Europe, Asia, Mediterranean
Rush skeletonweed (Chondrilla juncea)
All stages are tiny and best viewed with a microscope. Eggs are spherical and white. First-stage nymphs are pale yellow and 0.10 mm long while second-stage nymphs are humpbacked, orange, have four legs, and reach 0.17 mm. Adults are worm-like and yellow-orange. While adults of many other mite species have eight legs, Aceria chondrillae adults have only two pairs of legs that are located near their heads. Males are up to 0.18 mm long while females are 0.26 mm.
Overwintering adults attack shoot buds when rush skeletonweed bolts in spring. Feeding leads to the formation of contorted galls, each of which may contain several hundred mites. Females lay 60–100 eggs within the gall they occupy. There are two nymphal stages prior to the adult stage. One generation can be completed in 10–12 days, depending on temperature, and there are multiple generations per year. Mites spread with wind-dispersed seeds throughout the growing season via silk strands that act as parachutes. Mite populations increase until skeletonweed dies back in the winter.
Mite feeding induces the formation of characteristic galls and can also create a deformed “crook” appearance on the stem. The galls stunt shoot growth, reduce rosette and seed production, reduce root carbohydrate reserves, and often result in seedling death. Though this typically does not kill older plants, mite galling can help reduce the rate of rush skeletonweed spread.
Milan, J., J. Littlefield, C.B. Randall, and J.E. Andreas. 2022. Rush Skeletonweed 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-2022-13-RUSH SKELETONWEED-A. https://bugwoodcloud.org/resource/files/25241.pdf