Thrips infestations are difficult to predict because their migrations vary so much from year to year based on rainfall and the time that weeds dry up. In addition, little is known about the relative attractiveness and susceptibility of fruit in different stages of development. Some varieties may be more prone to damage than others, but this has not been well studied or documented. The migratory nature of WFT also makes spray timing difficult because a spray applied one day may have little effect on adult thrips arriving in the orchard a few days later. Further complicating treatment decisions is the fact that spays applied during bloom or post-bloom period pose a serious risk to bees present in orchard during this time period. Many insecticides known to be effective against thrips are also toxic to honey bees.

Until these damage symptoms can be shown conclusively to be caused by WFT, and thrips biology control are better understood in cherries, the following measures may help reduce the risk of damage and use of unnecessary insecticide treatments for thrips:

If you think you may have to spray for thrips this season, work with your PCA to develop a treatment plan. Well in advance of bloom and moving bees into the orchard, discuss your plan with your beekeeper and make appropriate plans to protect bees in the event treatment is needed.

Use yellow (usually sold as whitefly traps) or blue thrips “sticky card” traps to monitor WFT activity as bloom approaches and through (at least) the first few weeks of fruit development. For orchards located near fields of other known thrips hosts or open non-cultivated areas, placing some traps at the edge of the orchard may provide an indication of incoming migrations. There are no known treatment thresholds for WFT in cherries, but traps should provide some indication of thrips presence, movement and populations.

The main insecticides used for thrips control in other crops are spinosyns and pyrethroids. These insecticide groups include some of the same products relied upon heavily for control of spotted wing drosophila and other cherry pests. As such, their use for early season thrips control needs to be weighed carefully in the context of resistance management and label restrictions on the number of sprays or amount of product applied per season.

By Joe Grant, Farm Advisor, San Joaquin County; and David Haviland, Entomology Farm Advisor, Kern County