Tuned protection of aphids by ants against a predatory hoverfly

1. Aphid-tending ants that feed on honeydew have evolved strategies
against aphidophagous insects and tune their aggressive behaviour according to the
level of danger for their trophobionts. Here we investigate how Lasius niger Linnaeus
(Hymenoptera: Formicidae) ants react to different instars of Episyrphus balteatus De
Geer (Diptera: Syrphidae) hoverflies which vary in their voracity and defensive abilities.
2. During pairwise encounters, early syrphid instars (eggs, L1, and L2 larvae) elicited
lower aggression scores compared to third larval instars (L3), which was intensively
bitten by ants. L3 tried to escape from ants by releasing a sticky and toxic secretion over
biting ants that died or underwent severe morbidity.
3. In a standardised system including the host plant, aphid, tending ant, and hoverfly,
the ability of ants to protect an Aphis fabae Scopoli (Hemiptera: Aphididae) colony was
evaluated. Early E. balteatus instars placed onto the plant elicited no mobilisation of
ants, which often removed the hoverfly successfully. Eggs and early instars appeared as
the weak links for integrated pest management by hoverfly auxiliaries.
4. In contrast, L3 induced the number of ant patrollers to increase at a local scale
without any further recruitment from inside the ant nest. L3 syrphids were quite efficient
at gluing ants with defensive secretions and at resisting to removal attempts by ants.
5. While supporting the assumption that ants tune their defensive response to the
aphidophagous predator, the present results also showed a lack of efficient protection
of their trophobionts from the most voracious late syrphid instar.