Walnut husk fly

Husk fly trap hanging on rope lowered from walnut tree to check for flys.

In our last column we talked about coddling moth and what a pest it can be to walnut growers. Another insect walnut farmers have to watch out for is husk fly.
The husk fly is an actual fly that lays it’s eggs in the walnut husk. The eggs hatch and small maggots emerge to eat the husk. They do not actually eat into the nut itself, just the husk. But when they eat through the husk it turns all black and mushy which stains the nut a dark brown or black.
Also nuts with husk fly damage are more likely to have moldy nut meat. Either of these conditions makes the nut unmarketable. Therefore controlling them is important.
As with many insects if left alone, husk fly will build up numbers year after year until they will attack nearly every nut in the orchard. They were first noticed about 40 years ago, when substantial portions of the crop were being destroyed by the pest. Some growers lost over a third of their crop before adequate control measures were put in place.
Husk fly have only one generation per year. The adult fly deposits eggs into a nut husk. The eggs hatch and the maggots feed on the nut, eventually dropping to the ground and forming a pupa where they stay until the next year.
Unfortunately they do not all come out at the same time. The generation emerges anywhere from the first of July to the end of September.
Treatment of the flies must be done consistently to be effective. Most sprays only last a couple of weeks so repeat applications are necessary.
Usually an insecticide is mixed with fly bait, such as molasses, and then sprayed on the trees. Because bait is used only every other row needs to be treated. The flies are monitored using a yellow sticky card hung in a tree. They are attracted to the card with a lure and caught in the glue. They can then be counted to see if it is necessary to treat the orchard.
Recently a method has been developed to more accurately time the spray. The flies caught in a trap can be divided up by sex. Then the females are checked to see if they have any eggs. If they do, then the farmer knows it is time to spray.
Controlling insects is not the only thing walnut farmers do in the summer. Irrigation is another big task.
Walnut trees use a tremendous amount of water this time of year. That water has to be constantly replenished to keep the trees from stressing.
Some crops can tolerate a lack of water for a period of time. That is not true for walnuts, however. There is no time that walnut trees can go without water and not be damaged.
We use sprinklers on three-inch aluminum pipe that have to be moved every day. It runs into a fair amount of work for the farmer, but the trees like it.