The Influence of Seed Harvesting Ants in Annual Ryegrass Pastures and Their Possible Effects on the Epidemiology of Ryegrass Toxicity

Abstract

In the disease known as annual ryegrass toxicity, galls induced by a nematode, Anguina agrostis, replace seeds in ryegrass (Lolium rigidum) plants. Often these galls are colonised by a bacterium, Corynebacterium rathayi, which makes them toxic and frequently fatal to grazing animals. The ecology of ryegrass pasture ants was studied to determine their role in the epidemiology of this disease. Pitfall traps run over one year in ryegrass pastures revealed nineteen species of ants, all of which foraged maximally between December-March; the period corresponding to that when most seeds and galls are shed. Artificial depots of ryegrass seeds, placed out in pastures, indicated that 22-30% of seeds were removed within 24 hours. The principal ant species taking seed, presented in decreasing order of importance, were Pheidole sp. J.D.M. 155, Pheidole sp. J.D.M. 37, Melophorus sp. 1 (ANIC) and Rhytidoponera inornata. Seed/bacterial gall/nematode gall choice experiments indicated that ants were unselective harvesters of these items. Most seeds were stored around nest entrances, although lesser amounts were retained beneath the soil. Certain species were more prone to store seed than others.