Examination of data claiming Drosophila C virus is beneficial do not support this claim
Drosophila C Virus (DCV) is often claimed to be a beneficial virus based on some of the older literature (see references below). However, this data does not actually show that DCV increases host fitness. To establish any effects on host fitness related traits data were collected from two papers in which DCV is reported to enhance two fitness related traits, taking the mean values for each line/population studied and looking at the overall effect of DCV on ovariole number and development time.
Data was collected from:
Gomariz-Zilber, E. and Thomas-Orillard, M. (1993), Drosophila C virus and Drosophila hosts: a good association in various environments. Journal of Evolutionary Biology, 6: 677–689.
Thomas-Orillard M. Modifications of Mean Ovariole Number, Fresh Weight of Adult Females and Developmental Time in DROSOPHILA MELANOGASTER Induced by Drosophila C Virus. Genetics 1984;107(4):635-644.
The xls file contains data taken from these two papers and plots of these data. There appears to be no overall effect of DCV on development time. There is a small effect on ovariole number (note lifetime fecundity was not measured) with an increase of ~2 ovarioles in infected flies. Note oviariole number can increase in starved flies (dx.doi.org/10.1186/1471-2148-6-57) so this may be due to infection induced anorexia; DCV is known to trigger a nutritional stress response due to intestinal obstruction (dx.doi.org/10.1128/JVI.02320-14). Even if this does relate to an increase in fecundity, it is likely a life history shift (parasite induced fecundity compensation dx.doi.org/10.1098/rspb.2004.2959), and would equate to ~2-4 extra eggs a day which is unlikely to compensate for the lifespan of the flies being vastly reduced due to being killed by DCV in only a few days.