If you have an overabundance of apple fruitlets, you can thin excess fruit by hand. It is precise, but time consuming.
Another way to thin excess fruit is with the use of Sevin™ (carbaryl) insecticide. It is an insecticide that also mimics the natural plant hormone, auxin, that regulates plant growth and development. When applied between bloom and 30 days post-bloom this material can cause a percentage of the developing apples to drop from the trees. The results are not fully predicable. Lots of factors influence the effect, but it is a technique to consider if you have a lot of fruit and little time to thin by hand.
Factors that influence fruit-thinning results
Some factors that influence the amount of fruit thinning that occurs include:
- Stress level of the tree — heavy crop load, minimal moisture, high temperatures.
- Temperature — Thinning effects are greatest at temperatures above 70 degrees F. Over-thinning can occur if temperatures exceed 80 degrees F.
- Fruit size — Smaller fruit tend to drop readily, fruit over 16 mm becomes difficult to thin
- Amount of developing fruit — If pollination was good and the crop load is heavy, thinning may be easier. When pollination was poor and fewer fruit were fertilized and develop, chemical thinning may have less effect.
- Variety — Some varieties are naturally harder to thin: Golden Delicious, in particular, as well as Empire, Macoun, and Spy.
- Tree age — Trees less than 5 years old drop fruit more easily and may overthin
A “helpful hand”
The ideal time to thin apple fruitlets is when they are between 12mm and 16mm in diameter. This is usually one to three weeks after bloom. A handy size reference is your own hand. On an average hand, 12 mm = the tip of you pinky, 16mm = the tip of your pointer finger. Ideally you would like to keep the “king” bloom, that first and usually largest fruitlet, in the cluster and remove the rest.
Carbaryl is an insecticide that is toxic to bees. Do not use it when blossoms are present on trees and ground cover plants, or during active bee flight.
Carbaryl is also toxic to desirable predator mites. Restricting use to early in the season and directing it toward the tops of trees will do the least damage to beneficial mites. Consider this in your overall tree management strategy.
Use carbaryl as a thinner only on apples, it can cause deformed pears when applied within a month of bloom.
It does not act as a thinner on stone fruit.
If additional fruit thinning is needed, you can come back and hand-thin in the weeks ahead. At least you will have less hand work remaining.