What's eating these endangered orchids?

September 20, 2018, Kobe University
The Golden Orchid blooms (left) and J. tokunagai on the flower. Credit: Takuto Shitara

A species of seed-feeding fly is critically damaging the seed production of multiple orchid species, as revealed by a group of Japanese researchers. If the damage caused by this fly is occurring long-term and across Japan, these already-endangered orchid species could become unable to reproduce using seeds, and their dwindling numbers will take a large hit.

The survey was carried out by Project Associate Professor Kenji Suetsugu (Kobe University Graduate School of Science), Shigeki Fukushima (Head of the Chiba Prefectural Agriculture and Forestry Research Center) and Masahiro Sueyoshi (Principal Investigator at the Forestry and Forest Products Research Institute). The findings will be published on September 21st in the online edition of Ecology.

With over 20,000 species classified, orchids are one of the most diverse groups of flowering plants, and the unique shape of their flowers has entranced people for many years. Unfortunately, this popularity has led to orchid overharvesting. Combined with loss of habitat caused by development, this means that over 70 percent of Japan's native orchid species are classified as endangered by the Ministry of the Environment.

From the perspective of genetic diversity, it is better for endangered species to reproduce via seeds than cloning. This means that to save these orchids, researchers should identify the insects that pollinate them and the parasites that prevent -based reproduction. Therefore, Professor Suetsugu's research team has been collecting information on organisms related to orchid reproduction.

At a glance this fruit looks healthy, but it has been infested by J. tokunagai. The brown area indicated by the arrow is fly pupae. When the fruit is dissected, we can see that there are no seeds left. Credit: Kobe University

During a survey to identify the pollinators of orchid species, the team found that in pollinated fruit that should be able to produce seeds under normal circumstances, a seed-feeding fly known as Japanagromyza tokunagai was destroying the seeds of multiple orchid species (see figure 1).

When orchids bloom, J. tokunagai lays its eggs in the young fruit, and the insect young grow up eating the seeds in the fruit. They become pupae inside the fruit, and when they hatch into their winged forms, they make a hole in the fruit in order to exit. Fruit that is parasitized by these flies grows to the same size as normal fruit, so at a glance, it looks healthy. This means that researchers have probably underestimated the caused by these flies. In many cases, fruit parasitized by the flies produces no seeds at all (see figure 2). Scientists have known about the damage caused by seed-feeding flies since the 1980s, but they do not know the impact on seed production in concrete terms.

In this study, after artificially pollinating five species of orchids in Japan's Kanto region, the team covered some specimens with bags to prevent the J. tokunagai from entering, and left others uncovered. Afterward, they compared the quality and amount of the seeds produced by each plant. This is the first study to quantify the reduction to orchid seed production caused by J. tokunagai. Results showed that in all five species, damage caused by J. tokunagai reduced seed production by over 95 percent. It is not yet clear whether this is occurring across Japan or over an extended time period. However, if this situation continues, the affected will become unable to reproduce via , threatening their already dwindling numbers.

It is also possible that damage caused by J. tokunagai may be intensifying in recent years. This could be for two reasons: First, the flies are non-native species that have been introduced into areas where they lack natural enemies; second, the populations have become fragmented, also reducing the population size of natural enemies.

Professor Suetsugu says, "Going forward, we want to shed more light on the damage caused by J. tokunagai. We plan to do this by quantifying the damage in other areas of Japan, and by testing the theory that J. tokunagai is a non-native species through genetic analysis."

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