- Author: Patricia Lazicki
General situation
The colder weather has delayed thrips population development by a few days, but overall the outlook hasn't changed much from last report. The oldest tomatoes have been in ground for more than a month, and planting is progressing rapidly.
Thrips population
Most thrips are currently 1st-generation adults (Fig. 1). These adults are feeding and laying their eggs; second generation egg hatch is predicted to peak in the next 1-2 weeks. Generation 2 adults are predicted to peak in the second week of May.
Figure 1. Predicted thrips population development, based on current temperatures and seasonal long-term averages
For newly hatched thrips to acquire TSWV, the larvae have to feed on an infected plant (Fig. 2). Only thrips that have acquired TSWV as larvae can spread it by feeding as adults. In known hotspots, promptly controlling winter weeds like little mallow (cheeseweed), mustards, and prickly lettuce which may have served as overwinter hosts for TSWV reduces the likelihood of these early thrips generations acquiring the virus and spreading it to young tomato plants.
Figure 2. Thrips life-cycle and relationship to TSWV transmission. Time at each stage is shorter in hotter weather. **The pupal stage, which is in the soil and does not feed, can last much longer in cold weather. Graphic courtesy of Prof. Diane Ullman. Photos by Jack Kelly Clark, UC IPM Program emeritus.
- Author: Patricia Lazicki
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 2nd-generation egg hatch and adults are predicted to peak at 4/25 and 5/11, respectively, and 3rd generation adults predicted to peak in early June.
- Author: Patricia Lazicki
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 (1st-gen egg hatch peaked 2/23, and adults are predicted to peak 3/29). Currently 2nd-generation egg hatch and adults are predicted to peak at 4/22 and 5/8, and 3rd generation adults predicted to peak in early June. Thrips control is most effective in the 2nd and 3rd 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.
- Author: Neil McRoberts
- Author: Patricia Lazicki
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 1st generation egg hatch (peak 2/23). Generation 1 adults are predicted to peak at the end of March (3/30). Currently 2nd 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 3rd and 4th 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.
- Author: Neil McRoberts
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.