8. General discussion and conclusions, Danish Environmental Protection Agency

Entomophthorales on cereal aphids

8. General discussion and conclusions

8.1	Background
8.2	General Perspectives

8.1 Background

The background for this work was the need to develop alternative methods for pest insect control. Due to a general wish from the society to minimise the use of chemical pesticides, biological control may offer a sound, environmentally friendly alternative.

Choice of system

The cereal aphid/fungus system was chosen as a model system based on:

1) The importance of cereals in Denmark, 2) The difficulties in controlling aphids in cereals without chemical pesticides, and 3) Knowledge of the natural occurrence of entomopathogenic fungi obtained in an earlier project.

Aphids will in the future still be significant pest insects, and the desire to minimise the use of chemical pesticides has certainly not diminished during the project period (eg. the "Bichel Report" to the Ministry of Environment 1999).

The project proceeded in three ways: 1) New scientific information was obtained during the project period based on the specific experimental work, 2) Relevant literature was reviewed, and 3) A high level of information exchange was maintained with other research groups (in Denmark and internationally) during the period. The report contains both specific new data from our studies, a compilation of data from the literature, and the newest available data from other research groups, including the most recent findings reported at conference in August 1999.

8.2 General Perspectives

The national dimension

For the Danish cereal system, the set of data obtained represents a solid basis for future work directly addressing biological control. Two types of biological control are potentially possible: 1) A direct release of Entomophthorales as biopesticides and 2) An enhancement of the natural control of aphids with Entomophthorales. Both strategies were given attention in our studies. With respect to 1), the fungus P. neoaphidis has the highest potential at present. We were able to characterise, isolate and grow this fungus in vivo and in vitro, perform bioassays and develop parts of a model for this species in relation to cereal aphids.

However, before the fungus can be used directly as a biopesticide, a range of further experiments must involve: more efficient growth in vitro, formulation, release in the field, and evaluation of field release. Our studies have shown that the fungus should be subjected to further investigation in order to develop outdoor biological control.

Our attention on the winter survival of Entomophthorales provided us with significant information for a deeper understanding of the plant-aphid-fungus system. Based on the results we do not hesitate to propose that future use of Entomophthorales to control aphids should not solely be based on the biopesticide approach, but that strategy 2) should certainly also be seriously considered to enhance natural control (eg. earlier initiation of epizootics). Additional experimental work should in such cases include more in depth studies on the importance of cropping systems, surrounding hedges and other landscape structures for the development of epizootics.

The European dimension

During the project period, attention on Entomophthorales as biocontrol agents of aphids has increased significantly. In particular, the group at ETH, Switzerland, and the group at Rothamsted Exp. Station, UK, should be mentioned. We maintained close contact with these groups and performed experimental work in co-operation (e.g. growth in vitro). The increased attention on Entomophthorales has led to a European network cooperation from year 2000 (COST Action 842). Results from the Danish studies on cereal aphids contributed to the success of the application.

In the coming years, we plan to use the European network to ensure that our results will be used by the scientific society to assist in well-designed experiments for both the cereal aphid-Entomopthhorales system and for other insect-fungus systems as well.

In summary, conclusions from the project as a whole are as follows

-	Morphological, pathobiological and molecular characterisation methods were developed for P. neoaphidis, and proved that these sets of methods are now operational for ecological studies and can be transferred to other systems (chapter 2)

-	The natural occurrence of Entomophthorales in different ecosystems relevant to cereal aphids was described, and proved that a number of species from Entomophthorales occur on cereal aphids with P. neoaphidis and E. planchoniana being the most important species (chapter 3)

-	The winter survival structures of Entomophthoralean fungi infecting cereal aphids were investigated. It was shown that the soil is an important reservoir for infective units during winter, and that the number of fungal species occurring on cereal aphids on their winter host plants is higher than in the crop (chapter 4)

-	Methods for in vivo and in vitro isolation and growth of P. neoaphidis were implemented and further developed. It was shown that in host transmission studies, in vivo cultures are sufficient although effective growth in vitro can also be obtained (chapter 5)

-	The virulence of P. neoaphidis against R. padi and S. avenae was investigated and it was shown that the different morphs varied with susceptibility and that the lethal time was strongly dependent on temperature (chapter 6)

-	The interaction between P. neoaphidis and S. avenae was described by a biological conceptual model. The model confirmed that P. neoaphidis has potential for biological control of S. avenae (chapter 7)

-	The set of data obtained are of special relevance for the cereal aphid - Entomophthorales system, however the methods used (eg. characterisation methods) may prove useful in other studies on insect host - fungal pathogen systems, e.g. ecological studies on the performance of a fungus following release in the field