Unravelling Maryland’s Knotweed Problem, One Psyllid at a Time

This spring, the Maryland Department of Agriculture (MDA) has begun to raise the knotweed psyllid, Aphalara itadori, for eventual release on knotweed (Fallopia spp) plants in Maryland. The first 500 psyllids arrived in April from colleagues at the Philip Alampi Beneficial Insect lab, the New Jersey state facility, and became the start of the Maryland bio control lab’s knotweed psyllid colony. These tiny, plant sucking, aphid-like insects, are a welcome addition to existing controls for the invasive knotweed.

Knotweed, which was introduced as an ornamental and for erosion control in the late 1800’s, has spread throughout Maryland. The knotweed found in Maryland is usually Fallopia japonica but Fallopia sachalinensis occurs sporadically. A hybrid between the two, Fallopia bohemica also now occurs. Japanese knotweed, Fallopia japonica, is prevalent throughout Maryland, occurring in all counties, while giant knotweed, Fallopia sachalinensis, and possibly Bohemian knotweed, Fallopia bohemica, are also present in more than half of Maryland’s counties. All three species share the same invasive characteristics.

 The plant can produce viable seed depending on environmental conditions but the primary mode of dispersal is vegetative. Even small pieces of rhizomes root easily, and will colonize river banks to the exclusion of other plants. Because it has no fine root structure, it is a poor soil stabilizer and affects species composition and habitat quality along riparian corridors.  The plants prefer sunny, open locations such as river banks or highway right-of-way corridors. Once established, the plant is difficult to eradicate or control. Currently chemical controls with herbicide treatments are the best way to manage smaller patches of the plant. However combating a plant that is considered one of the world’s worst weeds, means using a number of tools, including biological control.

Knotweed psyllid has been released in parts of the UK, and in Canada. Permitting was finally granted for US releases in 2020. Early data from the Pacific NW and some Eastern US states have been mixed on the initial success of the psyllid. Often the success of bio control agents takes several years of releases, for establishment as well as experimentation with release protocols. The psyllid does successfully overwinter in a number of different substrates and is effective at stunting early vegetative growth of knotweed. All life stages, except the egg, feed on knotweed with piercing sucking stylet mouthparts. The psyllid can inflict fatal feeding damage, especially on young growth. Feeding signs are deformed and curled leaves that eventually brown and die.

Currently the MDA lab in Annapolis is successfully raising the Kyushu strain of knotweed psyllid. The psyllid is in the order Hemiptera or “true bugs”, and goes through a gradual change with no pupal stage. Like most bugs they start as a very tiny egg and pass through 5 nymphal stages, slowly gaining an adult appearance. Adults have clear wings marked by varying degrees of dark mottling. Late season psyllids respond to decreasing day length by darkening and will overwinter as adults in tree bark or even in woody debris and mulch.

Maryland is joining numerous other state and federal agencies in releasing the knotweed psyllid, including neighboring West Virginia and the C & O Canal National Park which runs through large portions of Maryland.