Evaluation of prey for the spider Dicymbium brevisetosum Locket (Araneae :Linyphiidae) in single-species and mixed-species diets

The objective of this study was to asses effects of dietary mixing of prey of different quality for a generalist predator. prey of three qualities wer e tested in single-species and mixed-species diets: the cereal aphid Rhopal osiphum padi as a low quality prey, and two qualities of fruit flies Drosop hila melanogaster representing intermediate quality (Normal flies) and high quality (Enriched flies) prey. The two types of fruit flies were obtained by rearing the flies on different media. It was expected that aphids might contribute positively to the diet of intermediate quality flies, but contri bute nothing or negatively to that of high quality flies. The value of prey was assessed by fitness parameters in an egg production experiment. Female s of the linyphiid spider Dicymbium brevisetosum were assigned to one of 6 diet treatments: 1) Normal D. melanogaster, 2) Normal D. melanogaster + R, padi, 3) Enriched D. melanogaster; 4) Enriched D. melanogaster +R. padi, 5) R, padi, and 6) R. padi added to Normal D. melanogaster until the first eg gsac appeared, then only R. padi. The following parameters were recorded: n o. of egg sacs per female, no, of eggs/sac, and hatching success. Females a n single-species diets of aphids produced fewer eggsacs containing fewer eg gs than spiders on fruit fly diets. Normal flies supported a high egg layin g rate but low hatching success compared to Enriched fruit flies. Mixing ap hids with Normal fruit flies had no effect on the measured fitness paramete rs, whereas mixing aphids with Enriched flies resulted in a lower total pro duction of spiderlings suggesting a toxic effect of aphids on spiders. A survivorship experiment with hatchlings was conducted in order to investi gate the effect of maternal diet on the offsprings' ability to utilise a lo w quality prey (R. padi). Two hatchlings from each of the first eggsacs pro duced by females in the egg production experiment on diet treatments 1) Nor mal D. melanogaster, 2) Normal D. melanogaster + R. padi, 3) Enriched D, me lanogaster, and 4) Enriched D. melanogaster + R. padi, were kept individual ly on a diet of R. padi and survival time was recorded. Maternal diet affec ted survival of those offspring feeding exclusively on R. padi. A lower sur vival was found on offspring from females reared on Normal fruit flies dcom pared to Enriched fruit flies, thus quality of offspring may vary with that of maternal diet. The results emphasise that effects of dietary mixing dep end on the characteristics of the prey types composing the diet. Negative e ffects of adding aphids to Enriched flies was found by a lowered hatching s uccess, while positive effects of adding aphids to Normal flies was found i n the survival of offspring.