What should we do when we encounter a nut fly in our garden?

The nut fly is a serious pest that can cause damage to produce amounting to 80% of the yield. It is the most destructive species of the genus Rhagoletis, along with the European cherry fly (Rhagoletis cerasi Loew) and the apple worm (Rhagoletis pomonella Walsh). Its consequences may not only be economic but also environmental. Its invasion can displace native species or compete with them for resources. It comes from North America, but it is already well enough to spread to be found almost everywhere. Pesticide control of walnut fruit flies in gardens near houses is not allowed. We are a company that likes to deal with means close to nature and those that are consistent with and support its development. For this, we will reveal some tricks that will help many of you cope with this task, but with very gentle measures. Approaching this problem responsibly, the first thing to start with is knowing the life cycle of this insect. For this, we will start with it and clarify the possible consequences of its invasion, as well as the appropriate measures to deal with it.

The walnut fly is a pest that causes serious damage to walnut production. It overwinters as a pupa in the ground and hatches between June and August. The life of a nut fly can last up to 40 days and it is possible to fly up to 800 m. This is important information about its distribution, which shows that it can very easily reach neighboring lands. Mating begins 6 to 8 days after their emergence and then the females lay their eggs. She can lay up to 15 eggs in or on the green shell of each nut. But it is also possible that two females attacked one nut, which means that up to 30 larvae can be found in it. Over a period of several weeks, each female walnut fly distributes up to 400 eggs on about 30 fruits. Depending on the temperature, the eggs hatch within 3 to 10 days. The nut fly can have one generation per year, but in rare cases, there are two.

You can recognize the places where the eggs were laid by the presence of small round black dots located on the green shell of the walnut. They can easily be confused by symptoms of fungal diseases. By looking closely, it can be seen that a black exudate is coming out of these small dark spots. This is a sure sign that there is a nut fly on the tree. The larvae that hatch from the eggs begin to feed on the green shell of the nuts, causing it to rot, blacken and disintegrate. It soaks well, becomes sticky, and turns black to such an extent that even the core (hard shell) begins to mold. Subsequently, it begins to dry and adhere to the hard shell of the walnut. This makes it difficult to remove. In severe infestations, the larvae completely destroy the pericarp (green shell), leaving only a black and withered exocarp around the shell. The earlier the walnut fly lays eggs, the more damage will be done to the walnut production. An attacked nut weighs about 20% less than a healthy walnut. Quality also makes a difference due to the presence of dry rot. Infected nuts become deformed and look dirty due to the rotting shell. If timely measures are not taken, the walnut fly can quickly take over the entire garden and those in the neighborhood as well.

In fact, the larvae of the walnut fly cannot pass through the hard shell of the walnut and reach the nut, they only damage the green shell. And this is good because there is a chance for the nuts that did not fall from the tree too early to be eaten without problems. When walnuts are grown for industrial purposes, this pest causes more damage, since the attacked nut does not have a commercial type, and additional cleaning is required. The black coating comes off easily when soaked in water and subsequently scrubbed with a brush or using a high-pressure cleaner or sandblasting.

In order to hibernate, the pupae fall to the ground or get there through the fallen fruits, after which they are buried in the soil to a depth of several centimeters (5 - 10). The following summer, the majority of adults (about 90%) emerge from the ground and repeat this cycle. The remaining 10% spend another season underground and emerge later. The nut fly can spend about 8 – 10 to 18 m in the ground as a pupa and then emerge again between May and August (rarely September) as an adult fly and begin its development. There are two main periods when the eggs can hatch - around the beginning of June when the nut falls from the branch long before it has reached its maturity. At this time of fruit development, its green shell is severely damaged, and the nuts cannot develop further and often fall from the tree before they are ready. The nut is in most cases inedible also because it has rotted due to the presence of too much moisture around its shell. The next period is around the first half of August. If the pest starts its attack now, there will be less damage and the nuts will still be edible. At this time, the egg-laying peak of walnut flies begins. But if the attack simply continues as the next stage in the development of these pests, then the damage will be even greater. This insect does not have many natural pests to stop its population, apart from some parasitic wasps (Biosteres sublaevis (Braconidae), Coptera occidentalis (Diapriidae) and Trybliographa sp. (Figitidae)) which do not significantly reduce their numbers. And in most cases, they are also enemies of people. Human intervention is needed to help the tree cope with this struggle.

Like other Rhagoletis species, the dispersal of adults is possible only over short distances, it can fly about 800 m. Longer distances are caused mainly by the transport of infected fruits or discarded shells of infected fruits. The spread of these pests through transport is a little more difficult, but not impossible. This is the case for several reasons, usually, walnuts are de-shelled before they are transported. Also, during the harvest period, there are not many larvae in the shell, and last but not least, the transportation of the fruit takes place at lower temperatures, making it difficult for it to survive in such conditions.

According to research carried out specifically related to the spread of the fruit fly, it is established that the temperature and humidity of the air are important for its population, whether there is a farm with animals nearby, whether the trees grow alone or are part of a large farm. It likes to feed on fruit juice, exudates, nectaries, dew and also yeast, and bacteria.

When the temperatures have relatively lower daily values, the development stage of the imago (adult fly) is prolonged, and the time for incubation of the eggs, as well as for the development of the larvae, is prolonged. Laboratory studies have shown that temperatures higher than 35°C, combined with low relative humidity, reduce the life span of the imago, and a temperature of 40°C is lethal in a period of only a few hours. The distribution of the population decreases when the temperature value also decreases. Relatively lower daily temperatures slow down the development stage of the walnut fly, and at a temperature below 8°C it freezes. Average daily temperatures for the development of imago and larvae are best reflected when their values are between 16 and 28°C.

In the presence of this disease on the tree, it is necessary to pick up and discard carefully and daily all fallen fruits. This is of course impossible to happen in large walnut orchards. If the attacked fruits are ready for consumption, they are cleaned with water. First, they are sour, and then they are rubbed well. Infected nuts and their husks cannot be composted, but stored in separate waste containers or burned.

If you know that there is a nut fly in your walnut garden, we offer you a trick that can be done in a smaller area and without the use of preparations. After mid-June, the ground under the crown of the tree can be covered with netting or black nylon. There are two benefits to this method. The first is that the hatching walnut fruit flies are kept on the ground and do not infect the plant above. And the second is that the temperature under the foil can rise so much that the insects simply die.

Nut fly larvae die at temperatures around 40 °C and at temperatures below 8 °C. If you have had this problem in your walnut garden, it is good to turn the top layer of the soil if possible in colder weather (below 10 degrees to have the greatest effect). So if there are pupae buried in the soil, they will come to the surface and die, their spread will stop and you won't see them again in the spring.

If your fruits are attacked by a nut fly, it is good to expose them to the strong sun for a few days after harvesting. Spread a tablecloth on the ground, carefully spread the walnuts on it, and turn them over for a few days. It is good to put them back in the evening and take them out again in the morning, in this way the finished product is preserved from moisture or from some animal such as a squirrel or other rodent that can steal part of the collected product.

Another alternative, eco-friendly, and very successful ideas that have a huge effect are raising chickens.Set free on the ground under the walnut, they would devour every single nut fly pupa. In this way, every tree will be freed from this scourge. This will have a completely positive effect if it happens from the beginning of spring to autumn ie. from July to September and into the following year, this disease will be eradicated from your garden. Choosing this way to deal with the walnut fly can provide more than one benefit. The first is that the hens will work for you, devouring the larvae found in the ground, and you will only have to watch the fruit on the crown. The second is that chickens will naturally enhance your garden and last but not least, they will bring you eggs and maybe meat every day.

• On yellow support, it can be made of cardboard or paper, a foil or nylon is stuck, on which a layer of glue is applied. Fish flavoring can also be added if you have some on hand. The glue is made from the following mixture. Melt linseed or castor oil, resin, or rosin in a water bath. As bait, you should take glycerin, honey, or sugar. Everything is mixed and applied to the already-made trap. And you may also find ready-made baits in some commercial establishments. 

• Another option, which would also achieve a positive effect by capturing part of the pest, is made with a plastic bottle.A yellow funnel is placed on it. Inside it is filled with "bait" - this is a mixture of 250 ml. liquid ammonia (unflavored) and raw fish waste. This bottle hangs on the tree we want to protect. Insects are attracted by the yellow color of the trap and by the smell of fish, and then find themselves hostages inside.

• It would be even more interesting if a luminous trap were invented for these insects. It should contain an LED in which batteries should be placed. It can be found in a ready-made version, but you can also make it yourself. There needs to be a base to attach the LEDs to (this could be a plastic bottle, bucket, sphere, or whatever you have on hand that is suitable for hanging on a tree). Then an outer layer of foil is wrapped, on which a layer of glue is applied. Is ready! It can be attached to the tree we want to treat. In this case, the trap becomes really irresistible because of the light that attracts insects, and then they find themselves glued to it. In this case, there is a risk that some useful species of insects, not only pests, will fall into our trap.

The improved version of this type of trap is a solar insect trap. It is a jar that hangs on the tree. The cap has a solar lamp that charges from the sun, making it the most eco-friendly alternative to a nut fly trap possible. It is only necessary to lay the appropriate bait and monitor it periodically. These traps will certainly protect nut production and if necessary it is desirable to increase their number.

Glue traps attract most insects because of their color and scent. The big advantage is when the food bait can be added. It attracts only the desired type of insects while choosing the option with only glue you risk catching beneficial insects as well.