Weather outlook: After further atypical cool, showery weather over the last week or so, the forecast for the next ten days shows a more settled period of dry weather with partial cloud cover and daytime highs gradually increasing towards the low 90'sF. Thrips population development can be expected to be steady over this period.
Thrips population timings: In our last update on 4/16 we projected Generation 3 adults to peak on June 7th and Generation 4 to peak on June 30th. The period of cool weather has slowed development further, delaying the projected generation times by a few days. The projected peak date for Generation 3 is now June 9th, with Generation 4 now projected to peak on July 2nd. We are still not hearing reports of any TSWV in the area, and together with the delay in thrips development, the overall risk level in the upper San Joaquin Valley is low. Depending on crop development stage we recommend targeting either Generation 3 or Generation 4 for any intended thrips treatments. Later planted crops may still be at early, vulnerable growth stages as Generations 3 and 4 emerge; stay on top of the situation and check in with your UCCE adviser to find out if TSWV is being reported as the season progresses.
Weather outlook The next 10 days will see consistent warm, dry weather in the San Joaquin Valley. Highs in the upper 70's or low 80's and overnight lows in the mid to low 50's will allow steady thrips development, and we should see numbers starting to build up.
Generation peak timings since our last update a couple of weeks ago our predicted peak date for generation 3 has moved forward by one day to June 7th, but generation 4 has remained unchanged at June 30th. If you are concerned about TSWV risk these are the two generations where control will have the biggest impact. Knocking thrips numbers down while the populations are still relatively small and while the crop is still small enough to allow good penetration of sprays is a good tactic. Keeping thrips under control at this stage also allows the crop to grow past the really vulnerable stage before and during flowering before there is much TSWV around and helps to reduce the yield impact if the virus does show up. The UC IPM guidelines for thrips control can be found by following this link. Further advice on treatments is available from your local UCCE adviser.
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 8th with Generation 4 peaking around June 30th. 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.