Management Requirements: Control of cocklebur requires active management once it becomes established in an area.
MECHANICAL CONTROL
According to Weaver and Lechowicz (1983), young plants of XANTHIUM STRUMARIUM regenerate readily from the lower nodes if trampled, clipped, or otherwise injured. Fruit on older plants or shoots which have been cut or damaged will continue to ripen provided fertilization has occurred prior to the injury. Burs may persist on dead plants for up to 12 months (Parsons 1973). Since plants can regrow, mowing is not an effective control measure for cocklebur.
Physical removal of the plants by hand pulling or hoeing them is effective if done prior to flowering. If left until after seed development, plants should be carefully removed so as not to dislodge the burs, piled, and burned (Parsons 1973).
PRESCRIBED BURNING
Burning is an effective means of destroying cocklebur seeds, but prescribed fire has seldom been used for this purpose.
BIOLOGICAL CONTROL
Much work has been done on the insect pests of XANTHIUM STRUMARIUM (Kelly 1931, Wilson 1960, Hare 1977, 1980, Hare and Futuyma 1978, Foote 1984). Hilgendorf and Goeden (1982, 1983) provide good reviews of sap and foliage feeding (phytophagous) insects associated with X. STRUMARIUM. They list 60 different species that attack it in different parts of the world. Although many of these also attack cultivated plants, eight feed only on plants in the Heliantheae Tribe (ragweeds and cocklebur).
The insect fauna of XANTHIUM species in the central U.S. is richer in species and trophically more specialized than in California (Hilgendorf and Goeden 1983). In California, these insects probably switched from ragweed when cocklebur reached the state. Nine insect species feed on cocklebur as immatures.
Several species of insects have been introduced to Australia to control XANTHIUM STRUMARIUM, but results have generally been disappointing (O'Connor 1952, 1960, Wilson 1960, Wapshere 1974b). The most promising control species there appears to be NUPSERHA ANTENNATA Brun., a beetle native to India and Pakistan (Haseler 1970). Insect species associated with X. STRUMARIUM have also been studied in Pakistan (Baloch et al. 1968) and India (Wilson 1960).
Hilgendorf and Goeden (1983) suggest that OEDOPA sp. nr. CAPITO (Diptera) is probably the only insect species worthy of study as a potential biocontrol agent for XANTHIUM STRUMARIUM. OEDOPA is restricted to the genus XANTHIUM, feeding on its roots. Baloch and Ghani (1969) suggest that a combination of insect species, with different feeding habits, would improve the chance of suppressing XANTHIUM populations.
Weaver and Lechowicz (1983) list 14 species of fungi that infect XANTHIUM in the U.S. and Canada. The rust PUCCINIA XANTHII Schw., which occurs throughout the U.S., southern Canada, parts of Europe, and India, is an obligate parasite on species of XANTHIUM and AMBROSIA (Conners 1967, Hasan 1974, Alcorn 1975, Jadhav and Somani 1978). It attacks all aerial parts of the plant except the flowers. Infected plants mature more rapidly than healthy plants and show decreased transpiration, dry weight, bur production, and percent germination (Hasan 1974, Julien et al. 1979). The spores overwinter on dead plant parts. Fungal and bacterial pathogens have had some success in controlling X. STRUMARIUM in India (Deshpande 1982). Kalidas (1981) induced rapid wilt in X. STRUMARIUM by using phytopathogenic toxins from seven different fungal and bacterial agents. Plant death was evident within 6 to 8 hours with each toxin. Sharma (1981) also describes a powdery mildew that infects cocklebur in India.
Nematodes reported from X. STRUMARIUM are APHELENCHOIDES RITZEMA-BOSI Schmidt (Weaver and Lechowicz 1983) and MELOIDOGYNE HAPLA (Siddiqui et al. 1973). CUSCUTA PENTAGONA (dodder) is a higher plant parasite that has been found on cocklebur (Munz and Keck 1973). OROBANCHE RAMOSA L. (broom rape) is another parasitic plant found on a variety of cultivated and weedy plants, including XANTHIUM (Polunin 1966, Munz and Keck 1973).
CONTROL BY GRAZING
Because of its toxicity and unpalatability, grazing is not a viable control method for XANTHIUM STRUMARIUM.
CHEMICAL CONTROL
Cocklebur is susceptible to a wide variety of soil- and foliar- applied herbicides commonly used for the control of broad-leaved weeds (Weaver and Lechowicz 1983), but certain XANTHIUM complexes are more susceptible than others (Anderson 1982). Dr. Jim McHenry (personal communication 1985), of the University of California, Davis, recommends the following herbicides for cocklebur control in California's preserves:
(1) 2,4-D amine, a phenoxy-type herbicide used for broadleaf weed control, should be applied to plants at the 3- to 5-leaf stage of growth. Application should be at the rate of 1 to 1.5 lbs/100 gallons of water, with one quart of surfactant/100 gallons. (Surfactants lower surface tension of the spray and increase the herbicide's effectiveness.) 2,4-D does not affect grasses.
(2) Dicamba (Banvel) is a broad spectrum herbicide used against perennial broadleaf weeds. It may persist in the soil for up to eight weeks. The suggested mixture is 0.5-0.75 lb/100 gallons water, with one quart of surfactant/100 gallons, and an application rate of 0.5-1.5 pints/acre (not to exceed 2 gal/acre in growing season). Dicamba is more selective than 2,4-D.
(3) Bromoxynil (Buctril, Brominal) is a contact herbicide which affects only the plants or portions of a plant actually contacted by the chemical. Therefore, adequate distribution of the chemical over the foliage is essential. Bromoxynil should not be used on grazed lands but is effective in controlling a wide variety of broadleaf weeds, including XANTHIUM STRUMARIUM. The suggested rate of application is 0.56-1.12 kg/ha (Beste 1983).
(4) Selective weed oils. There are several petroleum oils used for weed control. The herbicidal use of oils depends on their chemical and physical properties. Most contact oils evaporate slowly and owe their plant toxicity to their high content of aromatic compounds. Spraying oil on cocklebur will be effective only if entire plants are coated.
APPLYING HERBICIDES
Herbicides can be applied uniformly over an area (for large infestations) or by spot spraying individual plants. Dr. McHenry recommends using a flat-fan nozzle (Spraying Systems Co. #8003 or #8004 nozzle tip) rather than the cone nozzles available on most garden sprayers, as cone sprayers produce greater atomization of the chemicals and increase the chance of drift into unwanted areas. Spraying should be done on calm days when plant surfaces are dry.
Management Research Needs: The origin of XANTHIUM STRUMARIUM needs to be determined. Additional research is needed on the possible toxic effects of X. STRUMARIUM on wildlife, on biological control measures, and on the effects of prescribed fires.