3/30). Currently 2^nd generation egg hatch and adults are predicted to peak at 4/22 and 5/9,
respectively. This puts us a few weeks ahead of where we were in 2023, where Generation 1 egg hatch and peak adult populations were 3/15 and 4/23, respectively. Since this estimate is based on both current weather and average normals, these predictions may shift depending on what the weather is like in the next few weeks.

TSWV isn't transmitted to the eggs, and juveniles must acquire it while feeding on infected plant tissue. Because of this, risk of TSWV transmission begins with the second generation. If we see TSWV early, the 3^rd and 4^th generation are generally the ones to target. Additionally, tomato plants which are infected early will be at highest risk of yield loss. This means that intervention may be most useful in late-planted fields near historic hotspots, since these plants will be at greatest risk of infection by viruliferous thrips. The figure below shows how the early generations of thrips have varied in their timing over the last couple of years compared with our current projects for 2024.

General situation  After a slow, late start to the season, summer has finally arrived and is staying. Northern counties will see temperatures building over the next few days and settling into an extended period with 100+ highs.  All that heat will really accelerate thrips development and increase their activity. 

Thrips generations and TSWV  We have extended the population projection out to September 1.  It looks as though  generation 4 adults probably peaked a few days ago. Generation 5 adults should peak around July 26, with generation 6 following in mid August.  Based on recent scouting, thrips numbers have been low until now, but TSWV is relatively easy to find in crops across the northern counties, albeit at relatively low incidence.  Testing of samples has shown that the strains of the virus now present in the region are all resistance-breaking strains, so varieties with the SW-5 gene will need protection in situations where thrips pressure is high.  

In normal seasons most of the crop would be far enough along that our main concern would be generation 4, but there are quite a few late-planted fields and generations 5 and 6 could bring in TSWV before these crops are big enough to shrug off virus strikes.  Late-planted crops might require additional protection this year, particularly in locations with a history of TSWV damage.