Nutrient dynamics in a tritrophic system of ants, aphids and beans

The interactions between ants (Lasius niger), aphids (Aphis fabae) and plants (Phaseolus vulgaris) were studied in a laboratory experiment with the following treatments: application of N-fertilizer for plants, supply of mealworms or sugar solution or their combination as alternative food sources for ants. Three main questions were studied: (1) Do ants reduce tending to honeydew-producing aphids when an alternative sugar or protein resource was available? (2) Is aphid predation/protein consumption by ants higher when additional carbon is offered to maintain the carbon/protein balance? (3) Does fertilizer treatment propagate in the food web? For the experimental analysis stable isotope techniques were applied. delta N-15 served as a marker for the pathway from plants to higher trophic levels. Low delta N-15-value of fertilizer spread from plant shoots to aphids and ants. To trace which sugar-/protein source was consumed by ants, the different C-13/C-12-ratios of C-3- and C-4-plants were used with aphids feeding on C-3-plant material, while mealworm food and sugar solution originated from C-4-plant material. Fertilizer application had no effect on biomasses of plants, consumers or microflora. Ant biomass was significantly higher when additional sugar solution was offered. Higher contents of C-13 indicated a high incorporation rate of additional sugar. Additional protein had no effect on colony biomass and no increasing predation on aphids could be observed when carbon was in excess. However, due to the lack of queens and newly produced larvae, protein requirements of experimental colonies were lower than in natural systems. Ants positively affected aphid populations, but reduced tending, whilst having access to an alternative sugar resource. When sugar/protein was offered to ants, the host plant had an increased root/shoot ratio. This indicates that decreasing aboveground activity of ants could lead to reduced plant growth of aphid-infested plants, presumably due to higher fungal attack on shoots.