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Competition with filamentous fungi and its implication for a gregarious lifestyles in insects living on ephemeral resources

Rohlfs, M., Obmann, B., Petersen, R.
Ecological entomology 2005 v.30 no.5 pp. 556-563
Drosophila melanogaster, molds (fungi), Aspergillus niger, Alternaria alternata, Penicillium, interspecific competition, larvae, aggregation behavior, population density, mortality, insect development, temporal variation
1. Recent studies have demonstrated the existence of positive density dependence in the survival and development of Drosophila (the so-called Allee effect); however the underlying mechanisms of such Allee effects have remained elusive. Competition with filamentous fungi have often been suggested to be involved in causing high mortality at low larval density, but it has not yet been explicitly tested if the well known spatial aggregation of insect eggs yields a fitness benefit for the developing larvae in the presence of noxious moulds. 2. Using Drosophila melanogaster, the present study tested whether larval survival is greater in aggregations when confronted with various combinations of three representative mould species (Aspergillus, Alternaria, and Penicillium) and a head start for fungal development. 3. High rates of fungal-dependent mortality and significant positive density-dependent larval survival (i.e. Allee effects) were observed when larvae were confronted with food resources containing established colonies of Aspergillus or Alternaria. Neither the simultaneous transfer of Aspergillus or Alternaria spores with larvae to food patches nor food infections with Penicillium affected insect larval development. 4. Significant correlations between mould growth and larval survival could be identified, although the patterns that emerged were shown to be inconsistent when the effects were compared between fungal species and fungal priority. Because mould growth only partly explained larval survival, the influence of other fungal-borne factors, e.g. mycotoxins, needs to be elucidated in order to understand the mechanistic basis of insect-mould interactions. 5. These results are the first to argue convincingly for moulds being involved in mediating Allee effects for insects on ephemeral resources; however they also demonstrate an unexpected diversity in insect-mould interactions. Considering this diversity may be important in understanding insect spatial ecology.