Euphorbia heterophylla
(wild poinsettia)

E. heterophylla originated in the tropical and subtropical regions of America but is now distributed throughout tropical Africa, Asia and the Pacific in a total of at least 65 countries.

E. heterophylla grows in moist tropical and subtropical regions on a wide range of soils, principally in shaded waste places and in cultivated areas (Parsons and Cuthbertson, 1982).

E. heterophylla has an annual, spring-summer cycle.

Germination

Seeds are produced in great quantities with high viability. Seeds recently shed are dormant. Light and alternate temperatures (25/3°C) stimulate germination (Kissman and Groth, 1993). Each fruit bears three seeds which are expelled when the fruit is ripe. In Brazil, seeds germinate and seedlings emerge throughout most of the year. Seed longevity is high, and seeds may remain viable with a low dormancy level after being eaten by birds (Kissman and Groth, 1993).

Bannon et al. (1978) found that significant differences in germination occurred between two seed lots from different years. An alternating temperature regime of 25 and 35°C was optimal for germination and no effect of light was noted at these temperatures. However, light increased germination at constant temperatures of 25 and 35°C, and decreased germination at alternating temperatures of 10 and 35°C. Storage of E. heterophylla seeds at 36°C for 2-12 weeks caused a significant decrease in dormancy as compared to corresponding storage at 5°C. Seed moisture levels below 7.7% did not affect viability at 5 or 25°C after a storage period of 3-9 months. Seed viability decreased rapidly when seeds were stored for 3 months at 25°C with a moisture content of 10.8%, or at 5 or 25°C with a moisture content of 18.6%. Seeds buried in the autumn at a depth of 5 cm germinated in the field 9 months later. Field germination decreased as sowing depth increased.

E. heterophylla seeds germinate over a wide range of conditions, which explains why the plant is becoming an increasingly serious problem; germination was at least 95% when exposed to a solution of pH 2.5-10 or a solution with osmotic potential of up to -0.8 MPa. Light was not required for germination. Seed germination occured at temperatures ranging from 20-40°C with maximum germination (97%) at 35°C (Brecke, 1995). Etejere amd Okoko (1989) reported 95% seed viability. Seed weight increased by 63% after 36 hours of water imbibition. Optimum germination of the seeds occurred at 30-35°C. The young seedlings emerged from depths of up to 9 cm with maximum emergence at 1-3 cm.

Dorney and Wilson (1990) found that seedlings established best when left on the soil surface, particularly when covered by mulch. Seeds had no dormancy period and germinated in response to sufficient water. Machado-Neto and Pitelli (1980) studied the effect of sowing depth on emergence rate and found that the highest emergence rates occurred on the 5th day from seeds located at depths up to 2 cm, on the sixth day at 4-6 cm, and the seventh day at 8-10 cm. Average percentage germination was 21.3% with sowing at 0 cm and 79.0-84.7% at 2-10 cm; similar results were obtained by Cerdeira and Voll (1980). The data clearly demonstrated the ability of the weed to germinate from considerable depths and this is considered a factor in the aggressiveness of the plant.

Growth

A regrowth of the axillary buds of young plants is observed if a mechanical treatment or contact herbicide destroys the upper leaves. This process occurs if enough light is received (Kissman and Groth, 1993). Furthermore, Langston et al. (1984) have shown that E. heterophylla has unusual and remarkable ability to regenerate by adventitious buds developing from below the cotyledonary node after shoot excision. There was 100% recovery after such excision at cotyledon and 4-leaf stages. Of 17 weed species studied, the only others to show even partial recovery were Aeschynomene indica and A. virginica.

E. heterophylla is a C4 plant and its growth habit is highly dependent on light intensity. Paliwal and Ilangovan (1988) performed autoecological studies on several species, including E. heterophylla. They showed that photosynthetic processes and the rate of photosynthesis decreased with increasing leaf age. For E. heterophylla a good correlation was evident between the photosynthetic rate, stomatal resistance, protein content, transpiration rate, biomass, photosynthetic pigments and nitrate reductase activity.

Soyabean cultivars in competition with E. heterophylla at three densities and two periods of occurrence were studied by Chemale and Fleck (1982) in Brazil. Soyabean seed yield was reduced by weed competition; the number of pods and seeds decreasing with increasing weed density. Only the highest density reduced stem diameter and node numbers.

Weed populations varied most markedly with changing seasons and different levels of fertilizer application in different crops. E. heterophylla was among the species most promoted by fertilizers (Marnotte, 1984).


Allelopathy

Remison (1978) found that in the glasshouse, cowpea competition with E. heterophylla decreased the height of the plant, the number of nodes, peduncles and green leaves and the weight of pods and seeds and the decrease was greater with increasing density of the associated weed. In competition with the weed, cowpea did not respond to the application of fertilizers. In field experiments, competition from the natural weed flora affected the number of days to 50% flowering as well as other yield components of four cowpea cultivars studied. The yield of the climbing variety, Dinner, was least affected by competition whilst the semi-erect variety, Ife brown, was affected most.

Mohamed-Saleem and Fawusi (1983) studied the allelopathic effects of plant material of several plants, including that of E. heterophylla, on tomato, pepper and sorghum. They found that increasing amounts of decomposed weeds significantly reduced germination and seedling growth, although E. heterophylla had the least effect.

Allelopathic effects of seven weed species on pumpkin and aubergine were studied under greenhouse conditions by Almodovar-Vega et al. (1988a, b). Root exudates from the roots of several plants, including E. heterophylla, decreased the vine length and dry weight of pumpkin seedlings when added to their growth medium.

Gutierrez and Gonzalez (1993) reported that in garlic crops, Echinochloa colonum and E. heterophylla were found harbouring the mite Rhizoglyphus setosus. Lee and Subbarao (1993) found that all single-spore isolates of Diaporthe phaseolorum var. caulivora originating from diverse, soyabean-growing locations in the USA formed perithecia on substrates of plant origin. Zeigler and Lozano (1983) demonstrated that cross-inoculations of Elsinoe and Sphaceloma species pathogenic on cassava were also pathogenic on several Euphorbia species, including E. heterophylla.

Nymphs and adults of the mycophagous thrips Euphysothrips minozzii were found feeding on fungi infecting Lablab purpureus, Pennisetum glaucum and E. heterophylla in the field at Coimbatore, Tamil Nadu, India (David et al., 1973).

Laycock et al. (1977) reported that weed control in Ghana with herbicides significantly reduced the percentage of maize plants attacked and the number of larvae per plant of the stem borer Sesamia botanephaga [S. nonagrioides botanephaga]. Observations by Ba-Angood and Ba-Angood (1977) in the Sudan showed that Poekilocerus hieroglyphicus [P. bufonicus hieroglyphicus] preferred Calotropis procera and E. heterophylla. On a dry matter basis, it consumed 6.02 g of E. heterophylla in 6 days.

Topham and Beardsley (1973) found that E. geniculata and E. heterophylla were preferred to the previously known species as nectar sources for the Papua New Guinea sugarcane weevil parasite, Lixophaga sphenophori.

Total longevity of adults of the hemipteran Euschistus heros fed on E. heterophylla was significantly higher than when fed on other host plants (Panizzi et al., 1988). Inserra et al. (1989) reported that weed hosts of Rotylenchulus reniformis in ornamental nurseries of southern Florida included several weeds, among them E. heterophylla.

A survey of wild plants in and around cotton fields at two localities in Thailand showed that several weeds, including E. heterophylla, were acting as reservoirs or providing shelter for Bemisia tabaci, which is an increasingly important pest with the extension of irrigated cotton cultivation. Evidence was found that E. heterophylla can act as important sources of populations of B. tabaci that infest the 'out-of-season' cotton (Nachapong and Mabbett, 1979).

The potential of E. heterophylla as a food plant for Tetranychus urticae was studied in Cuba by Perez et al. (1987). P. bufonicus hieroglyphicus preferred Calotropis procera, Vicia faba and E. heterophylla, as reported by Ba-Angood et al. (1975) in a comparative study of the food plants of some species of Acrididae.

E. heterophylla has been reported as a host plant for R. reniformis by MacGowan (1989). Reproduction of Meloidogyne javanica in weeds, including E. heterophylla, was investigated by Lordello et al. (1988).

Debrot and Centeno (1985) determined in Venezuela that a striking yellow mosaic frequently observed on E. heterophylla was caused by Euphorbia mosaic bigeminivirus. The virus was transmitted to healthy E. heterophylla and E. prunifolia by B. tabaci, but not through seeds of E. heterophylla. It was also mechanically transmitted from both these Euphorbia species to Datura stramonium. The local cultivar Tacarigua of Phaseolus vulgaris was resistant to the virus on mechanical inoculation or transmission by B. tabaci. Kim and Fulton (1984) suggested that the Euphorbia virus that infected Datura stramonium in Arkansas, USA, belongs to the group of whitefly-transmitted geminiviruses.

Environmental

• Host of pest

Materials

• Poisonous to mammals

E. heterophylla is not readily confused with any other common weedy species, with the exception of E. cyanthophora which has similar variable leaf shape but the bracts mainly red (as opposed to green in E. heterophylla), nectaries sub-sessile with elliptical opening (stalked with round opening in E. heterophylla), fruits ovoid-angular (v. trigonous) and seeds tuberculate, without caruncle (prismatic tuberculate, with caruncle in E. heterophylla) (Oliviera and Sa-Haiad, 1988). Its relationship to other Euphorbia spp. in Taiwan is described by Lin and Hsieh (1991).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Chemical Control

E. heterophylla is not easily controlled by chemical methods, but reasonable to adequate control may be achieved with the following herbicides:

Ametryn, ammonium glufosinate, bromacil, chlorimuron-methyl, 2,4-D, fomesafen, imazaquin, metribuzin, oxyfluorfen, paraquat and terbacil. Also mixtures of ametryn with atrazine, diuron, 2,4-D or MSMA; asulam plus diuron; bentazon plus MCPA or paraquat; diuron plus 2,4-D, hexazinone, MSMA or paraquat; 2,4-D plus glyphosate, ioxynil, MCPA, picloram or propanil.

A useful review of the limitations and potential of the older herbicides was prepared by Wilson (1981). This notes the general unreliability of atrazine and linuron in maize and other crops, but 'general satisfactory' control by 2,4-D. In broad-leaved crops, trifluralin and related compounds are also ineffective, but compounds giving some useful control included fluridone, oxadiazon, cyanazine, 2,4-DB, bentazon and acifluorfen. Oxadiazon gives less reliable control than metribuzin, but may be useful on soils where metribuzin cannot be used (Nester et al., 1979).

Willard and Griffin (1993) studied the response of E. heterophylla to imazaquin, fomesafen and acifluorfen, and chlorimuron. Based on reductions in weed height and lateral branch number 28 days after treatment, E. heterophylla was controlled more effectively by herbicide applications at 5-7 cm height than at 15-20 cm in both greenhouse and field studies; however in the field study, weed biomass reduction gave variable results. With application of all herbicides at 15-20 cm, seed production of the weed was equal to the untreated control.

According to Harger and Nester (1980), the most effective soil-applied herbicide, metribuzin, gave 70-90% control, but later germination of E. heterophylla necessitated the use of post-emergence herbicides. E. heterophylla germination was reduced by sowing the soyabeans in early May, as shading from the soyabean canopy lowered soil temperature. Bentazone with a surfactant was the most effective overtop herbicide treatment when applied before E. heterophylla seedlings were 10 cm tall.

Biological Control

Biological control has been successfully used in Brazil using Helminthosporium sp. spores in an inundative technique (Yorinori, 1985).