General situation: Things have warmed up recently. Our reference weather station for Merced is running about a week ahead of the last of year and a whole 12 days 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. Things are set to cool off again slightly toward the end of the 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 31st, Generation 4 will peak on June 24th, and Generation 5 on July 15th. 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: We haven't had any reports of TSWV in the central San Joaquin Valley yet this season, but with crops well established and 2 to 3 generations of thrips probably now active since the winter, it's possible that any TSWV that over-wintered in weeds will have had the opportunity to start to move into crops. Although we've never had a detection of resistance-breaking TSWV as far north as Merced Co., Resistance-breaking TSWV has already been found further south in the Valley this season. In the long-term, spread of resistance-breaking strains is a concern. 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 are just past the peak for generation 2 (May 6th) Unless you are seeing early TSWV symptoms, or consider crops to be at high risk, it's unlikely that generation 2 will be carrying much virus, but if you are concerned about early attack please contact a UCCE adviser to discuss your situation. Generation 3 adults are predicted to peak on 6/3 (although, with hot weather on the way this might come forward a few days), with generation 4 following in the last week of June. Those two generations are the ones we would typically expect to be carrying the first significant levels of TSWV. 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 second 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.
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 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.