Parthenium weed (Parthenium hysterophorus L.) – seed biology of Australian biotypes

Parthenium weed (Parthenium hysterophorus L.) is an aggressive weed that has become a serious problem in the beef producing areas of central Queensland as a pasture weed. It is now also emerging as a major weed of cultivated crops such as sunflower, sorghum, wheat and sugarcane. 

Two introductions of parthenium weed into Australia have occurred, one in central Queensland and the other in south-east Queensland. The central Queensland infestation has been much more aggressive and troublesome than the south-east Queensland infestation and some ecological differences between the two populations have been noticed in the field. Several differences were detected in the growth and phenology of the parthenium weed plants from the two populations. In particular, the plants from the central Queensland population grew much taller and were significantly larger than those from the south-east Queensland population. There were also differences in the seed size, seed colour and percentage of seeds filled when plants from the two populations were compared. Hence, it does seem that these two populations should be regarded as two distinct biotypes. These differences have since been confirmed by DNA analysis of the two populations.

Germination temperatures for Parthenium occur across the 8 to 30°C range with the optimum germination temperature being 22 to 25°C. Persistence tests demonstrated that more than 70% of parthenium seeds buried at 5 cm below the soil surface survived for at least 2 years whereas surface-lying seeds survived for no longer than 6 months. Parthenium weed seeds were found to be very persistent in the soil and there was relatively little change in their abundance over an 18 month period. Within the top 3.5cm of soil, parthenium seed accounted for between 47% and 87% of the total seed present with estimates in population varying between 15 and 25 million parthenium weed seed per hectare at one site and 150 and 340 million seeds per hectare at a second site, depending on the intensity of the weed infestation and the time of year. The germination rate of parthenium weed seeds was also significantly faster than that of all other species present. 

Examination of the effectiveness of the biocontrol agent, Epiblema strenuana, the stem-galling moth, found that the moth did significantly reduce the flower and seed production of the weed, particularly if the moth attacked the weed when it was young (35 days old). In this situation the weed’s seed production was reduced by about 75% and the weed’s height decreased by about 34%. If the moth attacked the weed at a later stage of growth (after 55 days), there was no effect on the weed’s height and only a 39% reduction in seed production. The effectiveness of the moth was even greater if it was applied to the weed when it was grown in competition with buffel grass. In this situation the weed’s seed production was reduced by about 90% when compared to plants that were not attacked by the moth.

The response of parthenium to enhanced carbon dioxide levels found that grown under higher carbon dioxide parthenium plants were much taller and larger than those grown under present levels of carbon dioxide. Parthenium plants grown in competition with buffel grass at present levels of carbon dioxide found that the dry weight of buffel grass plants was about 7 times greater than that of parthenium weed. However, at the enhanced level of carbon dioxide the dry weight of parthenium weed was about the same as that of buffel grass. These experiments suggest that parthenium weed may become more competitive in the future, as the level of carbon dioxide in the atmosphere continues to rise.

Finally, the potential distribution of the weed was modelled using CLIMEX, a computer program that determines the climatic suitability of sites for the growth of the weed. This modelling was undertaken using present day and predicted future climates for Australia and the entire world. The model showed that parthenium weed has the potential to spread throughout large areas of Australia and also to many other countries. This will be backed up in the future using more complex computer models to give a more accurate prediction.

For more information please contact Sheldon Navie
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