Europe and Central Asia
Hoary cress (Lepidium draba)
All stages of this species are tiny and best viewed through a microscope. Eggs are spherical and white but turn slightly yellow with age. While adults of many other mite species have eight legs, Aceria draba nymphs and adults have only two pairs of legs; these are located near their heads and have feather claws at their tips. Nymphs and adults are elongated and translucent to opaque white, but may turn yellowish-orange at maturity. First-instar nymphs are smaller and have reduced features, while second-instar nymphs are similar to adults but lack a genital opening. Adults are 0.2– 0.3 mm long.
As hoary cress plants bolt in spring, mite nymphs and adults feed on plant tissue, inducing the formation of contorted galls on stem buds and nodes, as well as floral buds. Females lay eggs within the gall tissue, and there are two nymphal stages prior to the adult stage. One generation can be completed in 10–14 days, depending on temperature, and there are multiple generations per year. As hoary cress plants senesce during summer, mites migrate down to the roots where they overwinter. Throughout the growing season, mites primarily spread via wind, although some are also carried on the bodies of visiting insects.
Galls induced by mite feeding create a characteristic deformed appearance. The galls stunt shoot growth and reduce or eliminate seed production.
Littlefield, J., C.B. Randall, and J. Milan. 2023. Hoary Cress 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-2023-31-HOARY CRESS-A. https://bugwoodcloud.org/resource/files/27963.pdf