Distribution, invasiveness, biology and control of rubber bush (Calotropis procera) in northern Australia

Summary

This project investigated the weed risk posed by rubber bush (Calotropis procera) on rangelands in northern Australia.

Rubber bush establishes most prolifically in disturbed environments where there is reduced plant competition. Mature plants have remarkable survivability and seed profusely. Mating relies on specific insect (Hymenoptera) pollinators during warm months (October-February). Fruit production is aided by self-compatibility, peaking in medium-density (250-500 individuals/ha) stands. Low but similar fruit production per plant occurs at low stand densities, but in large dense stands pollinator limitation causes low fruit set per plant. Each fruit contains ~430 wind-dispersed seeds. Some seeds (~7.5%) are blown >1 km making range expansion of 1 km every 2-3 years possible. Seeds have high germinability (85-100%), no dormancy, and germination success is temperature and moisture dependent. Seeds exposed to high soil surface temperatures rarely germinate, while buried seeds germinate without fail.

Overall, rubber bush exerts considerable propagule pressure resulting in significant invasive potential. Distribution modelling shows that currently it has not saturated its potential range. The milkweed bug Oncopeltus fasciatus affects the fruit, and natural dieback events also reduce populations in some areas. Several herbicides can be used effectively to control rubber bush using a range of techniques. Mechanical methods require that the root system is severed 10-20 cm below ground, and that follow-up control is undertaken to treat new plants favoured by the disturbance.