Walnut husk fly
The walnut fruit 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.
In appearance and lifestyle, the walnut fruit 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 fruit fly belongs to the fruit flies (Trypetidae, Tephritidae) among the large group of flies.
The walnut fruit 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.
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.
The original home of the walnut fruit fly is southwestern North America. In Europe, it was first caught in Switzerland in the late 1980s. In the following years it was also found in Italy, Slovenia, Croatia, Germany, France and finally in Austria in 2008. However, it probably arrived in Austria several years earlier, as can be assumed on the basis of oral communications, and it was not until 2008 that the first official record was made.
Prevention and control
- With yellow sticky traps (cf. measures for cherry fruit fly), the start of flight can be determined by means of individual traps and a part of the adult fruit flies can be intercepted
- Treatment or destruction of infested fruit is the most important measure (killing the larvae, e.g. by freezing or burning the fruit).
- Another countermeasure is to cover the ground under the walnut trees, if possible, on the one hand before fruit fall, so that the larvae cannot pupate or overwinter in the ground, and on the other hand also in spring or summer (from about the end of June), to prevent the flies from hatching or flying out.
- Plant protection products for the control of the walnut fruit fly are listed in the register of plant protection products approved in Austria
- Variety selection: certain varieties seem to be better able to maintain fruit quality in case of infestation than others (cf. Solar et al. 2020 as well as Agroscope fact sheet)
We continuously conduct surveys on the occurrence of various important fruit fly species in Austria. For example, surveys on the walnut fruit fly were conducted from 2008 to 2010. In addition, since 2020, the seasonal occurrence in different federal states has been surveyed in cooperation with the official plant protection services of the federal states or the chambers of agriculture.
Lethmayer, C., Egartner, A., 2019. invasive flies in domestic fruit growing. Better Fruit (8), 7-8.
Last updated: 15.12.2021