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.