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 increasing towards the mid 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 May 27th and Generation 4 to peak on June 21st. The period of cool weather has slowed development somewhat, delaying the projected generation times. The projected peak date for Generation 3 is now June 1st, with Generation 4 now projected to peak on June 24th. There are reports of low TSWV incidence in the area. We also detected SW-5 resistance-breaking strains of TSWV earlier in the season, but the overall risk level remains low to moderate at this time. Depending on crop development stage we recommend targeting Generation 4 for any intended thrips treatments if they have not already been made. 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 Joaqin Valley. Highs in the upper 70's to upper 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 May 27th, and generation 4 has advanced by one day June 21st. 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.
Resistance-breaking TSWV Strains of TSWV that can break the SW5 virus resistance in both processing and fresh market tomatoes have been found in Fresno county for the last couple of years. It would not be advisable to rely on this resistance to keep TSWV at bay, and a disease management plan should be followed to reduce the risk of yield loss. Contact your local UCCE farm adviser for more information.
General situation After a dry and warmer weekend, the weather will return to a slightly cooler, unsettled pattern next week, with temperatures in the mid-to-high 70's and slight chance of 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 May 30th with Generation 4 peaking around June 22nd. 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.
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.