During the summer, strange formations can be found on some species of Goldenrod. The stems become enlarged and form a hardened golf-ball like object called a gall. Cut into this weird sphere and you'll quickly find what causes the plant to create such a strange object: the larvae of the Goldenrod Gall Fly.
The Goldenrod Gall Fly (Eurosta solidaginis) is a parasite which uses the Goldenrod for protection and nutrition for a whole year while it grows and pupates. When it does, it can seriously damage the Goldenrod, even prevent the plant from flowering and producing seeds. This is a serious blow to the plant, so the Goldenrod doesn't just take this kind of damage laying down. When the flies are flitting about searching for rods to lay their eggs in, the rods in turn try to avoid their pests in a very recognizable way: they duck.
Scientists had found that during the few weeks that adult flies spend mating and laying their eggs, some Goldenrod stems undergo an odd behavior: they stop growing upward and instead curl and grow downward, creating a "candy-cane" like stem (see L). Once the mating season is over, they straighten up again, just in time to flower and seed.It had been found that this odd downturn made the rods twice as resistant to the flies, but scientists weren't sure if it was the behavior causing this resistance or something intrinsic about those plants that made them resistant and turn down.
Luckily, they discovered that bent plants straighten when placed in the shade, and this gave them the unique and rare opportunity to investigate causation. Researchers took both straight and bending plants and place them in a greenhouse both in the sun and in the shade. As the shaded bent plants un-curled, they set Eurosta solidaginis upon the lot, and recorded which ones ended up parasitized.
The straight variety were parasitized heavily whether in the sun or the shade. But the 'ducking' plants completely avoided parasites - if they were still bent (see R). Those that began to straighten by being placed in the shade were parasitized just as much as the non-bending variety, strongly supporting the hypothesis that the bending behavior itself created the observed resistance to parasitism.
Moreover, the experiment explained why the ducking behavior works. Unlike when people duck, the plants did not cause the flies to miss. Indeed, female flies often landed on the bent stems. However, the researchers explained, the flies appeared confused or disoriented by the plant's behavior. It's possible they didn't recognize the bud where they usually lay because of its orientation, or interpreted the bend as a damaged or unhealthy plant that won't last long enough to nurture their young. For whatever reason, the female flies decided that the plant was not a good choice, and moved on to other plants instead of depositing eggs.
The researchers have yet to understand, though, why the bending behavior is relatively rare in Goldenrod populations. Their future research hopes to learn more about how this behavior occurs.
Wise, M., Abrahamson, W., & Cole, J. (2010). The role of nodding stems in the goldenrod-gall-fly interaction: A test of the "ducking" hypothesis American Journal of Botany, 97 (3), 525-529 DOI: 10.3732/ajb.0900227
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