General situation After a dry and warmer weekend, the weather will return to a cooler, unsettled pattern next week, with temperatures in the mid 60's to low 70's and significant chance of further rain showers on several days.  This pattern will prolong the slow start to the season, with thrips developing relatively slowly at the forecast temperatures.  We have projected thrips development out to the end of June, based on current conditions. This takes in the first four generations of thrips and covers the period we would typically think of as the most critical for controlling thrips numbers.  Because we are projecting ahead by two to three months, based on relatively cool early season conditions, we can expect the projected dates for generations to peak to vary by a few days as we settle in to the summer season.

Specific details Generation 2 adults probably peaked a few days ago but a few will still be emerging.  Since there are no significant sources of TSWV this early in the season, thrips are not a risk at the moment.  We are expecting to see Generation 3 adult numbers peak around June 8^th with Generation 4 peaking around June 30^th.  Given the slow start to the season, at this stage we would expect Generation 4 to be the key generation to target to suppress TSWV until the pre-flowering high risk period is past.  We will be actively monitoring the TSWV situation and will be posting updates, especially if anything happens to change our current advice.

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 mostly in the 70's F but with a dip to the low 60's on Thursday as a cold front with rain passes through the region.

Thrips populations and TSWV risk  At the moment the second post-winter adults are projected to peak around the first week of May.  If things warm up significantly that generation 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.  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 high 80's to mid 90's F - perfect conditions for thrips development.  Late-planted crops could have a long way to go before flowering and fruit set, which is when the threat of serious yield loss from TSWV decreases.  The take-home is that with thrips activity certain to increase and crops relatively late, the risk of damage from TSWV could be high if the virus shows up in the near future.

Thrips generation timing We have extended the projections out to the end of July.  Generation 3 adults are projected to peak in the next 5-6 days.  Generation 4 will peak in the last week of June, with Generation 5, peaking around the third week of July.  If you are planning to apply treatments to control thrips/TSWV, the next 4-6 weeks will be the period that will provide the best result for the investment.  Since reported TSWV levels across the area are low, suppression of Generation 4 with treatments applied soon will delay the development of TSWV and allow crops a chance to grow past the most vulnerable period. 

General situation With the cool, wet spring crops and thrips alike are somewhat behind their usual schedule. Temperatures over the coming week to 10 days will start to climb and we will see mid 90's F days around mid-week, with temperatures in the low to mid 80's F the rest of the time.  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 adults will peak around May 9th. We are currently projecting generation 3 adults will peak in the first week of June, with generation 4 adults possibly peaking right at the end of June.  So far this season TSWV incidence is still low so generation 2 adults probably do not represent a high risk of spread of the virus.  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, but 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.  We will be providing updates as the season progresses.  If you want more information about the situation, contact your local UCCE adviser.