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.

General Outlook

Post-winter thrips numbers are building up now.  This week the second generation post-winter is projected to peak.  Following the pattern of the last couple of years for May, TSWV incidence in Fresno Co. is still generally low this year, but one or two crops have shown slightly higher levels.  There is still an opportunity to get ahead of the disease by suppressing thrips numbers.

Projections and thrips management

The weather over the coming week is predicted to have highs in the mid 90s in the next few days then cooling slightly to the mid 80s F which means that thrips will be fully active.  Generation 3 adult numbers are peaking now, with generation 4 projected on June 16.  The build up in thrips numbers over the next 3 weeks will be very fast.  For this reason suppressing thrips now will be important to staying ahead of TSWV and avoiding economic damage in processing tomato.  Any insecticide treatments planned for thrips control and TSWV management should be targeted at these two generations.  Later treatments are likely to be much less effective. Immediate action is strongly recommended.

Protection of sw-5 resistance

TSWV resistant varieties rely on the sw-5 resistance gene.  The availability of effective TSWV resistance in processing tomato varieties is a huge bonus for the industry and a valuable shared resource that needs careful management.  Growing sw-5 varieties without the benefit of insecticide protection to suppress thrips increases the chances of resistance-breaking strains of the TSWV virus developing.  The larger the acerage of sw-5 varieties grown without insecticide protection, the greater the risk of resistance-breaking developing.   For this reason, we strongly recommend that sw-5 varieties are treated like susceptible varieties and, if at all possible, you do not rely on the resistance alone to manage TSWV.

A cool front will move through the region over the next 24 hours with a some chance of rain. Temperatures over the next week are likely to see highs in the high 70'sF to low 80'sF, meaning that thrips development will be steady. The second post-winter generation of adults is emerging around this time and peaked around 4/20.

Currently, there are no reports of serious TSWV outbreaks and virus levels appear to be generally low - as is usually the case at this time of year.  The third and fourth generations of adults are projected to peak in late May (5/23) and mid-June (6/15) respectively. Control focused on these generations is likely to keep TSWV levels below economic damage.  Late planted crops will be more sensitive to TSWV so extra care should be given to protecting any crops which have not reached flowering by late May. 

Welcome to the UC Davis Western Flower Thrips generation time projections for the Fresno area for 2015. The phenology clock is telling us that overwintering adults are present at this time.  Thrips degree-day accumulations are running slightly ahead of the 30 year average so far this year and we're projecting the peak dates for the first two post-winter adult generations at March 26th and May 5th.  Generally, TSWV doesn't begin to increase until the third and fourth generations of thrips are present. We'll be posting regular updates on the situation from now on.