Hello.  This is the first update for 2019 for the UC western flower thrips/tswv outlook for processing tomato.

General outlook  As a reminder, the base temperature we use for estimating thrips development is 45F.  Average temperatures have been at, or above, 45F since early January (although they cooled off significantly when the atmospheric rivers started arriving in February), so we haven't had any significant accumulation of degree days for again until the last week or so.  The 10-day outlook is for further unsettled weather with average temperatures into the high 50's to low 60's, but daily minimum's at, or below, the development threshold.  The take home is that we're not expecting any sudden acceleration in thrips development soon.  The relatively cool, wet soil conditions this spring also probably mean that there has been high mortality in over-wintering soil-borne thrips pupae, so initial numbers of thrips will be low.  If you are seeing things different from this please let us know.

Development forecast  We are currently running the projection model out to the end of April.  The first 2019 generation of adult thrips is currently projected to peak around March 24th.  We're expecting virus levels to be low in the first couple of generations (until thrips pick up tswv from infected weeds or there iinitial infection in early tomato crops).  Depending on when soil conditions allow planting to get underway it's hard to say at this stage of the season how early we'll see significant TSWV starting to appear; watch this space.

SW-5 breaking TSWV strains We have continued to track the spread of resistance-breaking TSWV strains.   To date, all of the detections have been in the San Joaquin Valley, with nothing so far detected in the Sacramento Valley.  We'll be posting a more detailed update in the near future.

General situation  It has been an up and down start to the year for thrips development.  Through February the relatively dry winter, and somewhat warm start to the year in the San Joaquin Valley meant we were running ahead of the 30 year average for degree day accumulation. The last couple of rain events have slowed things down, but we are still ahead of the long-term average.  Temperatures in the week ahead are predicted to be in the mid 70's to low 80's F, with dip into the high 60's on Thur/Fri as a cold front passes over the region (it will probably drop some rain in the Sacramento area).

Thrips populations and TSWV risk  At the moment the second post-winter adults are projected to peak at the end of the month, with the third generation peaking around May 28th.  If things warm up significantly those two generations may peak a few days earlier.  Currently we don't have any reports of TSWV activity in overwintering hosts and with the very low thrips populations now, the overall risk is low.  Our best guess at the moment is that we won't see significant thrips/TSWV activity in tomato until the third generation this year, which will be the first one to target, depending on whether TSWV has appeared, but please speak to your local UCCE adviser for more specific advice if you are concerned.

Resistance-breaking TSWV  Strains of TSWV with the ability to overcome SW5 resistance are now established in the San Joaquin Valley and you should expect to see them throughout in the Central Valley in the near future. Resistance breaking TSWV has been picked up from spring lettuce this season and there are some patchy outbreaks in that crop.  Do not assume that SW5 varieties will be able to escape TSWV damage without additional disease management practices.  Our advice is to target early generations of thrips entering tomato (see above) and, where possible, to rogue out infected plans showing symptoms of TSWV early in the season.  Remember, the impacts of TSWV are much smaller in plants infected after fruit set, so the aim should be to delay infection as much as possible, to avoid economic impacts.

General outlook After the extended cool spring, it looks like we have settled into typical summer weather for the Central Valley.  Temperature projections for the next 7-10 days are in the mid 90's to low 100's F - good conditions for thrips development. There are numerous reports of TSWV in processing and fresh-market crops across the area.  TSWV incidence has increased substantially over the last 10-12 days and there are reports of obvious TSWV in SW5 resistant varieties.  In addition to TSWV we are also seeing some curly top virus and alfalfa mosaic virus.  Treatments to suppress thrips and prevent further spread of TSWV should be applied without delay.

Thrips generation timing We have extended the projections out to the end of July.  Generation 3 adults peaked about  one week ago.  Generation 4 will peak around June 16th, with Generation 5, peaking around the first week of July.  If you are planning to apply treatments to control thrips/TSWV, the next 2-4 weeks are a critical period for getting effective control on thrips populations.   Since reported TSWV levels across the area are sporadic, but on the increase, suppression of Generation 4 with treatments applied soon is really important in preventing a large build up of TSWV.  Effective control now will delay the development of TSWV and allow crops a chance to grow past the most vulnerable period. 

General situation With the wet spring crops and thrips alike are somewhat behind their usual schedule. Temperatures over the coming week to 10 days will climb into the 90's and might peak at 100F on Thursday.  Combined with an absence of prolonged rain, this pattern should see thrips development accelerate.  There is a possibility of scattered thunderstorms and rain showers over the next 10 days, but it's not clear at the moment if the low pressure area that would cause those will track across northern California.

Thrips generation timings  We have extended the projection interval out to the end of June.  The current projection shows generation 2 egg hatch peaked around mid April and generation 2 are peaking now. We are currently projecting generation 3 adults will peak in the last week of May, with generation 4 adults possibly peaking around mid May. TSWV has been detected in the Fresno region this season, but incidence is still low so generation 2 adults probably do not represent a high risk of spread of the virus.  However, resistance breaking strains of the virus are known in this area, which means that good thrips control is even more important than it has been in the past few years.  Remember, controlling thrips numbers early in the season delays the spread of TSWV and gives crops a chance to get past the most susceptible stages, pre-flowering, before the virus incidence begins to increase.  If you're planning to invest in thrips treatments this season, treatments will be most cost-effective if applied to control generation 3 or possibly generation 4.  Treatments can be scheduled from now onward through the end of June.

Update on resistance-breaking TSWV  So far, resistance breaking strains of TSWV have only been detected in Fresno Co, not in the upper San Joaquin Valley or Sacramento Valley.  We know from surveys this year that the resistance breaking strains did over-winter, so there are likely to be some further outbreaks this year, most likely close to the areas where they occurred last year.  Action to remove weeds that can harbor TSWV has been taken in the hotspot areas. We will be providing updates as the season progresses.  If you want more information about the situation, contact your local UCCE adviser.