Walnut husk fly

Rhagoletis completa


The walnut husk fly, also known as the American walnut shell fly, is one of the most important animal pests of nuts. Infestation of the fruit can not only turn the nut shell black and unsightly, but also affect the nut kernel itself. In case of heavy infestation, this can significantly reduce nut quality and cause crop losses.

Aktuelle Warnungen

Current warning: Flugbeginn der Walnussfruchtfliege in Kärnten

Regions: Kärnten
Time period: 12.07.2024 - 20.10.2024

Der Flugbeginn der Walnussfruchtfliege (Rhagoletis completa) wurde mittels gelben Klebefallen (Gelbtafeln) festgestellt. Die erste erwachsene Walnussfruchtfliege wurde vor wenigen Tagen in Kärnten gefangen.

Der Flugbeginn in weiteren Bundesländern wird mit weiteren Warndienstmeldungen bekannt gegeben.

Für Nussbaumbesitzer in dieser Region wird daher empfohlen, (spätestens) ab jetzt am eigenen Standort mit Gelbtafeln den saisonalen Flugverlauf der erwachsenen Fliegen zu beobachten. Achtung: Da auch Nützlinge und andere Insekten abgefangen werden, Gelbtafeln nur für die Dauer der notwendigen Beobachtungen bzw. Bekämpfung verwenden!

Je nach regionalen Gegebenheiten (v.a. Temperatur und Niederschläge) wurde der Flugbeginn in Österreich in den letzten Jahren meist zwischen Anfang Juni und Ende Juli beobachtet. Fallenfänge traten nachfolgend bis in den Herbst auf.

Die jeweils aktuellen Meldungen zu Fängen der Walnussfruchtfliege sind auf obstwarndienst.lko.at (https://obstwarndienst.lko.at/5127/Walnussfruchtfliege) ersichtlich.

Nach Paarung und Eiablage verursachen die Larven in den Früchten die typischen Schädigungen, welche im Kapitel „Schadsymptome“ auf der AGES-Schaderreger-Seite ersichtlich sind. Zu beachten ist, dass oftmals ein früher Befall der Früchte zu einer höheren Schadwirkung führen kann, wodurch das rechtzeitige Setzen von Maßnahmen besonders relevant wird.

Die Beobachtungen zum Flugverlauf am Standort dienen dazu, zeitgerecht Maßnahmen zur Befallsreduktion (z.B. vermehrte Anbringung von gelben Klebefallen) durchführen zu können. Ebenso sind diese für die Planung des richtigen Zeitpunkts (z.B. Flughöhepunkt) eines möglichen Einsatzes von Pflanzenschutzmitteln, welche im österreichischen Pflanzenschutzmittel-Register (www.baes.gv.at)(Schadfaktor: Walnussfruchtfliege) ersichtlich sind, notwendig. (Weitere Maßnahmen: siehe Kapitel „Vorbeugung und Bekämpfung“ der AGES-Schaderreger-Seite)

Show warning details Aktuelle Warnungen abonnieren


In appearance and lifestyle, the walnut husk fly (Rhagoletis completa) is very similar to the European cherry fruit fly(Rhagoletis cerasi), to which it is also related.

The orange-brown colored flies are about 4 to 8 mm in size and are characterized by their conspicuous wing markings (black bands in the wings) and a yellow dorsal shield.

The larvae (maggots) are whitish-yellow in color.


The walnut husk fly belongs to the fruit flies (Trypetidae, Tephritidae) among the large group of flies.

The walnut husk fly forms only one generation per year, with pupae overwintering in the soil. Depending on the temperature, the adult flies hatch from about the end of June. Their main flight period is in the summer months (especially August), but they can still be observed in trap catches until late autumn. The eggs are laid in the green fruit shell. The larvae live gregariously in the fruit shell and feed on the pulp, turning it soft, black and slimy. Over 25 larvae can be found in a single fruit shell. After feeding for three to five weeks, the larvae either drop themselves out of the nut or drop to the ground with the nuts to bury themselves in the soil where they pupate. The new generation of flies does not follow until the next year.

Damage symptoms

Due to the feeding activity of the larvae in the flesh of the fruit, it becomes soft, black and slimy. When the fruit or the fruit shell is opened, the brightly colored maggots can be found in the flesh. In case of heavy infestation, not only the nutshell can become black and unsightly, but also the nut kernel itself can be affected. For example, (depending on various factors such as the time of infestation and the choice of cultivar) there may be a reduced nut weight and a higher proportion of nuts without developed kernels. Likewise, a darker (sometimes black) coloration of the kernels has been increasingly reported.

There are possibilities of confusion with fungi (Marssonina leaf spot fungus) and bacterial diseases (bacterial walnut blight), which also cause externally black fruit shells. Other light-colored fly maggots may also be found in infested (rotten) fruit flesh, but these are usually present only secondarily as detritus or decay feeders.

Since fruit flies only infest fruit, there is no damage to other parts of the plant such as leaves, trunk, branches or roots.

Host plants

The host plants are nuts(Juglans spp.); in Europe, the true walnut(J. regia) and the black walnut ( J. nigra) are particularly relevant; under certain conditions, peach(Prunus persica) also serves as a host plant.


The original home of the walnut fruit fly is the south-west of North America. In Europe, it was first caught in Switzerland at the end of the 1980s. In the years that followed, it was also found in Italy, Slovenia, Croatia, Germany, France and finally in Austria in 2008. However, it probably arrived in Austria a few years earlier, as can be assumed on the basis of verbal reports, and the first official detection was only made in 2008. Surveys from 2020 onwards showed that the walnut fruit fly is present in Austria - albeit with varying intensity - in all federal states. Current data on the seasonal occurrence can be found in the latest report on the AGES warning service walnut fruit fly.

Prevention and control

  • With yellow sticky traps (see measures for cherry fruit fly), the start of the flight can be determined by means of individual traps and some of the adult fruit flies can be intercepted (infestation reduction). Please also note the regional warnings from the AGES plant protection warning service regarding the start of the flight.
  • The early treatment or destruction of infested fruit is an important measure (killing the larvae, e.g. by freezing or burning the fruit) to reduce the occurrence of the walnut husk fly in the coming season. This measure is only effective as long as the larvae can be found in the infested fruit.
  • A rather rarely used countermeasure is to cover the ground under single walnut trees, if possible, on the one hand before the fruit falls so that the larvae cannot pupate or hibernate in the ground, and on the other hand in spring or summer (from around the end of June) to prevent the flies from hatching or escaping, for which purpose the cover must be tightly sealed at the edges. It should be noted that this measure cannot prevent walnut fruit flies from flying in from the neighbourhood.
  • If plant protection products are currently authorised against this pest, they can be found in the plant protection product register (see list of authorised plant protection products in Austria).
  • Variety selection: certain varieties appear to be able to maintain fruit quality better than others in the event of infestation (see Solar et al. 2020 and Agroscope leaflet)

Specialist information


We carry out ongoing surveys on the occurrence of various important fruit fly species in Austria. For example, surveys on the walnut husk fly were carried out between 2008 and 2010. In addition, since 2020, in cooperation with the official plant protection services of the federal states and the chambers of agriculture, the seasonal occurrence in different federal states has been surveyed and, based on this, warning service reports on the first occurrence will be prepared from 2023.


Lethmayer, C., Egartner, A., 2019. Invasive flies in domestic fruit growing. Better Fruit (8), 7-8.

Last updated: 29.11.2023

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