We're seeing some early TSWV in areas of western Fresno Co.  Here is up-to-date information from local UCCE adviser, Tom Turini:

In some specific areas within Fresno County, which include Five Points, Huron and Cantua Creek, treatment for thrips is suggested based on presence of the virus and the stage of thrips population development.  

Within parts of western Fresno County (Five Points), Tomato spotted wilt virus (TSWV) was initially detected in tomatoes in early April. By 16 Apr, areas within a few processing tomato fields had as much as 5% of the plants showing symptoms. In the area north of Huron and west of Five Points, the virus was detected in lettuce earlier in the season. The TSWV strain in the tomatoes and in the lettuce can move within and cause disease of tomato plants with the TSWV-resistance gene (Sw5). Therefore, the pathogen that poses a risk to tomato production is present in this area. 

Based on the Western flower thrips predictive population development model, peak egg hatch for generation two was on the 20 April and a substantial population of nymphs now exist. Peak for second generation adults is expected in early May. Thrips must feed on an infected plant as a wingless nymph to be capable of transmitting the virus as an adult. The winged, mobile adult can spread the virus within fields and to other fields. Therefore, an application of insecticides such as Radiant, Success or dimethoate to target the nymphs feeding on infected plants has the greatest potential of reducing spread of this virus. 

General outlook  We're heading into to a run of slightly warmer weather which will last about a week and accelerate thrips development.  With some planting still in progress and many crops at an early stage of growth we're already seeing some TSWV in the area, so we'll be keeping a close eye on the situation this season.  Please contact your UCCE local adviser if you see TSWV symptoms in your crop.

Thrips and TSWV situation The projection model is showing generation 2 egg hatch peaked a few days ago and adult numbers for generation 2 will peak in the first week of May.  Generation 3 adults are likely to peak right at the end of May.

Low risk situations
If you don't know of any local sources of TSWV and crops have just been planted it might be worth delaying treatment until mid June.  This would be our regular recommendation in typical years.  The risk is that it will allow generation 2 and 3 adults to breed and lay eggs, but if there are no local sources of TSWV the chances are that delaying thrips treatments to target generation 3 will be sufficient to delay TSWV from building up and causing severe damage.

High risk situations
If you do have local sources of TSWV, if you've planted early and are worried about early entry of TSWV to your crop, or if you do not want to take the risk of allowing generation 2 to bring TSWV into the crop, consider treatments in the first half of May.  Treatments around this time will target generation 3 juveniles developing on plants and also the tail end of the generation 2 adults.

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.

General situation:  Things have warmed up recently. Our reference weather station for Fresno is running about 3 days ahead of last year and a week ahead of the 30-year average for heat accumulation, so far this season.  We're into day 2 of a 4 day mini heat wave and that is sure to speed up thrips development.  Friday is forecast to see the intense heat break a bit, but there might be gusty winds, so may not be suitable for some field operations. Things are set to cool off again for most of next week, but temperatures will still be right in the sweet spot for thrips development, so we can assume that we're into the period of continuous vector activity from now on.  For those who check the website, we updated the projection graphs out to the end of August today, to give you a look at how the season is projected to play out at this point.

The current projections for generation peak times are as follows: Generation 3 adults will peak on May 27th, Generation 4 will peak on June 19th, and Generation 5 on July 9th.  Generations 3 and 4 represent the major risk of TSWV getting into crops before fruit set, so if you're planning to use insecticides to knock down thrips numbers, the next month or so is an important period.  

TSWV: Remember that Fresno Co. is the hotspot for development of resistance breaking TSWV. We have already seen TSWV in some fields in the county and tests done at UC Davis have confirmed that the virus is a resistance-breaking strain. Given the weather conditions and the confirmation of resistance-breaking TSWV already this season, we are anticipating that at least some locations will see heavier damage this season than in previous years.  Now is the ideal time to suppress thrips populations by using a suitable insecticide.  Protection of crops at this time of year delays the infection until the most susceptible crop growth stage (pre fruit set) is past.  If you see TSWV symptoms, especially if the variety carries the SW5 resistance gene, please let your local UCCE adviser know and arrangements can be made for samples to be collected and tested for TSWV and resistance-breaking at UC Davis.