Spotted wing midges are invasive midges introduced from Southeast Asia well known as pests of fruit crops.

Invasive midges lay their eggs by breaking down the mechanical protection of the fruit’s skin, providing an entry point for subsequent infestations. Egg laying and inoculated microbes then accelerate decay and as a result the fruit rots and becomes inedible.

Although this midge is known to cause massive economic damage in agriculture, little is known about its ecological impact on more natural ecosystems such as forests.

Invasive midges compete with native frugivorous insects

A recent study by Swiss scientists from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL and Ökobüro Biotopia, published in magazine NeoBiotaconcluded that spotted-winged midges compete strongly with other frugivorous species and that their presence could have dire consequences for ecosystems.

The research team assessed the use of potential host plants at 64 forest sites from mid-June to mid-October 2020, checking a total of 12,000 fruits for midge egg deposits.

How big is the problem?

To determine whether midge attacks trigger fruit decay, scientists recorded symptoms of fruit decay after egg laying. In addition, they monitored the fauna of other midges (drosophilids) in the area, hypothesizing that spotted wing midges would outnumber and compete for food with other fruit-eating insects.

The authors found egg deposits on the fruit of 31 of the 39 fruiting forest plant species they studied, with 18 species showing an attack rate of over 50%.

In addition, more than 50% of affected plant species showed severe rot symptoms after egg laying. Ovipositions can alter the attractiveness of fruits by changing their chemical composition and visual cues such as color, shape, and reflective patterns, which in turn can cause seed dispersers such as birds to consume fewer fruits.

Invasive midges make fruit unappetizing to birds

Given the large number of infested fruits, significant ecological impacts can be expected. “Rapid decay of fruit attacked by the spotted wing fly (Drosophila suzukii) results in a loss of fruit available to other species competing for this resource and may disrupt seed dispersal mutualisms due to reduced fruit consumption by dispersing animals , such as birds,” says Prof. Martin. M. Gossner, entomologist at WSL.

“If the midge reproduces in large numbers, both seed dispersers and plants could suffer,” he added.

Global warming can amplify the unwanted effects

The authors also found that invasive midges were highly represented and dominant in trap catches and showed that the more abundant they were, the less abundant native drosophilids were. This suggests additional negative effects of invasive species on native communities.

With ongoing climate change, these potentially severe ecological impacts could be amplified in temperate forests, as higher mean and winter temperatures will most likely lead to shorter generations and lower winter mortality, which will ultimately further increase the pressure on berries and the competitiveness of spotted-winged midges on native drosophilids, the authors note, according to

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