During the summer of 2023, flower bud galls were abundant across Missouri on American elderberry (Sambucus nigra subsp. canadensis) plants. These galls appear as enlarged florets on umbels. The florets never develop into fruit, causing yield loss (Figure 1).
Figure 1 Flower gall on 'Bob Gordon' collected June 6, 2023, in Fordland, Missouri. Photo courtesy of Patrick Byers
A yellowish-orange larva feeds on the floral tissue inside the gall during early June. When examined, the larvae are about 3.5 mm long and have a brownish spatula on the thorax when viewed in a ventral position (Figure 2). Several functions of the spatula have been suggested, including its use to dig or scrape plant tissue for feeding, to move the larva backward inside a gall, for clearing a space, for preparing an emergence hole to exit the gall, or for moving through soil.
Figure 2 The head capsule and the brown, sclerotized spatula (red arrow) located on the thorax of a larva found inside a flower bud gall on American elderberry in Missouri.
On the larva, about six dorsal papillae are visible on the first three segments of the body. Two pair of pleural papillae are also present on segments one to eleven (Figure 3). Two upwardly curved hooks are flanked by two anal papillae on the posterior segment (Figure 4). Unfortunately, adults were not collected, so a definitive identification of the insect species was not obtained. However, Raymond Gagne, Adjunct Scientist at the Smithsonian National Museum of Natural History has identified this midge species as Schizomyia umbellicola from the photographs of larvae shown below.
Figure 3 A dorsal view of an orangish-yellow larva removed from an American elderberry flower bud gall in Missouri.
Figure 4 The posterior segment with two brown, sclerotized hooks on a larva extracted from an American elderberry flower bud gall in Missouri.
In Europe, a flower bud gall occurs on European elderberry (Sambucus nigra subsp. nigra), common privet (Ligustrum vulgare), and European fly honeysuckle (Lonicera xylosteum) plants, which is similar in appearance to that observed in Missouri. However, the gall inducer in Europe has been identified as Placochela nigripes on several websites. The larvae of P. nigripes resemble those collected from flower galls in Missouri when viewed without magnification. However, the morphology of P. nigripes larvae from Europe was somewhat dissimilar to those collected in Missouri, when the latter specimens were examined under a microscope. Adult P. nigripes are long-legged flies with wings 2.0 to 2.5-mm long and simple venation (Figure 5).
Figure 5 Female (left) and male (right) adults of Placochela nigripes that induce flower bud galls on European elderberry in Norway. Photos courtesy of Hallvard Elven
Flower galls have also been observed on blue elderberry (Sambucus cerulea) in the United States. Larva of this midge species are pale green with dark lateral stripes on body segments (Figure 6). However, according to Raymond Gagne it is a lepidopteran species. Apparently, this blue elderberry gall inducer is misidentified as Schizomyia umbellicola on some internet sites.
Figure 6 A green-colored larva with black lateral stripes on segments within a flower bud gall on blue elderberry near Morrow Bay, California.
Other elderberry flower galls have been noted. In 1901, Arnoliola sambuci, Kieffer, was described, but this species has white larvae. Another flower bud gall on European elderberry is induced by a yellow, jumping larva, known as Contarinia sambuci. Also, in the eastern European country of Georgia, another gall has been reported on danewort (Sambucus ebulus).
Clearly, more work needs to be done to properly identify the midge species that induces the enlarged florets on American elderberry plants. In the future, adults may be reared for morphological descriptions. DNA analyses are also needed to compare the insects found in Missouri with those of known species. Finally, field experiments are needed to determine if adult midges can be controlled in the spring before larvae enter elderberry florets.