Over the past few seasons, a new kind of fruit damage has been observed in some cherry orchards. As fruit begin to reach full size and approach harvest, small (around 1 mm diameter) circular or slightly oval-shaped scars or “dimples” become apparent on the fruit surface. The marks are “dry” and superficial – only the fruit epidermis is affected and there is no penetration or wounding of the underlying fruit flesh. In some cases, the marks or dimples are more numerous near the blossom end than the stem end of affected fruits. When magnified (with a high-powered hand lens or dissecting micro-scope) the marks appear as a thin layer of dead epidermal cells, usually with a small hole near the center, with a small, smooth and very shallow cavity or depression underneath.
The leading theory is that this damage is caused by punctures from the ovipositor of western flower thrips (WFT) during early stages of fruit development. This theory is based on the type and size of damage, prevalence of thrips during the period of early fruit development, and based on experience from other crops like grapes and nectarines that can also be 'stung' by WFT. Damage to cherries is similar to what is observed in these other crops, except that there is more variation in dimpling. This, however, can be explained by the timing of the puncture: stings prior to rapid fruit expansion are more likely to have pronounced dimples than fruit that has already expanded before being attacked.
WFT over-winter primarily as adults and, as weather warms in early spring, they move to alfalfa or grain fields, weeds, and other ground vegetation to reproduce. As weedy hosts start to dry up, WFT migrate to alternate hosts, including cherries, and are particularly attracted to flowers. Depending on the weather, thrips migrations can coincide with cherry bloom or early stages of fruit development. The theory is that female thrips — that are initially attracted to cherry flowers — probe small fruit with their ovipositors to determine if they are a suitable location to lay an egg. It is possible that the abundance of late winter rainfall we have experienced in recent years — and the above-normal growth of winter annual vegetation it has promoted — may explain recent observations that this type of damage is becoming more prevalent.
Bees at risk
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