Drosophila suzukii: the fruit fly

Drosophila suzukii: the fruit fly

In recent years the crops of cherries, strawberries, blueberries, raspberries, blackberries and vineyards are threatened by the fruit fly (Drosophila suzukii), an insect of the order Diptera and of the Drosophilidae family, native to Southeast Asia. This small insect has only recently spread to Italy, probably brought with goods from the Far East. Here it found a favorable environment and proliferated at the expense of small fruit crops.

The gnat causes serious damage and irreversible on various fruit-producing species with thin skin. Drosophila is spreading undisturbed, in the absence of effective natural antagonists. The insect has already been identified in 12 Italian regions and in 13 European countries.

It is not easy to protect crops from fruit fly, as Drosophilaaffects the fruits during ripening, a phase in which treatments cannot be carried out due to the impending harvest. What makes the insect particularly dangerous for orchards and vineyards is also its reproduction speed: females can reach 10 generations in a year.

Today, knowledge regarding the behavior of this eastern gnat is inadequate: there is still too little experience, given its relatively recent spread on our territory. The first findings in Italy date back to 2009, in Tuscany and Trentino.

If interventions with chemicals are often impossible to perform or ineffective, some biological methods have provided excellent results. In particular, the combination of the mass capture of adult individuals (operation that requires the use of biotrap) with correct agronomic practices of preventive character, it allows to effectively defend the crops from the attacks of the Diptera.

Drosophila suzukii: characteristics of the insect

The adults of the fruit fly are about three millimeters long and can be recognized by the rather large red eyes and the golden-brown body, characterized by the presence of dark bands on the dorsal abdominal segments. Males have black spots on the wings, which are also found on the toothed ovipositor of the females.

The insect overwinters on local spontaneous plants. In fact, another factor that has contributed to the spread of the eastern gnat in Europe is the presence of many plant species that can act as intermediate hosts for the winter. Afterwards, the females lay eggs of small size on various fruit species, ensuring the development of 10 generations per year and showing a decidedly higher capacity for oviposition than the native gnat. There are 2-3 eggs laid on each fruit and 350-400 those laid by a female during the entire life cycle.

Eggs take 12 to 72 hours to hatch, while larvaethey turn into pupae in 3-14 days. The pupae in turn become adults in a period of time between 3 and 15 days. The duration of each stage and therefore the entire life cycle depends on the temperature.

Damage caused by the gnat

The attacks of Drosophila suzukii begin at the moment of oviposition. With the serrated ovipositor, the females cut the exocarp of the fruits when they have matured and lay their eggs in the mesocarp.

Subsequently, the larvae emerge from the eggs which, feeding on the pulp, cause irreparable damage to the ripe fruit, also causing bacterial infections is fungal. After 2-3 days, the fruits affected by the insect present depressed and soft areas, in correspondence with the incisions of the ovipositor. In a short time, the fruits undergo decay.

What plants does it affect

In Italy, the gnat has caused serious damage to stone fruit (cherries, peaches, plums, apricots) and ai berry fruits (blueberries, raspberries, blackberries, strawberries), as well as kiwis, persimmons, figs and grapes. The females of the dipteran also hit apples and pears already damaged.

How to defend the orchard

As already anticipated, the fruit fly is difficult to combat with chemicals: even if you want to use them, treatments should be done when the fruits are ripe and this would make harvesting risky due to the presence of residues in the fruits. For those who still want to use insecticides, it is essential to alternate the active ingredients to avoid the onset of resistance phenomena.

In organic farming, the most effective method of fighting Drosophila suzukii is based on traps with food bait, which allow to capture adult specimens and avoid their reproduction. Some agronomic practices, such as good pruning, are also very useful in preventing the problems caused by the dipterus.

Monitoring and mass capture of the insect

To minimize the damage caused by the fruit fly, the most effective methods are the monitoring and mass capture of adult individuals through the use of food chromotropic biotraps. The traps do not contain harmful substances or pheromones, but food baits that are easy to prepare even at home. Traps specifically designed for capturing Drosophila suzukii,Red Tap Trap and Vaso Trapthey are made of plastic, resistant, versatile and capable of attracting the gnat by exploiting the red color, a favorite of the insect. The traps are applied as caps to containers filled with a food bait.

Before setting up the traps you must prepare the bait, which is nothing more than a blend of apple cider vinegar, red wine, and brown sugar. This recipe was tested and recommended by the Edmund Mach Foundation of Trento.

Later, we proceed to hang the biotrap to plants. You can use a plastic bottle with a red label in combination with the Tap Trap cap or a glass jar combined with Vaso Trap, which can also be placed on top. The ideal is that the biotrap is at eye level and well exposed to the sun.

To perform an efficient capture, it is best to wait for the mid April, a period in which Drosophila suzukii usually begins to manifest. To establish the right time for planting traps, you must always consider both the cultivation area and the cycle of the fruit plant to be defended. In general, the best thing to do is to check for the presence of the eastern gnat by first fielding some Vaso Traps with monitoring function and then several additional Tap Traps.

When a strong presence of Drosophila suzukii is noticed, it is advisable to install Tap Trap for mass capture of the gnat. It mounts instead Vase Trap for selective capture of the insect. Vaso Trap in fact has a funnel with numerous holes that allows only the fruit fly to pass inside the trap and therefore facilitates the detection of the dangerous dipteran.

Other methods of biological control

Traps alone are not enough to contain the damage of Drosophila suzukii. Therefore it is advisable to resort to some preventive agronomic measures which create a favorable environment for the rapid ripening of fruit. Ripening quickly, the fruits will be less prone to attacks by the eastern gnat.

It turns out to be very useful prune fruit plants in such a way that the light penetrates easily through the canopy and guarantees a more uniform ripening, allowing a more concentrated harvest. Other recommended row management practices are the correct execution of the peeling and thinning of the fruits and the timely elimination of rot. Furthermore, in some cases, it is possible to anticipate the harvest without waiting for the fruits to reach physiological ripeness and proceed with the harvest of the still unripe fruits.

Video: Spotted Wing Drosophila in Small Fruits