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 although now it is 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.
The abundance and persistence of parthenium weed seeds in the soil at
two sites in central Queensland was also investigated. 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 the parthenium plants were much taller and larger than
those grown under present levels of carbon dioxide. Parthenium weed plants were also grown
in competition with buffel grass at both of these concentrations of carbon dioxide. The
dry weight of buffel grass plants was about 7 times greater than that of parthenium weed
when they were grown at present levels of carbon dioxide. 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 modeled using
CLIMEX, a computer program which determines the climatic suitability of sites for the
growth of the weed. This modeling 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 where it is not yet present and
also to many other countries. In the future, the data generated in this project will be
used in other more complex computer models to give more accurate predictions.
For more information, please contact:
Steve Adkins
Tropical & Subtropical Weeds Research Unit, University of Queensland, St Lucia QLD
4072 Australia. Tel +61 7 3365 2072, Fax +61 7 3365 1177
Email: s.adkins@mailbox.uq.edu.au ,
Web-Site: www.weeds.uq.edu.au |