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Individual study: The effect of bitou bush Chrysanthemoides monilifera ssp. rotundata treatment with herbicide and weed removal on the rare endemic shrub Pimelea spicata, at Shellharbour, Illawarra, New South Wales, Australia

Published source details

Matarczyk J.A., Willis A.J., Vranjic J.A. & Ash J.E. (2002) Herbicides, weeds and endangered species: management of bitou bush (Chrysanthemoides monilifera ssp. rotundata). Biological Conservation, 108, 133-141

Summary

In Australia, weed invasion has caused the extinction of several endemic plant species and threatens others. Rare plants, which often have small population and range sizes, and are competitively inferior to invasives, are at particular risk. For example, Pimelea spicata is an endangered endemic shrub threatened by habitat loss and invasion by weeds, including the bitou bush Chrysanthemoides monilifera ssp. rotundata which infests about 70,000 ha of the New South Wales coastline. Bitou bush was first recorded in Australia near Newcastle, New South Wales around 1908, probably introduced through dumping of ships' ballast. From 1946 to 1968 it was planted along the coast to revegetate areas after mining. It is now regarded as one of the worst weeds in Australia because of its invasiveness and associated detrimental economic and environmental impacts.

The winter application of the herbicide glyphosate is used to control bitou bush, whilst other native plants such as sclerophyllous shrubs in the genera Acacia (wattles), Banksia (banksias) and Leptospermum (tea-trees), and large Lomandra (mat-rush) tussocks are resilient. However, no attempts at bitou bush control by glyphosphate application had been undertaken in areas with Pimelea as the effects of glyphosate on this shrub were poorly known. Observations at sites were wind had caused glyphosphate to drift over areas where Pimelea was present suggested that it suffered reduced growth and productivity. This study investigated the effect of glyphosate application and manual weed removal on Pimelea.

Study populations: Two large populations of Pimelea spicata to the south of Shellharbour, Illawarrawere were selected, such that only a proportion (20%) of the adult population would be affected by herbicide spraying. During September 1998, 40 flowering plants were selected throughout the known extent of each population. From these, 30 in each site were randomly selected, and the diameter of the tap root at its thickest point was measured. These plants were then grouped with their nearest neighbours in five groups of six plants. Typically, plants within a block and the blocks were separated by 2-3 m.

Seedlings: Pimelea seedlings were grown in glasshouses for 12 weeks. Each selected adult plant had two seedlings planted next to it, seedling leaf number and height were recorded, and the seedlings were left to settle for four weeks. After this time, all seedlings were alive and healthy.

Treatments: Treatments were applied to 1 m² plots around each plant (and encompassing the seedling). The herbicide glyphosate was applied at the recommended rate for bitou bush infestations (1:180) with a hand aerosol spray pack, with no application as a control. Weed removal was at one of three intensities:

1) Slash – weeds cut by hand and the biomass left in situ
2) Slash and remove – weeds cut by hand and the biomass removed
3) no cutting control

The experiment was fully factorial, giving 2 herbicide treatments x 3 weed removal treatments x 5 replicate blocks. Plots receiving both herbicide and slash treatments were sprayed first, the herbicide allowed to dry, and then cut. Treatments were applied on 'day 1' in late September 1998.

On days 13, 36, 54, 84, 126 and 169 (169 = late February), the sites were revisited and the plants allocated to one of five health categories:

1 = healthy plants
2 = 1-25% of above-ground biomass dead
3 = 26-50% dead
4 = 51-75% dead
5 = 76-100% dead

Also, on adult plants, the tap root diameter was remeasured, the number of reproductive shoots counted, and the number of green shoots counted on the first and last day.

Herbicide treatment: Of those Pimelea plants treated with herbicide, 57% had died by the end of the experiment, while all remaining plants suffered some damage. The application of glyphosate had a significant negative impact on the number of green shoots, the number of reproductive shoots, plant health and tap root diameter of adult plants. This negative effects became more pronounced towards the end of the experiment.

Weeding: There was no significant effect of weeding treatments on the number of green shoots, the number of reproductive shoots, plant health or tap root diameter of adult plants. However, when pooling the herbicide treatments (glyphosate and control) resprouting of plants was greater in plants in the slash and remove treatment (55% of plants), than the slash treatment (30%) and control (15%).

Seedlings: Only 17 of 60 (28%) of Pimelea seedlings survived until the end of the experiment. Furthermore, only one seedling survived the application of glyphosate.

Conclusions: Application of glyphosate killed Pimelea seedlings and half of the adult plants. Furthermore, reduction in tap root diameter suggests that further adverse affects might have been recorded if the experiment were longer-term. Consequently, the authors suggest that sustainable weed management is the best option for protecting this rare endemic from bitou bush and glyphosate application should be avoided in areas where Pimelea is present. (See also Case 381 for the efficacy of manual cutting in the control bitou bush).


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