Cherry fruit fly: identification & treatment

A cherry tree is a thing of beauty and tasting your first home-grown cherry is a very special experience. That said, finding a maggot inside is not. Learn how to keep your cherries maggot-free by understanding the cherry fruit fly and how to control it.

The European cherry fruit fly (Rhagoletis cerasi) is a pest of several species in the genus Prunus,including the sweet cherry (Prunus avium) and sour cherry (Prunus cerasus). It is closely related to the Western cherry fruit fly (Rhagoletis indifferens), which is found in Western United States.The flies are entirely harmless to humans, but their larvae can cause severe damage to a cherry crop. Unfortunately, it is only as harvest time approaches that you might become aware of these unwelcome visitors.

Cherry fruit fly: identification and damage pattern

It is easy to mistake the cherry fruit fly for a small house fly. On closer inspection, you will notice the following characteristics:

• A fly 3-5mm in length
• Transparent wings marked with black, zebra-like stripes
• Vivid yellow mark on their midsection
• Black body with fine, pale lines sometimes visible

 Symptoms to look out for include:

• Black flies on your cherry tree. Hard to spot but are most active on still, warm days.
• A small hole in the skin of the fruit.
• White maggots in the cherries. These are the larvae and are about 6mm in length.
• Soft, brown discolouration of the cherry skin.
• Rotting or tunnel patterns inside the fruit.

Other species of cherry fly exist. What appear to be tiny household fruit flies on the cherry tree may also be Drosophila suzukii, the Spotted Wing Drosophila (SWD). It is very different in appearance to the cherry fruit fly, although it causes similar fruit damage. Clusters of tiny black flies on your cherry tree are more likely to be aphids (Aphidoideae), which distort cherry leaves by feeding on their sap.

It is possible to detect and monitor cherry fruit flies by using traps, such as our Plantura Yellow Sticky Traps, in the branches of your tree. Take the traps down after about 3 weeks, when the first cherry fruit flies have been trapped, to minimise trapping beneficial insects. Almost no more cherry fruit flies will be caught at this point. Yellow sticky traps detect if cherry fruit flies are present and can give some idea of population levels. However, as they are attracted to the colour yellow, cherry fruit flies from neighbouring trees may also be drawn to the tree where the yellow traps are set up.

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Once you have correctly identified the cherry fruit fly, understanding its life cycle is both helpful and interesting. The pupae overwinter in the ground below the cherry tree. They are brown and about 4mm long. In May, before the harvest, the cherry fruit flies emerge from their pupae and fly up to the nearby cherry tree. There, they mature, mate, and feed on aphid sap, cherry fruit juice and even bird droppings. After about a week, the females begin laying a single egg beneath the skin of each developing cherry fruit, piercing the skin and leaving a puncture mark. The larva hatches from the egg inside the cherry. It feeds on the cherry flesh for up to three weeks. Finally, it tunnels to the surface of the fruit, leaving a small but visible hole in its skin. It drops to the ground below and burrows into the soil. The larva pupates until the next spring when the cycle begins again. It is a brilliant system that requires some persistence to combat.

Cherry fruit fly control

The organic control of cherry fruit fly is a process of management rather than immediate eradication and may require some patience. Methods of control can vary according to the size of your tree and the amount of time you want to spend protecting your crop. A combination of several techniques should produce good results if carried out over successive seasons. Pitting your wits against these clever creatures can also be a satisfying challenge once you begin to see results. Here are a few possibilities to consider when trying to control cherry fruit flies in your garden.

Harvest your cherries before they ripen

If the cherry fruit fly is discovered early in the season before any larvae have emerged from the fruit, then removing every single cherry from the tree before the traditional harvest period is an option. This intercepts the cherry fruit fly larvae before they can drop to the ground and continue their cycle. Doing this for at least 2 years in a row can be very effective in reducing the pest population. Of course, it means you are sacrificing the existing crop for the possible benefit of future harvests. Many gardeners will find this heart-breaking! However, if you are a curious and adventurous cherry grower, it can be an interesting experiment. This option is more achievable on smaller trees where the fruits are visible and within easy reach. It will be most effective if there are no neighbouring trees within flying distance that are also infested. Dispose of the cherries carefully to avoid possible reinfestation. Burn them or bury them in the ground at a depth of at least 30cm.

Tidy up at the end of the season

Whether or not you decide to harvest your cherries prematurely, at the end of the harvest season, remove all remaining fruits from the tree and from the ground below. They can still harbour flies or pupae and are a major source of future infestations. This measure can also keep fungal diseases such as brown rot (Monilinia) in check. Spreading a harvesting net around the base of the tree during the harvest period can prevent the larvae from any infested fruit from entering the soil if they fall to the ground. Removing every fruit from a particularly large or neglected tree can be tricky, so please do not risk a fall in pursuit of that just-out-of-reach cherry!

Cover your cherry tree with a fruit tree protector

Fruit tree protectors envelop the tree like a huge bag, securely fastened around the lower trunk. They are made from a very fine mesh and prevent the cherry fruit flies from reaching the tree and laying their eggs in the fruit. Light, moisture and air can still pass through. This method is more practical on smaller, compact trees. If you enjoy a challenge and have some time and a few cheerful helpers, it can also be an interesting project for larger trees as well. There are a few things to consider before embarking on this:

• Measure the height and width of your tree in advance. The cover you buy must exceed these measurements.
• Put up the cover after pollination but before the cherry fruit flies emerge from the ground.
• The cover will not only protect the tree from cherry fruit flies, but also from beneficial insects such as ladybirds, which can control aphids. Address any existing pest issues before using the cover.
• Apply the cover with care to prevent the mesh from tearing and unnecessary damage to the tree.
• The cover needs to be partially removed and carefully replaced every time you pick some ripe cherries. This is also a good opportunity to observe any changes to the tree, which will be less visible under the netting.

For a smaller scale trial of this method, bought or homemade fruit tree ‘sleeves’ of mesh or fleece can also be applied to individual fruit bearing branches of the tree.

Apply a fabric ground cover or green manure under your tree

A physical barrier on the soil at the base of the tree can interrupt the life cycle of the pest at both adult and pupal stages. This could take the form of a weed control fabric, which can be firmly secured with pegs around the base of the tree. A natural, living mulch is also an option. Green manures such as white or red clover can be sown around the base of the tree. Their sturdy and extensive root systems interfere with the activity of the cherry fruit fly and their larvae. Green manures also support soil life and attract beneficial insects.

Tip: insecticides containing Spinosad are not recommended. Although marketed as an organic spray for cherry fruit fly, the naturally occurring active ingredient is also highly toxic to bees.

Prevention methods

If you are thinking of planting a cherry tree or two or have so far enjoyed your home-grown cherries pest-free, then you probably want to know how to prevent an infestation in the future. Whilst all the control methods above will certainly discourage an attack, here are a few more preventative measures you can take to keep those maggots out of your cherry harvest:

• Choose an early maturing-variety such as ‘Bigarreau Burlat’. Late-fruiting trees sometimes suffer from heavier infestations of cherry fruit fly.
• Choose a compact variety, so you can observe the whole of your tree closely and notice problems early on.
• Establish and care for your cherry tree correctly. Just like humans, a strong, healthy and well-cared for tree is always in a better position to manage any stress.
• Encourage parasitic wasps by companion planting with fennel (Foeniculum vulgare), dill (Anethum graveolens), coriander (Coriandrum sativum)and other umbellifers (Apiaceae). These wasps are not harmful to humans, but they can help control cherry fruit fly. Birds also act as a deterrent, although they may be more interested in eating your cherries!
• Try using nematodes. Research suggests that the Steinernema feltiae species may be effective against cherry fruit fly larvae. Apply in June at the base of the tree.
• Talk to your neighbours! Do any of them have a cherry tree too? Although they are weak fliers, cherry fruit fly can sometimes spread to trees up to 100m away. If you can join forces to prevent them then so much the better. You might even pick up more tips and tricks for keeping your cherry tree happy and healthy.

If you feel inspired to give your cherry tree even more attention, why not go to the next level and learn about the art of pruning cherry trees in our separate article.