General situation: We're a little bit late getting the clock started this season, but here is the first post of the 2021 thrips/TSWV risk evaluation. The aim is to give an update on a roughly 7-10 day interval until the end of June and then we'll take a look how the season is shaping up and most likely, reduce the frequency until harvest starts.
It's no secret that we've had another dry winter and spring. There has been enough rain for the usual flush over-wintering weeds, some of which may act as sources for TSWV, but the soil has been dry enough that over-winter survival of thrips pupae was probably on the high side. Overall, things are set up so that we could see relatively large thrips populations early in the season this year.
We finished last season with further confirmations that SW5 resistance-breaking strains of TSWV are now common in western Fresno County. Without a detailed survey to establish what proportion of isolates is carrying the resistance mutation, or how far the resistance breaking strains have spread from their initial hotspots it's hard to give an accurate assessment of the risk that resistance breaking will show up this season in any specific area. If resistance-breaking strains have been confirmed previously in your neighborhood the chances are that they'll be there again this season. If you haven't yet seen resistance breaking, it's harder to say what will happen, but a cautious approach would be to assume that SW5 resistance might not be enough on its own to guarantee a clean crop. Please keep an eye on any varieties carrying SW5 resistance and report any symptoms that might be TSWV to your UCCE local adviser so that we can stay on top of the situation.
Immediate outlook: Planting is underway, or for some growers focusing on main crop harvests may well be done. The first post-winter generation of adults is peaking right about now. The next peak in egg hatch is predicted for April 16th and the peak of the second generation of adults at the start of the first week in May. Based on experience, we don't expect these two generations of thrips to be carrying much TSWV, but local hostspots could happen. Based on temperatures so far this year, as far as thrips are concerned, Fresno County is 4 days behind 2020, 1 day ahead of 2019 and about 6 days ahead of the 30-yr norm.