Harvest is now well underway and despite high numbers of thrips TSWV incidence remains low across the region. Only late-planted crops are now at risk from serious yield loss from TSWV. Yield loss is worst when plants are infected before fruit set. If any crops are still pre-set they should be considered as priorities when considering insecticide application over the next couple of weeks. Generation 5 of the thrips is expected to reach peak hatch in the next day or so and generation 6 will peak on August 13th.
Temperatures are predicted to peak at 100+ F at the end of this week before returning to the high 80s F early next week. Thrips will develop rapidly under these conditions.
Thrips numbers are now high throughout the San Joaquin Valley. In the Merced area generation 3 adults peaked on May 28th and generation 4 adults will peak around June 17th. TSWV incidence is still low so the the next 10 days to 2 weeks are an ideal period for insecticide application if you are considering treatment for thrips/TSWV. Spraying at this reduce generation 4 thrips and check the spread of any TSWV that is already present.
Recently planted crops are at higher risk from TSWV damage because thrips numbers are now high and the effects of the virus are more severe when plants are infected while still young.
The temperature has started climbing over the last week and the next seven days will see figures in the mid to high 80's F. This is the ideal range for thrips development so we can expect the current (second) generation to develop right on schedule. Peak egg hatch was projected to occur on or around April 17th and adults from that hatch will reach peak numbers around April 28th, about 10 days earlier than further north in the Sacramento Valley and delta. However since these thrips are the first generation which will have had a chance to grow on tomato as juveniles this year, with very low levels of virus around, there is only a small risk of any immediate spread of TSWV. Depending on how much TSWV we see in crops over the next few weeks we'll be able to give an indication of which generation should be targeted for any insecticides that are planned this year. Watch this space.
The first generation of post-winter adult thrips were expected to peak (i.e. reach maximum size) on March 28th. We are currently projecting that peak egg hatch for generation 2 will occur around April 16th. Generation 2 juveniles from that egg hatch will be the first opportunity for thrips feeding on tomato to acquire TSWV if there is any present. Given the very low levels of TSWV in planting material the risk of this happening is likely to be low. The recent period of unsettled weather will give way to more stable, warmer conditions next week with temperatures in the mid to high 70s. We can expect thrips activity to start to increase under these conditions. At this point in the season we are 4 days ahead of 2012 and 11 days ahead of the 30 year average for accumulation of thrips degree days. This indicates that thrips activity may be above average in the early part of the season this year and the situation should be monitored for any indications of early TSWV.