Europe
Bull thistle (Cirsium vulgare), musk thistle (Carduus nutans), Italian thistle (Carduus pycnocephalus), slenderflower thistle (Carduus tenuiflorus), and (rarely) plumeless thistle (Carduus acanthoides)
Larvae are tan maggots growing up to 19 mm long. Adults are fuzzy with orange-tan hairs, large black eyes, and clearish wings. Adults can be up to 15 mm long, including their wings.
Adults emerge very early in the spring and deposit eggs on young leaves as host plants bolt. Larvae soon emerge and mine into shoots and stems. As the season progresses, larvae then mine into roots and continue feeding as they develop through three instars. Pupae overwinter in roots or in plant litter. There is one generation per year.
Damage similar to what is caused by this biocontrol agent has been observed in several native thistles species (Cirsium edule group). However, despite multiple surveys of native Cirsium spp. in California, C. grossa has never been observed causing nontarget impact in that state. Nevertheless, caution should be used when considering the introduction of this fly into new areas.
Larval mining interferes with water and nutrient transport, ultimately resulting in decreased seed production and sometimes plant death.
Randall, C.B., J.E., Andreas, J. Milan, and K. Gladem. 2024. Introduced Thistle Biocontrol Agents: History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2024-8-INTRODUCED THISTLES-A. https://bugwoodcloud.org/resource/files/29169.pdf