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.