Summary and conclusions, Miljøstyrelsen

Entomophthorales on cereal aphids

Summary and conclusions

Aphids are important pest insects on cereals in Denmark. The three most important species are: the cereal aphid (Sitobion avenae), the bird cherry-oat aphid (Rhopalosiphum padi) and the rose-grass aphid (Metopolophium dirhodum). Insect-pathogenic fungi from Entomophthorales establish natural epizootics in populations of aphids in cereals. The most common species on aphids on cereals are Pandora neoaphidis, Entomophthora planchoniana and Conidiobolus obscurus.

The project aimed to evaluate the potential of fungi from Entomophthorales for biological control of aphids in cereals, both for direct control ("biopesticides") and for indirect control (enhancing natural epizootics). The report was written based on results obtained in the experimental work and from literature studies.

Morphological, pathobiological and molecular methods were implemented and further developed for characterisation of aphid pathogenic fungi from the genus Pandora. It was shown that a correlation between RAPD-PCR profile and the geographical origin of the isolate was present, while no correlation between host species and profile was seen. These methods can be used to study other insect-fungus systems and furthermore for monitoring of isolates released for biological control.

Studies on natural occurrence documented that populations of R. padi on the winter host (bird cherry) were infected by a broader range of fungi than in cereals. The species Neozygites fresenii and Zoophthora phalloides were only found on host insects on their winter host. We consider the interaction between aphid host, summer and winter plant host, and aphids on other host plants as important for the development of epizootics. We found that soil is an important reservoir for winter survival structures of P. neoaphidis and C. obscurus and infection in spring is likely to begin from these structures.

Successful isolation and growth of P. neoaphidis was obtained both in vivo and in vitro. For small scale experiments in vivo cultures are sufficient, while in vitro propagation is necessary for experiments where large amounts of inoculum are needed (e.g. for field releases). The in vitro growth experiments focused mainly on the importance of agitation speed. High virulence of P. neoaphidis against S. avenae and R. padi was documented in bioassays. Calculated LC₅₀ revealed that alates were more susceptible than apterae. LT₅₀ for P. neoaphidis infected individuals was strongly dependent on incubation temperature.

A biological conceptual model was created to encourage a deeper understanding of a system consisting of S. avenae and P. neoaphidis. Our experimental data and data from the literature were used to develop the model, which then pointed out factors of significance in the development of epizootics.

It can be concluded that fungi from Entomophthorales possess great potential for future, biological control of aphids in cereals and other crops. Possible strategies include developing the fungi as biopesticides or using them to enhance natural epizootics.

It can be confirmed that particularly P. neoaphidis has potential as a future biopesticide. Further development depends on additional experiments, including technical studies involving production and formulation. Nevertheless documentation of the effects following outdoor release is still required.

The potential of a strategy for enhancing natural epizootics was also supported in the project. In particular, the importance of alternative aphid hosts, winter host plants for aphids and the soil was elucidated. Additional studies including factors we now hypothesise are important, e.g. landscape elements, are necessary before an operational strategy is possible.