Thrips numbers are building up and the second post-winter generation of adults is predicted to be peaking right now. The third generation will probably peak in mid-May (current peak date is projected to be 5/17) but lettuce production will mostly have left the area by then. It looks as though this year has continued the trend we've seen in the recent past, and there hasn't been a major issue with TSWV in the spring lettuce. Virus incidence appears to be generally low across the valley at the moment.
Risk to spring lettuce from TSWV is generally low in the San Joaquin Valley because virus incidence is reduced by the winter period and lettuce cropping is generally finished by the time the levels of virus start to build up again in late spring and early summer. All that being said, occasional and serious attacks can occur, particularly when thrips numbers are high early in the season. In that situation thrips feeding damage can be a problem in its own right, independent of whether there is a high level of TSWV. For more detailed advice on management of thrips refer to the UC IPM guide, which can be found by clicking on this text.
This year we're off to a generally warm start, which means thrips numbers will probably build up relatively quickly. The first generation post-winter is at peak adult numbers right now. The peak of egg hatch for the next generation will be early in the first week of April, with adult numbers peaking again about three weeks later (around April 24th).
The warm winter and spring weather means that Western Flower Thrips (WFT) degree-days are running about one whole generation time earlier than the 30 year average. The projection model for WFT running on the Five Points CIMIS weather station shows that adult numbers for generation 1 of 2015 probably peaked around March 14th. Extensive studies over the last five years in the Central Valley have shown that the level of TSWV is very low at this time of year, so even though thrips numbers are building, the immediate risk of widespread TSWV infection in spring lettuce is low.
Because the season is warmer than usual, WFT will have another generation peak before the end of the traditional spring lettuce period in April. The projected adult peak is around April 6th. The risk of TSWV transmission by this generation will be higher than for generation 1, but still relatively low, unless there is a localized source. If thrips numbers are high enough, treatment to prevent direct feeding damage might be justified.
Following the long cold snap and exceptionally dry winter thrips populations are currently low. The weather fronts which passed across northern California earlier in the month did stimulate a flush of germination in winter annual weeds, but sources of TSWV inoculum are currently not abundant. Overall risk to spring lettuce is low.