Fruit Fly Biology and Behaviour

Fruit flies are found anywhere where food is present and allowed to rot, including homes, restaurants and supermarkets. They are also important pests of many types of fruit such as caneberries, strawberries, and other fruit. Fruit flies are small and usually have red eyes. They are usually tan in the front part of the body and black in the rear part.

Fruit flies lay their eggs in ripe, overripe or rotten foods. When the eggs hatch, the fruit fly larvae feed on the surface or soft areas of the food. Fruit flies can lay up to 500 eggs in their lifetime, giving them huge reproductive potential and the ability to quickly establish large populations if given the proper environment to feed and reproduce. Their life cycle usually lasts about a week. The Pest Prophet app can be used to predict when different generations will occur, and is useful in agriculture settings.

Fruit flies prefer ripened fruit or vegetables to feed and reproduce, but can also feed on other forms of organic matter or food waste. Moist, fermenting plant material is the requirement for fruit fly development, which is why fruit flies are seen in homes or restaurants. In agricultural settings, this makes field sanitation very important.

Fruit flies are often considered a nuisance, but in agriculture they can damage fruit and in some cases can vector diseases. In soft fruit, fruit flies can lay their eggs in ripe fruit, allowing larvae to emerge later, post-harvest.

Fruit Fly Damage Prevention

The best method to control fruit flies is by prevention through sanitation practices. Discard any ripe or rotting fruit from the field. Soft fruits should be harvested in a timely manner. In the case of berries, care should be given to remove all fruit that may be hidden in the plant canopy. Culled fruit should be removed from the field completely.

Fruit Fly Sampling

In order to confirm the presence of fruit flies and assess the population level. Extensive sampling is needed. Pheromone based sticky traps can be used and should be checked and changed at least weekly, if not more often.

Spotted wing Drosophila is the most important fruit fly for many crops including cherries, raspberry, blackberry, blueberries and strawberries. It can also infest stone fruits or any other soft fruit. Spotted Wing Drosophila resembles Drosophila melanogaster,  also know as the common vinegar fly as well as other fruit flies that attack rotten or overripe fruit. The Spotted Wing Drosophila is a greater threat, however, because in certain crops, it will attack and feed on undamaged fruit, causing crop losses.

Spotted Wing Drosophila adults have red eyes with brown thoraxes. The distinguishing characteristic is the black spot on the wings present in adult males only. Larvae are tiny white maggots that feed on the fruit. They are very difficult to distinguish from other fruit flies.

The western cherry fruit fly, Rhagoletis indiferrens are another fruit fly pest on cherries and other crops. They can be distinguished from Spotted Wing Drosophila by their larger size and dark banding patter on wings. The western cherry fruit fly is a quarantined pests which occurs and Washington and other pacific northwestern states, but has not been established in California.

In moderate climates such as coastal California, Spotted Wing Drosophila may be active through most of the year. They are most active between 68F and 86F.

Researchers have developed the Spotted Wing Drosophila growing degree day model to help growers predict when subsequent generations will occur based on trap catch dates and weather data. The Pest Prophet app can be used to easily implement in the field without needing weather stations.

Spotted Wing Drosophila Damage

Fruit flies dame rotten and over ripe fruit, with exception of Spotted Wing Drosophila, which also attacks fully ripened fruit. Adult females penetrate the skin of the fruit with their long, sharp ovipositor, which lays eggs in the soft flesh of the fruit. Eggs are laid in clutches of 1 to 3 fruit, but females will deposit eggs on many fruit, and/ or the same fruit multiple times. These eggs hatch and devleop into maggots which feed on the fruit.

The flesh of the fruit usually turns brown and sofgt, with sunken areas where feeding has occurred. Damaged fruit canoften be infected by fungal pathogens that cause further rot and decay.

Spotted Wing Drosophila Management

There a many different types of traps that can be used against Spotted Wing Drosphila. A simple mix of yeast, sugar and water in a a jar or bottle trap has been very successful in catching SWD. 12 oz of water, 0.25 oz of baker’s yeast and 4 teapoons of sugar is recommended. This mix should be allowed to ferment for a day then transferred to 500 ml bottle with 4 holes drilled into the lid. Flies are attracted to the solution, enter the bottle and drown in the liquid.

Jars or bottles should be hung in the field in cooler or shady areas. One liquid emixture can last for a few weeks and can give a good idea of SWD pressure in a given area, but it is ideal to check and count SWD at least weekly.

Control of SWD can be very challenging because once eggs are depositted in the fruit, there is nothing that can be done to save infested fruit. Audlt flies need to be controlled before they are allowed to lay eggs, and breeding sites need to be reducded and controlled to prevent egg laying.

Attractant Bait Sprays

Bait sprays that target adult flies such as GF-120 are one option for adult fly control. Coverage is less important with baits thatn with other insecticides, and baits can be applied with low volumes and larger droplet size. This might help to reduce damage to beneficial insects and/ or bee populations. The efficicacy of baits decreases over time, so materials will nbee to be re-applied to sustain control. Trap counts can help to inform how well baits are working and when they need to be reapllied. Heavy infestations will need baits to be reapplied more frequently.

Chemical Control Sprays

There are several chemical sprays that are effective against Spotted Wing Drosophila, including malathion, pyrethroids and spinosyds. This can be used with or without baits added. However, using these pesticides add a lot of risk of damage to bees and other beneficial organisms and irresponsible use can have long lasting damaging effects. Spraying should also be done in conjunction with regular monitoring and these insecticides materials should only be considered when SWD populations are very high.

Sanitation

Sanitation is a major factor in Spotted Wing Drosophila control. Removing any overripe or damaged fruit completely from the field can reduce the populations of SWD. In cases of severe infestation, it is better to remove all ripe fruit and bury or dispose it in a closed container. Row furrows and other areas should be kept clean from fruit that has fallen.

It is important to work together with neighboring fields to remove all infestations before populations become overwhelming.

Sources:

Spotted Wing Drosophila. Cornell Fruit Resources.

Kanzawa, T. 1939 Studies on Drosophila suzukii Mats.,

Sakai, M. and Sato, R. 1996. Bionomics of Drosophila pulchrella (Diptera: Drosphilidae) Fukushima Fruit Tree Exper. Sta