Cape-Ivy / Weed Control User Tool (WeedCUT) / Methods for Managing Weeds in Wildlands

Overview

One biological control agent of Cape-ivy is permitted for release in California, the shoot tip-galling fly Parafreutreta regalis (Diptera: Tephritidae), which is native to South Africa. The Cape-ivy fly was discovered by USDA-Agricultural Research Service (ARS) scientists working with South African scientists in the early 2000s. After testing over 99 plant species in a quarantine laboratory to verify that the fly can make galls only on Cape-ivy (Balciunas et al. 2010) and demonstrating that the galls can reduce plant size by 30 to 50% in greenhouse tests (Balciunas and Smith 2006) a permit for field release was obtained in 2016 from USDA-APHIS and CDFA. The Cape-ivy fly is the first biological control agent in the world targeting Cape-ivy.

The Cape-ivy fly adults are about 1/8” – 1/4” long (Figs. 1-3), live up to 4 weeks, and females can lay up to 100 eggs (Balciunas and Smith 2010). Eggs are laid in the plant stem near a growing tip and stimulate the formation of a gall. Larvae feed on the gall tissue as it forms, and the galls increase in size to about that of an olive (3/4” – 1” long) (Fig. 4). However, gall size is variable and depends on the size and number of larvae inside the gall and the plant’s vigor/health. Larvae go through three growth stages (instars) and chew an ‘exit window’ (Fig 4, left gall) in the gall prior to pupating. Adults emerge after about two weeks and break through the ‘window’ to emerge from the gall. The Cape-ivy fly can complete one generation in about 2 months in a greenhouse at 75 °F. There are several generations per year, with a slowdown of activity during the winter months.

The Cape-ivy flywas released between 2016 and 2019 at 18 sites between Humboldt and Santa Barbara counties on both public and private lands (Moran and Portman 2020). Releases were initially conducted once in the fall or spring, but this technique had a low level of successful establishment. In 2018-2019 a new technique was used of performing five releases, at monthly intervals, at each site, using a release cage (Fig. 5) that confined adult flies for 3 to 4 weeks, then moving the cage and releasing new adults into the cage. This method led to establishment of the fly at four sites to date, including one just north of Santa Cruz (Santa Cruz County), two sites in Big Sur (Monterey County), and one site in San Luis Obispo (San Luis Obispo County) along streams and in coastal scrubs (Portman and Moran 2020). Fly populations are increasing rapidly, with some locations having gall densities of over10 galls per square yard. Additional Cape-ivy fly field release sites in Santa Barbara and Los Angeles Counties are being evaluated for population establishment by university scientists. In 2019-2020, a new release technique, involving the planting of greenhouse-galled plants at field sites (Fig. 6), was tested, with initial evidence of establishment at one site in Sonoma County and one site in San Mateo County. The impact of the fly on Cape-ivy is unknown, but studies are underway to determine its effects.

The leaf- and stem-mining moth, Digitivalva delaireae (Lepidoptera: Glyphipterigidae), which, like the fly, originates from South Africa, is being evaluated as a candidate biological control agent in the ARS laboratory in Albany, CA. It appears to have a narrow host range (Mehelis et al. 2015) but can make mines on two native Senecio species in the lab. Additional testing is underway to determine if the moth is sufficiently safe to use.

Biological Control Agents

Species	Common name	Distribution	Impact¹	Notes
Parafreutreta regalis	Cape-ivy shoot tip-galling fly	limited	N/A	First released in 2016 in CA.

¹Too early to evaluate impact in the field.

How the Technique Is Employed

The Cape-ivy fly is not currently available for re-distribution, as populations from the first field releases are still establishing and cannot be disturbed. It is anticipated, however, that re-distribution will become possible within the next few years. No Federal or state permit is required to move the Cape-ivy fly within California, but various private landowners and public agencies may have their own permitting requirements for releases on their lands.

Each field site should first be surveyed to determine if the Cape-ivy fly is already present, especially if the site is located between San Francisco and San Luis Obispo. Make a small (1 ft²) sampling square of PVC and throw it randomly into the Cape-ivy. Examine shoots for galls (Fig. 2), and consider counting number of galls with and without either intact exit ‘windows’ (indicative of pupation) or exit holes (indicative of adult emergence). Count the number of galls and total number of shoot tips (galled and ungalled). Collect data for at least 10 sampling square ‘throws’ per site. If the Cape-ivy fly is already present at the site, survey again in 3 months and look for signs of dispersal and/or galled shoot population density increase. If population increases are occurring, there is no benefit to releasing more flies.

To distribute the fly, collect mature galls (about 50) at established field sites with about an inch of stem below the gall. To keep galls fresh so that adults will emerge, stick the stem into moist florist’s foam and place them upright in a ventilated cardboard or plastic container. The container should be protected from extreme heat/cold and kept indoors. Adults can be collected from the containers using a hand-held insect aspirator. Adults can be held in a refrigerator for up to one week if not convenient for immediate release. To maximize the chance of release success, it is best to determine the number of female and male flies; this can be done easily be examining the tip of the fly abdomen from either the side or underside while flies are in vials (for easy counting limit flies per vial to 10). Females have a thick black structure (V-shaped when seen from below) at the end of the abdomen known as the ovipositor (Fig. 4), while males lack this structure (Fig. 5).

Vials containing adult flies (at least 20, even ratio of females:males) can be released onto healthy Cape-ivy shoots in the field. If possible, construct a cage made of PVC frame (square on bottom with 2 ft sides and 15 inches tall) and cover with a breathable fabric such as muslin (example shown in Fig. 5). Stake down the edges of the cages with dog or tent stakes, being careful not to crush Cape-ivy stems, and reach underneath the cage to release the flies or detached galls inside the cage before completing the stake-down process. Alternatively, adults or galls may be released without cages, but this approach makes it more difficult to track initial release success. The USDA-ARS has recently obtained preliminary evidence of the establishment success of a release method using potted galled plants, generated in a greenhouse colony, as sources of flies for field releases. Galled plants are being planted in a circle (Fig. 6) for about three months to let the adult flies emerge over time and colonize resident Cape-ivy plants. To release using this technique, it is necessary to first propagate Cape-ivy in a greenhouse, and expose plants after about 6 weeks of growth them to flies in greenhouse cages.

If releasing flies in field cages, remove the cage after 4 weeks. If more flies are available, move the cage to a new location at least 150 ft away and repeat the release procedure. Mark each cage location with pin flags. If no cage is used, mark the point of release. Two months after a release, count galls in the former cage location, or in a circular plot about 10 ft in diameter around the point of uncaged release. Make separate counts of galls with and without ‘windows’ (Fig. 4), opaque round spots which are chewed by larvae before they pupate. There may also be galls with open ‘windows’ (round holes), indicating that adult flies have emerged. By six months after the original release, if the release was successful, ‘second generation’ galls outside of the formerly-cage location(s) will be observed. Use the PVC square technique above to survey for galls outside the release location(s) along a random walk or on at least two linear transects in opposite directions from the release location. Check for overwintering and establishment the following spring using the same methods. If establishment occurs at the release patch, survey other non-connected Cape-ivy invasions at your site beginning the second year after release

Establishment is expected to reduce live Cape-ivy shoot tip density, percent cover, and spread into new habitats, but the USDA-ARS is still evaluating the impact of the fly. If flies disperse to your site or releases lead to establishment, perform annual monitoring of Cape-ivy stem tip density, percent cover, and abundance and diversity of other plant species.

Special Tips

Releases are best performed in the spring or summer (March to August).

If Cape-ivy occurs in more than one habitat type (e.g., shady riparian and open scrub/bluff), release in each habitat type and compare success of establishment.

Biological control of Cape-ivy can be complementary to herbicide treatments or mechanical control, but only if there are areas that escape the treatment (too difficult to access or too environmentally sensitive). Do not treat areas/plots at which the biocontrol agents have recently (within the past year) been released or have dispersed naturally, or at the very least, set aside ‘refugia’ plots at least 10 ft x 10 ft for the biocontrol agents to develop their populations.

Caveats

Do not release on wilted (drought-stressed) Cape-ivy, as it is inferior as a host for the fly. Do not release in fall or winter. Fall releases may involve drought-stressed plants, and releases in winter will expose the fly to low temperatures (frost can kill the shoot tips) and wet conditions that suppress adult activity and slow gall development.

Do not release along trails or other areas subject to trampling/disturbance.

The ability of Cape-ivy fly populations to survive flooding is not known. However, galls can be made on both ground-covering Cape-ivy and on stems hanging from shrubs and trees, so some of the galls are likely to survive.

Where Can I Get These?

There are no currently commercial sources for the Cape-ivy fly. The fly is not widely established but is expected to become well-established over the next few years. Contact the USDA-ARS authors for more information.

References

Balciunas, J., and C. Mehelis. 2010. Life history of Parafreutreta regalis (Diptera: Tephritidae): A candidate agent for biological control of Delairea odorata. Environmental Entomology 39:114-120. doi: 10.1603/EN09135, also available at https://naldc.nal.usda.gov/download/40006/PDF

Balciunas, J., C. Mehelis, L. Van Der Westhuizen, S. and Neser. 2010. Laboratory host range of Parafreutreta regalis (Diptera: Tephritidae), a candidate agent for biological control of Cape ivy. Environmental Entomology 39: 841-848. doi: 10.1603/EN08220

Balciunas, J. and L. Smith. 2006. Prerelease efficacy assessment, in quarantine, of a tephritid gall fly being considered as a biological control agent for Cape ivy (Delairea odorata). Biological Control 39: 516-524. doi: 10.1016/j.biocontrol.2006.08.019 Also available at https://naldc.nal.usda.gov/download/1870/PDF

Mehelis C.N., J. Balciunas, A.M. Reddy, L. van der Westhuizen, S. Neser, and P.J. Moran. 2015. Biology and host range of Digitivalva delaireae (Lepidoptera: Glyphipterigidae), a candidate agent for biological control of Cape ivy (Delairea odorata). Environmental Entomology 44: 260-276. doi: 10.1093/ee/nvu030

Moran, P.J., and S.L. Portman. 2020. Release and establishment of the Cape-ivy fly,Parafreutreta regalis, in Califoria – 2019 update. In: Pickett, C.H. (ed.), Biological Control Program Annual Summary, 2019. California Department of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, CA, pp.80-84.

Portman, S.L, and P.J. Moran. 2020. Cape-ivy galling fly established and thriving along the California coast. California Invasive Plant Council (Cal-IPC) Dispatches 28: 8, 14.