Two gardening topics I know quite a bit about are fruit trees and raspberries. For many years now I’ve raised Heritage raspberries, which are an ever-bearing variety. Ever-bearing means that they produce about the same time as the summer-bearing varieties and then again in the fall.
Because I only want a fall crop, I prune the canes just before the ground freezes to about 3 inches from the ground. This makes the pruning process much easier, and I believe it also makes the fall berries sweeter.
Some of the more common insects I’ve had to deal with over the years on my raspberries were yellow jackets, picnic beetles and ants. But four years ago a new and difficult insect became a regular in my raspberry patch. It’s the spotted wing drosophila fly, also called the SWD. It was first discovered in the western United States in 2008 and is now well-established throughout the United States and Europe.
Because the fly is so small, it is believed that it has been spread by human transport and the spread has been very rapid.
The insect uses its serrated mouth to inject its eggs into over-ripe raspberries, strawberries and blueberries. Grapes can be damaged too by the fly’s attempt to inject the grape skin. But because the skin is often too tough, the damage is more like a bruising that can ruin the fruit.
Each fly can lay 350 eggs in one lifetime, and it only takes 11 days to go from egg to adult fly, but only 4-5 days from egg to maggot.
The SWD prefers temperatures in the 55-88 degree range with a high humidity of 82-94%. This summer has been especially hot and humid, so the fly did some damage on the early raspberries.
I personally do not use insecticides on my raspberries because they can harm bees and other pollinators. The University of Minnesota is working hard to find a way of controlling this insect which has affected so many small fruit growers. About 20-25% of those growers have gone out of business already because of the SWD.
There are traps being researched and used to capture and kill the fly. The traps don’t attract more flies than would already be in the area. They should be used in early April to early May or before the fruit ripens, and changed out weekly.
So far there is no biological control for the SWD, but research is being done on a particular kind of wasp that may be a predator and would feed on the SWD. Exclusion netting is also a possible way to protect your crops, but it would keep out other pollinators, so is not very practical.
My customers like that I grow my fruit organically, so they’ve stuck with me. I pick my berries a little under ripe and they ripen nicely overnight. They are used mostly for jams or baking, so I’ve suggested that they are used right away or frozen for later use.
Jerry Vitalis is a Chisago County Master Gardener.