Effective control of western flower thrips (WFT) to prevent cosmetic scarring and contamination is important in spring lettuce crops, and now that INSV has been found infecting plants in Yuma lettuce, management becomes even more important.  For most of the growing season, WFT numbers have been below average based on sticky trap counts, field reports and population densities here at YAC. However, in the past week WFT populations have increased in most growing areas. This is not surprising as we typically observe “bioconcentration" of WFT during March and April on late lettuce as surrounding produce acreage declines. Each time a lettuce field is harvested, and disked, adult thrips disperse from these areas into the next available lettuce field.  As the number of lettuce acres becomes reduced near the end of the season, this creates a bottleneck effect that concentrates high numbers of thrips adults on the remaining fields under production. This can often make chemical control of WFT very difficult, particularly in March, as adults can continually re-infest fields following spray applications.  Unfortunately, bioconcentration is likely to be even more intensified with the increase in late lettuce we have this season.  So, what management approaches can you take to manage thrips and hopefully reduce the potential incidence of INSV.  The first line of defense should be sanitation. Growers should disc under producecrops immediately following harvest. The longer a harvested crop remains above ground, the more insects that can build up and move to adjacent lettuce fields, especially with the dry, windy, and warm weather we typically have in March and April.  So far INSV incidence is light, and on average less than 1% infections has been reported in fields. However, the longer the season progresses and thrips begin to move, the threat of INSV spreading increases.
Controlling WFT with insecticides is the best approach to minimizing cosmetic feeding damage and may reduce spread of INSV within fields. For adults, which can easily be found on the leaf surface, your best choices of insecticide are methomyl (Lannate) or acephate at a high label rate.  They are the most efficacious products against adult WFT, but are also effective against larvae. Radiant is not as consistently efficacious against adults, but a 7 oz or higher rate will provide the best knockdown and residual control of WFT larvae. Torac, via contact activity, is relatively effective against WFT, and is most efficacious applied by ground on smaller plants when populations are lighter.  Remember, the key to preventing cosmetic damage by WFT is to maintain larval populations at low levels.  The cryptic or thigmotactic behavior of thrips often makes them difficult to find on lettuce plants. Research has shown that if you can see a few adults and larvae on the plant, it means that there are likely 8 to 10-fold more thrips actually on the plant (hiding near the base of the plant between midribs).  This behavior also means that spray coverage is important, particularly with contact insecticides like Lannate. There are other insecticides such as Minecto Pro, Exirel, Movento and Beleaf that can provide suppression (50% control or less) against WFT larvae, but don’t expect much activity against adults. For more information on the identification, biology, ecology, and management of thrips on desert produce please visit Western Flower Thrips Management in Desert Produce – 2023.