IMPACT OF INSECTS AND FUNGI ON DOUBLEGEE (EMEX AUSTRALIS) IN THE WESTERN-AUSTRALIAN WHEAT-BELT

Biotic influences on doublegee (Emex australis Steinheil) seed product ion were investigated as a precursor to the introduction of new insect biological control agents for this weed, and to investigate the cause of doublegee decline in the northern and central wheatbelt of Western Australia since 1990. The symptoms of the decline are doublegee plant s of reduced size with distorted leaves, collapsed stems, and smaller achenes (the spiny seed-bearing fruit) that crumble when mature. Three sites were investigated in 1992 by surveys for insects and fungi, and insect and fungus exclusion experiments. Emex stem blight (Phomopsis emicis R. G. Shivas) was present at the 3 study sites. The Watheroo si te had comparatively high levels of dock aphids (Brachycaudus rumexico lens Patch) on doublegee plants, the Badgingarra site had a comparativ ely high density of dock sawfly (Lophyrotoma analis Costa) on doublege e, and very few insects were present on doublegee at the Wongan Hills site. Viruses were not detected in samples of plants showing the effec ts of decline. The exclusion experiment showed a significant effect of removing insects and fungi on achene dry weight at the Watheroo site. There was no treatment effect at the Badgingarra and Wongan Hills sit es. The biology of the fungus and the aphid lead to the conclusion tha t the primary cause of doublegee decline is the dock aphid. This indic ates that biological control against E. australis might be achieved by using insects that indirectly affect seed quality.