Devil Weed: Another Plant from Hell in Hawaii

By Franny Kinslow Brewer, Communications Director for the
Big Island Invasive Species Committee (BIISC)

Siam weed, bitter bush, jack in the bush, masigig, huluhagonoi, and triffid weed…no matter what common name you call it,
Chromolaena odorata is named as one of the world’s worst 100 invasive species on the planet. So, when it popped up on O‘ahu in 2011 this plant from hell was referred to as devil weed here in Hawai‘i.
Native Devil weed is native to South and Central America, the American Tropics, and the United States (Florida and Texas only ). It was first detected on O‘ahu by the Army Natural Resources Program during routine plant surveys at the Kahuku Training Area (KTA) in 2011. Following this initial detection, the Hawai‘i Pacific Weed Risk Assessment evaluated this plant species and calculated a score of 28 suggesting the potential to become highly invasive in Hawai‘i. This really isn’t a big surprise though. Devil weed has been rapidly invading lands from SE Asia, West Africa and South Africa, Australia and the south Pacific islands, moving around on agricultural and forestry products, seed, gravel, vehicles and equipment. It’s so good at being so bad… Devil weed can mature within a year, flowering between December and February and each plant can produce 800,000 seeds annually, for about a decade. As with many asters, they develop
puffy clumps of seeds that are easily distributed by wind in the local area. This allows for massive establishment of thickets, smothering other vegetation. It also has allelopathic properties, releasing chemical compounds in the soil that help suppress the growth of
other plants. In addition to this rapid localized spread, these seeds are like glue when they are wet, sticking to anything or anyone that brushes against the plant or picks up seed contaminated soil. This has allowed long-range introductions that we are now seeing across
O‘ahu…and most recently interisland. The preferred habitat of devil weed is well drained soils, open and disturbed areas, trail and roadsides, riparian and forest edges. The plant quickly invades dryland areas, pastures and farmlands, which is of great concern because it’s toxic
to livestock when ingested and can cause skin and respiratory irritation in humans. It also increases fuel loads for regional wildfires to burn farther and hotter.

     In areas of West Africa where management tools and other resources are limited, subsistence farmers have had to relinquish their farm and ranchlands to this aggressive plant. After the detection at KTA in 2011, the Army Natural Resources Program (ANRP) jumped into action setting up survey buffers, removing plants and implementing control measures. With such a large area to survey, ANRP
partnered with the O‘ahu Invasive Species Committee (OISC) to get more personnel in the field to pull plants out of the ground. After the KTA detection, an area in Kahana Valley was detected about 20 feet off a main trail in 2013 and a short time later was detected at
Schofield Barracks. Given the distance and topography from KTA, it’s unlikely that this population resulted from wind dispersal rather human activities were the most-likely the vector of introduction. Aggressive management by the ANRP and OISC has worked at suppressing
plant abundance regionally in  these initial areas, but both long-range and local dispersals have overwhelmed the available resources. ANRP continue to manage the infestation at Schofield Barrack and along with OISC manage the KTA/Kahuku Motocross Track on O‘ahu’s north shore. OISC also continues to work on the infestation in Kahana Valley with hopes of a localized eradication there. However, over the past five years a large area in the Pūpūkea-Paumalū Forest Reserve has become infested and numerous smaller populations have been reported
across the island.
This combination of increased detections and limited staff and resources is making island-wide eradication unlikely. What’s more, is
that in February of 2021 devil weed was detected in Hilo growing at the dragstrip and motocross area. The Big Island Invasive Species Committee (BIISC) and the Hawai‘i Department of Agriculture are working to find the extent of the infestation, removing all plants as they survey. The hope is that devil weed is still eradicable on Hawai‘i Island. (Hawai‘i Island residents should email suspect devil weed locations and photos to biisc@hawaii.edu.) What’s next? While devil weed is unlikely to be eradicated from O‘ahu, suppression of the plant is still a
priority given its threats as an agricultural pest, threats to native species, wildfire and human health risks. OISC
and ANRP continue to work on suppression at KTA, Schofield Barracks and Kahana Valley. OISC now has an outreach program that is focusing on surveying and removing plants from heavily trafficked trails to help stop some of these long-range dispersals. To do this, they’ve started the Devil Weed Crew program which is a network of volunteers conducting early detection surveys of O‘ahu’s trails and removing any plants found. OISC is also working with Conservation Dogs of Hawai‘i whose trained K9s are able to sniff out plants and able to cover more ground than a person visually surveying. its native range, there are approximately Is biocontrol an option? Maybe! In
21 known natural enemies to devil weed comprised mostly of herbivorous insects and some fungal pathogens. Not surprising that the lack of these natural enemies outside its native range is a huge contributor to devil weed being one of the top invaders on Earth. That
said, the plant has been so disruptive in other parts of the world that much biocontrol research has already been conducted and successful releases of several control measures have already been established in other parts of the world. In Hawai‘i, the candidate to undergo the evaluation process is a small, herbivorous fly Cecidochares connexa. It has had several successful introductions with positive results in
Asia, Africa and the Pacific, and host specificity testing for Hawai‘i is set to begin this year.
If you come across devil weed on O‘ahu, pull it out and throw it in the trash. Do not green waste or compost this plant. If you are a farmer or rancher that has a sizeable infestation, email OISC at oisc@hawaii.edu for specific management recommendations and
strategies to help you keep your ag lands free of devil weed. Surveying and removing plants before they flower and set seed is recommended. Being proactive in detecting and removing plants as soon as they are found can save a lot of time and money when managing this pest. This plant is a devil of a weed, but the fight isn’t over!

For more information on devil weed, how to ID the plant, and management recommendations visit: https://www.oahuisc.
org/devil-weed/ Franny Kinslow Brewer is the Communications Director for the Big Island Invasive Species Committee (BIISC)

For more information, visit: www.oahuisc.org/volunteer/devil-weed-crew/
Conservation Dogs of Hawai‘i have been training scent detection dogs for devil weed since 2018, working on both public and
private lands. www.conservationdogshawaii.org

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About the author  ⁄ Garrett Webb

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