Technical Service Center
Environmental Applications and Research Group — Publications
Biological Control of Leafy Spurge, Bureau of Reclamation Research Sites
Technical Memorandum No. 8220-99-06
Leafy spurge (Euphorbia esula) is an aggressive perennial that grows to a height of 3 feet and produces a milky latex sap that is irritating to cattle and humans. The deep root system of this non-native plant makes it very difficult to control by mechanical or chemical methods. While leafy spurge is primarily a rangeland weed, it occurs on Reclamation ditchbanks and rights-of-way. Weed control activities in these areas are heavily influenced by proximity to water, which restricts the use of certain herbicides. The resulting maintenance problems from the structural effects of the roots on the banks, safety issues due to skin and eye irritation from the plant's sap, and the need to avoid infesting neighboring agricultural fields and rangeland require action from Reclamation managers.
A number of insects have been introduced into the United States for the biological control of leafy spurge, each with its own habitat requirements. In an effort to gain experience locally in matching insect species to site conditions and monitoring results, Reclamation began a cooperative project with Jefferson County Weed and Pest Control (JeffCo) at a location near Pine, CO. The insects released in June 1995 were Spurgia esulae, Apthona nigriscutus, and Oberea erythrocephala.
In June 1997, personnel from both Reclamation (Denver, CO) and the Garrison Diversion Conservancy District (Oakes, ND) attended a leafy spurge biocontrol workshop held in North Dakota. At this workshop, we received instruction in release, monitoring, and collection techniques for Apthona flea beetles. At the close of the workshop we were given insects which we released at several Reclamation sites near Oakes, ND. The insect species released were Apthona nigriscutus, Apthona lacertosa, and Apthona czwalinae. Personnel from the Denver and Oakes offices will conduct monitoring of the insects and vegetation at these sites.
The following information about the biocontrol insects is taken from Biological Control of Weeds in the West (Western Society of Weed Science, 1996).
Spurgia esulae - Common name: leafy spurge tip gall midge. The delicate adult emerges in April and has a very short life span. Eggs are laid on the growing tips of plants, and the larval feeding causes the tip to form a gall. This affects leafy spurge's ability to flower and produce seeds. The midge prefers dense spurge growing on south-facing slopes, and does not do well near rivers or in windy areas.
Oberea erythrocephala - Common name: red-headed leafy spurge stem borer. The female beetles deposit eggs in spurge stems, and the larvae tunnel in the stem and roots, affecting the spurge plant's viability. There is normally one generation per year, but in cooler climates it may take two years to complete one generation. They seem to prefer areas with trees, and have established and increased best in riparian areas.
Apthona (flea beetles):
Apthona nigriscutus - Common name: black dot leafy spurge flea beetle.
Apthona czwalinae - Common name: black leafy spurge flea beetle.
Apthona lacertosa - Common name: brown-legged leafy spurge flea beetle.
The female flea beetles lay eggs on the plant stem near or below the soil surface. Larvae begin feeding on small roots and continue into the larger roots. This feeding impairs the root's ability to take up moisture and nutrients, reducing the potential plant height and retarding the flowering period. It also creates holes that open the roots to disease. High concentrations of the beetles often cause an "Apthona circle", a circle of very low density spurge within the weed infestation. These insects do best in warm, sandy or loamy soils where the smaller roots will be near the soil surface and easily accessible to small larvae. A. nigriscutus does better than the other flea beetles on hotter and drier sites, lighter soils and higher on hills. A. czwalinae/lacertosa are better able to tolerate cooler and moister soils and more shade than A. nigriscutis.
Reclamation release sites in North Dakota were chosen to fit criteria developed by the USDA: not at risk for flooding, well-drained loamy or sandy soil, and good sun exposure. Flea beetles were obtained through a workshop/field day sponsored by the North Dakota Department of Agriculture and were kept cool until they were released the following day. These insects had been collected by field day sponsors shortly after the adults emerged from pupation. Collection at this time ensures that the majority of their eggs are laid at the new field site.
Current standard practice for the release of Apthona is to mark a suitable site with a stake and release all the flea beetles at the stake, rather than scattering them about. Cages or tents are not necessary for this insect because of its slow movement or dispersal rate. A release of 500 - 1,000 beetles per site frequently results in successful establishment.
Initial monitoring methods were adapted from methods used to monitor purple loosestrife biocontrol sites. At the 1997 leafy spurge biocontrol workshop in North Dakota, experts described their monitoring methods for Apthona. Adopting the new methods for the 1998 field season did cause some gaps in the data collected, but new data collection forms have now been devised that should address the continuity of data (Appendix I, Table 1). Data for Apthona biocontrol will be collected in the categories: (plant) % cover, % flowering, % senescence, density, height, and (insect) numbers per five sweeps. Data will be collected at several points at 0, 5, 10, and 15 meters from the initial release marker because of the way Apthona expands populations in a circular fashion. Diameters of any "Apthona circles" that develop will be tracked. Data collection for other biocontrol insects will be based on this system, but will only be collected at a permanent quadrat, rather than in expanding circles, due to the differences in insect movement and expansion.
Monitoring of all sites will take place around July, with an additional visit to the Pine site in late May or early June to look for Spurgia galls. Monitoring at the Oakes site will be scheduled during peak abundance of Apthona adults. This has been found to occur between 1200 - 1500 growing degree days for sunflower (Appendix I, Table 2).
In an effort to gain experience locally in matching biocontrol insect species to site conditions and monitoring results, Reclamation began a cooperative project with Jefferson County Weed and Pest Control (JeffCo). In June of 1995, JeffCo purchased and released several biocontrol insect species at a private site near Pine, CO. Reclamation supplied two 12' x 12' x 6' screen tents for the releases, and will be conducting future monitoring of the site. The insects released were Spurgia esulae (300 galls), Oberea erythrocephala (300 adults), and Apthona nigriscutus (400 adults). The use of tents for Apthona and Spurgia was not absolutely necessary to confine the insects to a known starting location. Having a known starting location can make relocating the insects for monitoring purposes much easier, and can make it possible to document a rate of spread. However, since it was best to do a contained-release of Oberea, tents were used for all the insects.
In 1997, Reclamation took aerial photos of the release site for future reference and assessment of the effectiveness of control methods. We also conducted insect surveys and plant height/density measurements in 1996, 1997, and 1998 (Appendix I, Table 3). Plant data from these surveys show some decreases in height, density, and flowering (sites 3, 5) that may be a result of the biocontrol insects. Unfortunately, recovery of biocontrol insects at those sites was not strong enough to show a definite correlation.
In 1997, the only evidence of insect survival was galls of the type formed by S. esulae, and one O. erythrocephala adult. Evidence of both these species was found near their original release sites. We did not find any Apthona, either by using the sweep net or by digging up roots to look for larvae. It is possible that we missed the peak abundance times for these stages.
In 1998, surveys for insects again turned up galls of the type formed by S. esulae, and another adult O. erythrocephala was found in the vicinity of the original release. A number of plants in the latter area had stem damage of the type associated with O. erythrocephala. This insect has proved to be difficult to establish at other sites, so it is encouraging to find any adults and plant damage. It may take a long time for this insect to build to a widely damaging population, as there is only one generation every one or two years (depending on climate).
Surprisingly, a large number of Apthona were found at a site not near their original release. A number of conditions made it appear that the insects had recently been released at that point (high density close to a marker stake from previous years, very little feeding on the leaves by adults, and no sign of plant stress from larvae in roots). Inquiries to JeffCo and to the Colorado State Department of Agriculture did not turn up any information about a release, so the source of these insects is unknown. It will be interesting to note in 1999 if the flea beetles have established at that particular site and if the plants show stress.
In June 1997, personnel from both Reclamation (Denver, CO) and the Garrison Diversion Conservancy District (Oakes, ND) offices attended a leafy spurge biocontrol workshop held in North Dakota. At this workshop, we received instruction in release, monitoring, and collection techniques for Apthona flea beetles. At the close of the workshop we received insects (about 12,500 total of Apthona nigriscutus, Apthona lacertosa, and Apthona czwalina) which were released at several Reclamation sites near Oakes, ND (Appendix III). Release sites were chosen for good sun exposure, and good upperlevel root growth of the spurge plants. These sites were marked with stakes and insects deposited next to the pole. The positions of the release locations marked by the stakes were backed up by GPS readings, and photographs taken. Additional sites were established in 1998 using a total of 5,000 flea beetles obtained at another North Dakota field day and from Jamestown Dam.
The sites were closely monitored for flea beetle emergence by Garrison Diversion personnel, and on July 9, 1998 data were collected by personnel from Denver and Oakes offices (Appendix 1, Table 4). The insects had been on-site for one year, and while it generally takes about three years to notice effects on the spurge, results are promising. The flea beetles have established at all sites, and at one site ("pilot drain 2") they are found in high numbers.
Annual monitoring will continue for at least the next two summers, with relocation of insects to additional sites as populations warrant. Monitoring will be conducted based on the monitoring form shown in Appendix 1, Table 1 by personnel from both the Garrison Diversion and Denver offices. Timing of the monitoring will be based on research showing that the peak emergence of Apthona is between 1,200 to 1,500 sunflower growing degree days (Appendix 1, Table 2). Monitoring and collection of insects for redistribution should ideally be performed as soon as possible after peak emergence of the new adults.
Revegetation plans are probably unnecessary, as a good mixture of grasses is interspersed with the spurge and should fill in as the spurge is controlled. However, sites will be evaluated as the spurge declines, and revegetation with grass seed will be done if needed.
We would like to express our appreciation for the contributions and support from the following:
Bureau of Reclamation Research Program WATER Project EE007, "Development of Improved Aquatic Site Pest Management Methods"
Bureau of Reclamation Program Analysis Office
Garrison Diversion Conservancy District
Bureau of Reclamation, Dakotas Area Office
North Dakota Department of Agriculture
Jefferson County, Colorado - Weed and Pest Management Office