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.
General situation Welcome to our updates on Western Flower Thrips development for 2017. The cool wet winter and early spring mean that degree day accumulation for thrips is currently running behind the 30 year average. The wet weather and saturated soils are also likely to have caused higher than normal mortality for thrips pupae, so we're expecting numbers to be low, at least to start with this season - but let us know if that's not what you're seeing!
Projected thrips development As in previous years we're using January 1st as our start date for the biological calendar. Assuming that's approximately correct, the first post-winter generation of eggs probably hatched around February 21st and adult numbers peak around March 27th. Our current projection is showing generation 2 egg hatch around April 21st and generation 2 adults to peak around May 9th. Generation 2 is the very earliest we would expect to see thrips starting to move TSWV around. Again, let us know (through your local UCCE adviser) if you're seeing TSWV early in the season.
Resistance breaking TSWV Last season there were a number of confirmed cases of resistance-breaking(RB) strains of TSWV in mostly fresh-market tomatoes, but also in a processing tomato field, in the San Joaquin Valley. The RB strains of the virus that can overcome (break) the resistance conferred by the SW-5 gene that is now widely used in fresh-market and processing varieties. Based on last season's observations the RB strain was quite localized (Cantua Creek, Firebaugh and Rt. 198), but we are keeping the situation under scrutiny and will be surveying crops this season to assess how things are developing. We can report that we have detected the RB strain in weeds (sowthistle) with tomato spotted wilt symptoms from Cantua Creek and Rt. 198; thus, the RB strain has been able to survive the winter. We'll keep you posted on what we're seeing through these updates. If you want to know more about the situation contact your local UCCE adviser.
Daytime high temperatures will be in the high 70's to mid 80's over the next few days before things cool off a little on Friday, when the San Joaquin valley is forecast to see some rain. These temperatures are ideal for thrips development and the season has a little ahead of the rate we would expect based on the 30-year average temperatures; the second post-winter generation of adults is expected to peak on 4/30, with the third generation expected to peak around 5/31. So far we haven't had reports of TSWV outbreaks in the area, so there's a good chance to stay on top of the TSWV risk if you are planning to use an insecticide to suppress thrips development. With low virus levels in the area, treatments applied to knock down the second thrips generation (in late April) or the third generation (in late May) will likely have a good effect. Overall the immediate TSWV risk generally low.
This is the first update on western flower thrips for 2016.
It has been a warm winter so far. Warm weather means faster thrips development, so this year might see the optimum treatment period for thrips in processing tomato occurring slightly earlier than normal (whatever normal means these days...). In the Merced area the first generation thrips egg hatch happened in the third week of February, with peak adult numbers expected in about 18 days from now. The second generation egg hatch will happen around April 18th on current projections. The cooler, wet weather forecast for the next few days is likely to slow development and increase thrips mortality so despite the warmer general conditions thrips numbers might not build too quickly, at least in the immediate future. So far we have no reports of TSWV so risk of damage is currently low.
Which crops will benefit from thrips treatment?
Our last update indicated that thrips pressure was building and with some TSWV already out there, it was important to treat for thrips now to prevent TSWV really taking off. We scouted several crops in western Fresno Co. last week and generally found a low level of TSWV in non-sw5 varieties. Early-planted crops are already well into fruit filling and are unlikely to benefit from a thrips treatment at this time. However, late planted crops that are only just flowering or are pre-flowering are still at the most sensitive stage for TSWV infection. Any thrips control planned for this time should focus on late planted crops that have not reached mid fruit set.