Artificially prepared cow pats containing Ostertagia ostertagi eggs were deposited on two pasture plots in May, June and July 1986. Half of the cow pats, placed on one plot, were inoculated with 2000 conidia per gram faeces of the predacious fungus Arthrobotrys oligospora. On the other plot fungus-free control cow pats were placed at the same time. In the faeces generally fewer infective O. ostertagi larvae developed in the inoculated than in the control cow pats. On the herbage around the control cow pats deposited in May, June and July a maxium concentration of infective larvae was found at the same time on the 6th of August 1986. At that time the herbage larval infectivity around inoculated cow pats deposited in May, June and July was subject to a reduction of 48%, 89% and 46%, respectively, compared with fungus-free control cow pats. This experiment indicates that a concentration of 2000 A. oligospora conidia per gram faeces results in a significant lowering of the herbage larval infectivity during the grazing season in Denmark.

Three nematode-trapping fungi, one Arthrobotrys oligospora and two Duddingtonia flagrans isolates, were fed to Ostertagia ostertagi-infected calves to test their ability to destroy larvae of this parasite in faeces and consequently to reduce the transmission of infective larvae to herbage. The fungi had previously been selected for their capability to pass the alimentary tract of cattle without losing growth and nematode-trapping potentials. Dung was collected from three calves each fed one of the three fungi and placed as 1-kg cow pats on a parasite-free grass plot together with control cow pats from a calf that was not given fungi. The cow pats contained comparable concentrations of parasite eggs. The two D. flagrans isolates were highly effective in that they reduced herbage larval infectivity by 74–85%. In contrast, A. oligospora did not show any effect in the present experiment. Field experiments will demonstrate if D. flagrans represents a potential organism for biological control of bovine gastrointestinal nematodes under practical agricultural management conditions.

Artificially prepared cow pats containing Ostertagia ostertagi eggs were deposited on two comparable grazing plots at weekly intervals during June and July 1987. Before deposition the cow pats, on the one plot, were inoculated with 0·250 g mycelial fragments per kg of faeces of the predacious fungus Arthrobotrys oligospora. On the other control plot comparable numbers of non-inoculated cow pats were placed at the same time. Subsequently, in July, two equal groups of calves were turned out one on each plot to graze for two months after which they were necropsied. The number of infective O. ostertagi larvae in inoculated cow pats was reduced by 42% and herbage larval infectivity around them by 50–71% as compared with the corresponding parameters in control pats and surrounding herbage. After a grazing period of two months the calves on the plot with inoculated cow pats had acquired worm burdens that were on average 37% lower than those of calves on the control plot. The lower degree of parasitism in these calves was also reflected in lower egg counts, lower serum pepsinogen levels and higher body weight.

In field experiments, conducted on parasite-free grass plots in two consecutive summers, artificially prepared cow pats containing Cooperia oncophora eggs were inoculated with the nematode-trapping fungus Arthrobotrys oligospora. Numbers of infective C. oncophora larvae isolated from the pats as well as from the surrounding herbage were subject to an approximately teh-fold reduction as compared with numbers in fungus-free pats and herbage surrounding these. This reduction was undoubtedly a result of entrapment of the parasite larvae within the faecal pats.

Removal of the sheath of the ensheathed infective juvenile of Trichostrongylus colubriformis prevents capture by the nematophagous fungus Arthrobotrys oligospora. Exposure of the trap hyphae to a variety of saccharides, which may block a recognition system based on lectin/carbohydrate binding, failed to prevent capture but some saccharides did inhibit penetration and invasion by the fungus. Capture and penetration thus appear to be two distinct processes with capture being less specific than penetration. Carbohydrate residues were absent from the outer surface of the cuticle and the sheath but were present on the inner surface of the sheath. The limited accessibility of these lectin-binding sites may explain the slow process of infection of the infective juvenile by the fungus. The sheath does not protect the infective juvenile against attack by this nematophagous fungus.

The nematode-trapping fungus Arthrobotrys oligospora traps and invades all the free-living juvenile stages of the tri-chostrongyle nematode Trichostrongylus colubriformis. The processes of capture and invasion of the 1st-stage juveniles are described using differential interference contrast optics and 3-D reconstruction techniques. The adhesive responsible for capture is well preserved using a freeze-substitution technique for scanning electron microscopy. The invasion process of the ensheathed 3rd-stage juvenile of T. colubriformis takes much longer than in the 1st- or 2nd-stage juvenile and involves the formation of secondary infection pegs between the sheath and the cuticle which appear to penetrate the cuticle by physical pressure.

Most nematode-trapping fungi are dependent on specific hyphal structures on or in which nematodes can be trapped mechanically or by adhesion. These structures are a prerequisite for the ability of the fungus to invade a host and are thus crucial for survival as well as virulence. The diversity of trapping structures is large and highly dependent on the environment of the fungus. Within one single species, Arthrobotrys oligospora, not only adhesive nets are formed but also so-called conidial traps, hyphal coils around hyphae of other fungi, and appressoria in the rhizosphere of agricultural crops. In this article these structures and the conditions for their development are described. Since the trapping structures influence the survival and the virulence of their producer, it is important that we know more about the molecular background of their development and function. The application of genomics to understand the function and the development of infection structures, therefore, has substantially increased the potential of A. oligospora to become a model system for fungal morphogenesis.