- Author: Neil McRoberts
General Outlook Well, here we are, the 2022 season is open for business. Transplanting is under way (or soon will be). The first half of the winter was wet and cold, but since the turn of the year, we have seen next to no precipitation in the region. Temperatures have been more or less normal for the time of year and all in all it has probably been a reasonably good year for thrips survival as pupae in the soil over the winter. Unless we get some serious cold wet weather to round off the spring the prospect is that thrips numbers are likely to start building quickly over the next couple of months. The next 10 days or so will see changeable weather with a cool weekend and start to next week (with a small chance of rain showers on Saturday) giving way to a run of three or four days with temperatures in the low 90's. Those kinds of temperatures will accelerate thrips development.
Population projections Based on temperatures experienced at the Stratford monitoring site, the current projection is for generation 1 adults to peak around March 25, with generation 2 egg hatch peaking in mid April and generation 2 adults peaking in the first week of May. Unless you are in an area with a history of early TSWV infections and you suspect that the virus is already present in weeds or other crops close by, the chances are that the first couple of generations of thrips will be relatively free from virus and usually do not pose a serious threat to new tomato plantings. We've had a few reports of TSWV in lettuce in the area so be aware that there is some virus around and we might see it move into tomato early this year. Contact your local UCCE advisor if you are concerned about your situation and wondering when/if to start treatments to suppress thrips.
- Author: Neil McRoberts
General outlook A week of 100+ F daily maximum temperatures and warm nights will give thrips development a boost over the next week. A gradual return to mid 90's highs is expected by the second week of June. The current mini heat wave won't be enough to cause any significant mortality in thrips. Degree day accumulation for thrips this season is about two days behind last year and roughly 6 days ahead of the 30-year normal.
Population projections Generation 3 post winter adults peaked in the last day or so. Generation 4 adults are currently projected to peak around June 19th. These two generations are the first that are likely to have significant levels of TSWV in them, so any plans to apply insecticide specifically to control thrips and suppress TSWV should be scheduled sometime in soon. We've seen some TSWV already in a few areas. Levels are generally low (below 1%), but there are a couple of hot spots that show there has been some early secondary spread of the virus within the crop. Secondary spread would be indicated by seeing short runs of two or three plants with symptoms clustered together. Under these circumstances treatment to suppress further spread is recommended. If you are not seeing any signs of TSWV in your area you may want to consider delaying treatment and re-assessing the situation for generation 5. If you want further advice on management options for your situation please contact your UCCE advisor.
We are now finding SW-5 resistance breaking strains routinely in Fresno and Kings counties. These strains are now the dominant type in this region and relying on SW-5 resistance to avoid TSWV damage is not recommended. SW-5 varieties that have other favorable agronomic characteristics can still be used as part of an IPM approach for disease control.
Please report any unusual virus-like symptoms to your local UCCE adviser. In addition to TSWV there are a number of other viruses that might show up in tomato. Most of these will have minimal effects on yield in processing crops, but we are always on the look out for reports of viruses to help get a complete picture of what's happening across the region.
- Author: Neil McRoberts
- Contributor: Thomas Turini
We're seeing some early TSWV in areas of western Fresno Co. Here is up-to-date information from local UCCE adviser, Tom Turini:
In some specific areas within Fresno County, which include Five Points, Huron and Cantua Creek, treatment for thrips is suggested based on presence of the virus and the stage of thrips population development.
Within parts of western Fresno County (Five Points), Tomato spotted wilt virus (TSWV) was initially detected in tomatoes in early April. By 16 Apr, areas within a few processing tomato fields had as much as 5% of the plants showing symptoms. In the area north of Huron and west of Five Points, the virus was detected in lettuce earlier in the season. The TSWV strain in the tomatoes and in the lettuce can move within and cause disease of tomato plants with the TSWV-resistance gene (Sw5). Therefore, the pathogen that poses a risk to tomato production is present in this area.
Based on the Western flower thrips predictive population development model, peak egg hatch for generation two was on the 20 April and a substantial population of nymphs now exist. Peak for second generation adults is expected in early May. Thrips must feed on an infected plant as a wingless nymph to be capable of transmitting the virus as an adult. The winged, mobile adult can spread the virus within fields and to other fields. Therefore, an application of insecticides such as Radiant, Success or dimethoate to target the nymphs feeding on infected plants has the greatest potential of reducing spread of this virus.
- Author: Neil McRoberts
General outlook We're heading into to a run of slightly warmer weather which will last about a week and accelerate thrips development. With some planting still in progress and many crops at an early stage of growth we're already seeing some TSWV in the area, so we'll be keeping a close eye on the situation this season. Please contact your UCCE local adviser if you see TSWV symptoms in your crop.
Thrips and TSWV situation The projection model is showing generation 2 egg hatch peaked a few days ago and adult numbers for generation 2 will peak in the first week of May. Generation 3 adults are likely to peak right at the end of May.
Low risk situations
If you don't know of any local sources of TSWV and crops have just been planted it might be worth delaying treatment until mid June. This would be our regular recommendation in typical years. The risk is that it will allow generation 2 and 3 adults to breed and lay eggs, but if there are no local sources of TSWV the chances are that delaying thrips treatments to target generation 3 will be sufficient to delay TSWV from building up and causing severe damage.
High risk situations
If you do have local sources of TSWV, if you've planted early and are worried about early entry of TSWV to your crop, or if you do not want to take the risk of allowing generation 2 to bring TSWV into the crop, consider treatments in the first half of May. Treatments around this time will target generation 3 juveniles developing on plants and also the tail end of the generation 2 adults.
- Author: Neil McRoberts
General situation: We're a little bit late getting the clock started this season, but here is the first post of the 2021 thrips/TSWV risk evaluation. The aim is to give an update on a roughly 7-10 day interval until the end of June and then we'll take a look how the season is shaping up and most likely, reduce the frequency until harvest starts.
It's no secret that we've had another dry winter and spring. There has been enough rain for the usual flush over-wintering weeds, some of which may act as sources for TSWV, but the soil has been dry enough that over-winter survival of thrips pupae was probably on the high side. Overall, things are set up so that we could see relatively large thrips populations early in the season this year.
We finished last season with further confirmations that SW5 resistance-breaking strains of TSWV are now common in western Fresno County. Without a detailed survey to establish what proportion of isolates is carrying the resistance mutation, or how far the resistance breaking strains have spread from their initial hotspots it's hard to give an accurate assessment of the risk that resistance breaking will show up this season in any specific area. If resistance-breaking strains have been confirmed previously in your neighborhood the chances are that they'll be there again this season. If you haven't yet seen resistance breaking, it's harder to say what will happen, but a cautious approach would be to assume that SW5 resistance might not be enough on its own to guarantee a clean crop. Please keep an eye on any varieties carrying SW5 resistance and report any symptoms that might be TSWV to your UCCE local adviser so that we can stay on top of the situation.
Immediate outlook: Planting is underway, or for some growers focusing on main crop harvests may well be done. The first post-winter generation of adults is peaking right about now. The next peak in egg hatch is predicted for April 16th and the peak of the second generation of adults at the start of the first week in May. Based on experience, we don't expect these two generations of thrips to be carrying much TSWV, but local hostspots could happen. Based on temperatures so far this year, as far as thrips are concerned, Fresno County is 4 days behind 2020, 1 day ahead of 2019 and about 6 days ahead of the 30-yr norm.