What is in this article?:
- Western flower thrips suspected in cherry damage
- Bees at risk
- Over the past few seasons, a new kind of fruit damage has been observed in some cherry orchards.
- 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.
- Many insecticides known to be effective against thrips are also toxic to honey bees.
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.