Galling Flies report

Controlling Cape Ivy speaker

Using flies to control weeds

They could limit Cape Ivy’s damage

About 20 Cambrians eager to control Cape Ivy heard the good news about a fly that could help control this invasive weed at the Cambria Forest Committee meeting in August 2019.

Kirsten Sheehy, a research specialist in the Riparian InVasion Research Laboratory (RIVRLab), described this colorful fly, Parafreutreta regalis, and explained the process of vetting it before releasing it. It can’t kill the plant, but it can slow down its progress.

How it works

The fly works by laying its eggs at the growing edge of the plant’s shoots, the shooting tip. The eggs hatch, larvae grow, and the resulting gall prevents the plant from flourishing. Compare the plant on the left, which has been infested with the flies, with the normal growth of the plant on the right.

Cape Ivy is one of the worst invasive plants crowding out native plants along the California coast. It’s sometimes called the Kudzu of the West. Cambria has its share.

RIVRLab studies the impacts of non-native species in riparian and aquatic systems and develops methods to manage, if not eradicate, them.

Cape Ivy’s invasion

Cape Ivy, and its fly, are native to South Africa. The plant’s bright green, shiny leaves made it a popular houseplant in the 1950s. It escaped from those confined pots and started taking over the Central Coast landscape. As with many non-native plants, other organisms that control it in its native habitat aren’t here to stop it.

Cape Ivy now infests over 500,000 acres along the coast and some inland areas, from San Diego and Baja north to Washington. Even the Channel Islands have patches. It’s relentless, growing over power poles and railroad tracks, as fast as a meter and a half a month. The moisture in fog alone may be enough to keep it alive. Ms. Sheehy has even found a dry leaf on the floor sprouting roots.

Frost can kill it, but it doesn’t get cold enough along the Central Coast for that to happen. Weed whacking and herbicides can cut it back, but it grows back quickly. Controlling it manually and with chemicals is expensive, labor intensive and time-consuming

Cape Ivy damages the forest habitat by crowding out native plants. Displacing native plants, it deprives local wildlife of the food they need. Researchers think that as native plants disappear, the native insects that may have helped control invasive plants disappear. When it dries out in summer, it’s flammable.

The pyrrolizidine alkaloids it produces may endanger steelhead trout. It may be toxic to aquatic invertebrates and other plants. RIVRLab is conducting research on that point, using ostracods as an ecosystem indicator.


Biocontrol, using one organism to control another, can boost other methods of control. The Cape Ivy flies could reduce infestations, making mechanical and chemical control more effective.

Plants become invasive weeds when they are taken out of their native ecosystem, where other native plants, insects and animals kept them in check. Researchers went to South Africa, Cape Ivy’s home, to look for insects that could target Cape Ivy. Although they found hundreds of potential control insects, they narrowed that down to two.

No one wants to release an insect that will become a problem to other plants. Ms. Sheehy explained that the flies are tested in two ways. In the Choice test, flies are confined with Cape Ivy and other local plants, to determine whether the flies will complete their life cycle on them. They didn’t.

Next, they are confined only with the local plants, with no Cape Ivy available. “We want to see whether they would rather die than lay their eggs on something else,” she said. They didn’t.

Those results are presented to the Technical Advisory Group, then reviewed for approval by the US Department of Agriculture before the flies can be released. In the case of Cape Ivy flies, nearly a hundred local native plants were tested. The flies died rather than use them. That process took seven years.

The flies are effective in controlling Cape Ivy, and no danger to other plants. Next, learning to grow enough of them to release into Cape Ivy areas that need help.

Releasing flies

Ms. Sheehy is developing best lab practices for mass rearing of the flies. Hydroponic methods, rather than raising the flies on potted plants, increase production with less hands-on effort. Getting the nutrient solution just right has taken time.

Eventually, flies could be available to the public for weed control. With enough Cape Ivy around, the flies could be self-sustaining, or even produce enough galls that they could be shared with other infested sites. Whether the flies survive winter isn’t yet known.

A small number of flies were released on one San Luis Obispo site, on Tank Farm Road, and galls were found on the plants in June.

“We really didn’t expect establishment, since it was a small release a long while ago,” she said. “Certainly not the numbers we need to control the ivy yet, but with a few more releases we may get there.”

Future of flies

Cambria may be a site for release of the flies in the future. RIVRLab plans to release flies at single sites by 2020, adding sites later in the year. Several people volunteered their land for the flies.

After the release, Ms. Sheehy follows up to determine how the flies – and the Ivy – fared.

Because of past organism introductions that became invasive, researchers and the USDA are extremely cautious about testing before allowing the release of a non-native insect. The Stem Mining Moth, which actually kills the Cape Ivy plant, is currently being tested to make sure it can’t affect native vegetation.

That’s the one I’m waiting for.

Click here to see her Cape Ivy biocontrol slides.

— Christine Heinrichs

More information on Cape Ivy Galling Fly from the 2017 Cal IPC Symposium.

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