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.
General situation We are firmly into the 2020 season and the weather is starting to settle into its summer pattern. According to the data from our reference weather station the Merced area is running slightly ahead of the 30 year average. A high in the mid 90's is forecast for Thursday with temperatures building to that peak and then declining again slowly. That sort of temperature pattern will certainly accelerate thrips development so we might see some changes in predicted dates for generations as we pass through the next 2-3 weeks.
Thrips generation timings According to the degree-day projection, we passed the peak for generation 2 (May 2nd) in the last few days. We have already had some confirmed cases of trace levels of TSWV in crops in Fresno Co. This is quite early for the virus to show up (also see the Special Note, below). If you are concerned about early attack please contact a UCCE adviser to discuss your situation. Generation 3 adults are predicted to peak on 5/28 (although, with hot weather on the way this might come forward a few days), with generation 4 following around 6/20. Those two generations are the ones we would typically expect to be carrying the first significant levels of TSWV and with TSWV already confirmed in processing tomato, if you're considering the use of insecticides to control thrips/TSWV action in the next month is likely to be the most cost-effective time to make applications. Again if you want to discuss specifics, please contact your local UCCE adviser. The current projection runs until July 1. We'll be extending the projections this week and posting another update in 7-10 days.
Special Note: SW-5 resistance breaking strains of TSWV have been detected at multiple sites in Fresno Co in the last week or so. Although we haven't yet seen issues with resistance-breaking strains as far north as Merced, be aware that these strains of the virus are out there and likely to spread over time. If you see suspicious symptoms in SW-5 varieties please let your local UCCE adviser know. We can arrange for samples to be collected for testing.
We've been a bit slow getting the thrips risk and TSWV updates up and running this year. We hope you are all safe and well. We also want to say thank you to everyone involved in food production and agriculture in general for keeping the food chain flowing during the COVID-19 epidemic.
Current outlook After a warm, dry winter we're expecting thrips pupal survival over the winter to have been high and the numbers to pick up quickly. The recent late season cold/rain system that passed through over the weekend will have checked them somewhat. Based on the Davis weather station, we're currently projecting generation 1 adults to peak in the middle of April, with generation 2 in the first week of May and generation 3 in the first week of June.
Generally, generation 1 doesn't present much of a risk for carrying TSWV into processing tomato. In recent years, TSWV hasn't been present until generation 3 or 4, but we will gather up status report before our next update in case there's any virus out there early this year and action is needed on generation 2.
NOTE: The dataset for the Five Points CIMIS station (which we've been using for several years) is currently corrupted so we've switched to Stratford CIMIS station in the meantime.
As ever, if you're seeing high thrips counts or obvious signs of TSWV we'd like to hear from you via your local UCCE adviser. Best wishes for a good growing season and for as much normality as possible in these difficult times.
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.