Goldenrod Gall Fly: The Master Plant Manipulator

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   It is late summer, you are walking through a sunny meadow, and the goldenrods are blooming yellow. Peering at a goldenrod flower to get a closer look, you notice something odd about its stem. The stem is swollen into a ball shape towards the top. Looking around at the field, you realize the swellings are on many goldenrod plants. It turns out that these swellings, or galls, are produced by the goldenrod gall fly, Eurosta solidagnis. This tephritid fly completes its whole life cycle only on goldenrod plants.

                                                 Lifecycle 

Eggs are deposited into goldenrod stems by female gall flies in the spring. The eggs hatch after a couple days and larvae feed on the stem. The gall grows, leading nearby leaves to wither and die. However, there is no negative effect on the goldenrod plant overall, the plant blooms normally. The gall fly larvae eat the nutrient-rich tissue in the middle throughout the summer and go through two instar stages. In the fall the gall chews a tunnel to the gall's surface to make exiting the gall easier in the spring but leaves a thin layer. Over winter the larvae go through a third instar stage, and the gall acts as minor insulation. The cold weather leads the larvae to convert their glycogen into glycerol and sorbitol, both acting as an antifreeze. In the spring the larvae pupate inside the gall, push out of the covering of the previously dug tunnel, and leave the goldenrod to mate and lay eggs. (https://hort.extension.wisc.edu/articles/goldenrod-gall-fly-eurosta-solidagnis/)


Gall Production

    Goldenrod gall fly saliva is thought to contain plant hormones that induce cell division and expansion in the plant stem. A study done by Gabriela Ponce found that the phytohormones were localized in the salivary glands of the larvae. (https://doi.org/10.1007/s11829-021-09817-5) The hormones are secreted into the plant tissue to manipulate its growth. Auxin is a plant hormone that promotes cell elongation and maintains apical dominance. Cytokinin inhibits root growth, reduces apical dominance, and regulates cell division of the root apical meristem. If there is less auxin than cytokinin then a bud is induced to grow. If auxin and cytokinin are equal, then they cause cell division, leading to callus tissue formation. Higher than normal concentrations of cytokinin and auxin have been found in both gall-inducing insects and the galls themselves. 


Description

    Goldenrod gall flies have translucent wings with brown splotches and are smaller than a housefly. Females have an ovipositor extending from their abdomen. They are not strong fliers. They live about 10 to 14 days, enough to mate and lay eggs on the young goldenrod stems. Male goldenrod gall flies rock back and forth to attract a female. After mating a female will find a suitable stem to lay the eggs. Different races of the fly are specific to different species of goldenrod, including canada goldenrod (Solidago canadensis) and late goldenrod (S. altissima). The female fly finds the preferred goldenrod species through chemical receptors on its feet, antennae, and ovipositor. (https://www.fllt.org/inside-the-goldenrod-gall/)


Predators

    Although the gall acts as protection for the larval flies, other animal species have adapted to be successful predators of the gall fly. Two species of wasps (Eurytoma gigantea and E. obtusiventris) are parasitoids of the larvae. Adult female wasps deposit their eggs into the gall tissue with a long ovipositor. When the wasp eggs hatch they feed on the fly larvae and in the spring the wasps emerge instead of the flies. Downy woodpeckers and black-capped chickadees are also predators of the goldenrod gall fly by breaking the galls to eat the larvae. Thicker-walled galls do better against wasps, but woodpeckers are more likely to go for the thicker-walled galls. 


References 

Chao, M. (2020, January 28). Inside the Goldenrod Gall. Finger Lakes Land Trust. https://www.fllt.org/inside-the-goldenrod-gall/ 


Mahr, S. (n.d.). Goldenrod Gall Fly. Wisconsin Horticulture. University of Wisconsin-Madison. https://hort.extension.wisc.edu/articles/goldenrod-gall-fly-eurosta-solidagnis/ 


Ponce, G. E., Fuse, M., Chan, A., & Connor, E. F. (2021). The Localization of Phytohormones within the Gall-inducing Insect Eurosta solidaginis (Diptera: Tephritidae). Arthropod-plant interactions, 15(3), 375–385. https://doi.org/10.1007/s11829-021-09817-5 

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